NATIONAL COMPUTER
SECURITY CENTER
A GUIDE TO
UNDERSTANDING
CONFIGURATION MANAGEMENT
IN TRUSTED SYSTEMS
NCSC-TG-006-88
Library No. S-228,590
FOREWORD
This publication, "A Guide to Understanding Configuration
Management in Trusted Systems", is being issued by the National
Computer Security Center (NCSC) under the authority of and in
accordance with Department of Defense (DoD) Directive 5215.1. The
guidelines described in this document provide a set of good
practices related to configuration management in Automated Data
Processing (ADP) systems employed for processing classified and
other sensitive information. Recommendations for revision to
this guideline are encouraged and will be reviewed biannually by
the National Computer Security Center through a formal review
process. Address all proposals for revision through appropriate
channels to:
National Computer Security Center
9800 Savage Road
Fort George G. Meade, MD 20755-6000
Attention: Chief, Computer Security Technical Guidelines
____________________________
Patrick R. Gallagher, Jr. 28 March 1988
Director
National Computer Security Center
i
ACKNOWLEDGEMENTS
Special recognition is extended to James N. Menendez, National
Computer Security Center (NCSC), as project manager and primary
author of this document.
Special acknowledgement is given to Grant Wagner, NCSC, and Dana
Nell Stigdon, NCSC, for their constant help and guidance in the
production of this document. Additionally, Dana Nell Stigdon,
was responsible for writing the section on the Ratings
Maintenance Program. Acknowledgement is also given to all those
members of the computer security community who contributed their
time and expertise by actively participating in the review of
this document.
ii
CONTENTS
FOREWORD .................................................... i
ACKNOWLEDGEMENTS ............................................ ii
CONTENTS .................................................... iii
PREFACE ..................................................... v
1. PURPOSE ................................................. 1
2. SCOPE ................................................... 1
3. CONTROL OBJECTIVES ...................................... 2
4. ORGANIZATION ............................................ 3
5. OVERVIEW OF CONFIGURATION MANAGEMENT PRINCIPLES ......... 4
5.1 PURPOSE OF CONFIGURATION MANAGEMENT ................ 4
6. MEETING THE CRITERIA REQUIREMENTS ....................... 5
6.1 THE B2 CONFIGURATION MANAGEMENT REQUIREMENTS ....... 5
6.2 THE B3 CONFIGURATION MANAGEMENT REQUIREMENTS ....... 6
6.3 THE A1 CONFIGURATION MANAGEMENT REQUIREMENTS ....... 6
7. FUNCTIONS OF CONFIGURATION MANAGEMENT ................... 7
7.1 CONFIGURATION IDENTIFICATION ....................... 7
7.1.1 Configuration Items ......................... 8
7.2 CONFIGURATION CONTROL .............................. 10
7.3 CONFIGURATION STATUS ACCOUNTING .................... 11
7.4 CONFIGURATION AUDIT ................................ 12
8. THE CONFIGURATION MANAGEMENT PLAN ....................... 14
9. IMPLEMENTATION METHODS .................................. 16
9.1 THE BASELINE CONCEPT ............................... 16
9.2 CONFIGURATION MANAGEMENT AT MER, INC. .............. 18
9.3 THE CONFIGURATION CONTROL BOARD .................... 20
10. OTHER TOPICS ............................................ 23
10.1 TRUSTED DISTRIBUTION ............................... 23
10.2 FUNCTIONAL TESTING ................................. 24
10.3 CONFIGURATION MANAGEMENT TRAINING .................. 24
iii
10.4 CONFIGURATION MANAGEMENT SUPERVISION ............... 25
11. RATINGS MAINTENANCE PROGRAM ............................. 26
12. CONFIGURATION MANAGEMENT SUMMARY ....................... 27
APPENDIX A: AUTOMATED TOOLS ................................. 29
A.1 UNIX (1) SCCS ...................................... 29
A.2 VAX DEC/CMS ........................................ 30
GLOSSARY .................................................... 32
REFERENCES .................................................. 34
(1) Unix is a registered trademark of Bell Laboratories
iv
PREFACE
Throughout this guideline there will be recommendations made that
are not included in the Trusted Computer System Evaluation
Criteria (TCSEC) as requirements. Any recommendations that are
not in the TCSEC will be prefaced by the word "should," whereas
all requirements will be prefaced by the word "shall." It should
be noted that a TCSEC rating will only be based upon meeting the
TCSEC requirements. Recommendations are made in order to provide
additional ways of increasing assurance. It is hoped that this
will help to avoid any confusion.
v
1. PURPOSE
The Trusted Computer System Evaluation Criteria (TCSEC) is the
standard used for evaluating the effectiveness of security
controls built into ADP systems. The TCSEC is divided into four
divisions: D, C, B, and A, ordered in a hierarchical manner with
the highest division, A, being reserved for systems providing the
best available level of assurance. Within divisions C through A
are a number of subdivisions known as classes, which are also
ordered in a hierarchical manner to represent different levels of
security in these classes.
For TCSEC classes B2 through A1, the TCSEC requires that all
changes to the Trusted Computing Base (TCB) be controlled by
configuration management. Configuration management of a trusted
system consists of identifying, controlling, accounting for, and
auditing all changes made to the TCB during its development,
maintenance, and design. The primary purpose of this guideline
is to provide guidance to developers of trusted systems on what
configuration management is and how it may be implemented in the
development and life-cycle of a trusted system. This guideline
has also been designed to provide guidance to developers of all
systems on the importance of configuration management and how it
may be implemented.
Examples in this document are not to be construed as the only
implementation that will satisfy the TCSEC requirement. The
examples are merely suggestions of appropriate implementations.
The recommendations in this document are also not to be construed
as supplementary requirements to the TCSEC. The TCSEC is the
only metric against which systems are to be evaluated.
This guideline is part of an on-going program to provide helpful
guidance on TCSEC issues and the features they address.
2. SCOPE
An important security feature of TCSEC classes B2 through A1 is
that there be configuration management procedures to manage
changes to the Trusted Computing Base (TCB) and all of the
documentation and tests affected by these changes. Additionally,
it is recommended that such plans and procedures exist for
systems not being considered for an evaluation or whose target
evaluation class may be less than B2. The assurance provided by
configuration management is beneficial to all systems. This
guideline will discuss configuration management and its features
as they apply to computer systems and products, with specific
attention being given to those that are being built with the
1
intention of meeting the requirements of the TCSEC, and to those
systems planning to be re-evaluated under the Ratings Maintenance
Program (RAMP) (see Section 11. RAMP).
Except in cases where there is a distinction between the
configuration management of a trusted system and an untrusted
system, the word "system" shall be used as the object of
configuration management, encompassing both the system and the
TCB. It should be noted that the TCSEC only requires the TCB to
be controlled by configuration management, although it is
recommended that the entire system be maintained under
configuration management.
3. CONTROL OBJECTIVES
The TCSEC gives the following as the Assurance Control Objective:
"Systems that are used to process or handle classified or
other sensitive information must be designed to guarantee
correct and accurate interpretation of the security policy
and must not distort the intent of that policy. Assurance
must be provided that correct implementation and operation
of the policy exists throughout the system's life-cycle."[1]
Configuration management maintains control of a system throughout
its life-cycle, ensuring that the system in operation is the
correct system, implementing the correct security policy. The
Assurance Control Objective as it relates to configuration
management leads to the following control objective that may be
applied to configuration management:
"Computer systems that process and store sensitive or
classified information depend on the hardware and software
to protect that information. It follows that the hardware
and software themselves must be protected against
unauthorized changes that could cause protection mechanisms
to malfunction or be bypassed completely. [For this
reason, changes to trusted computer systems, during their
entire life-cycle, must be carefully considered and
controlled to ensure that the integrity of the
protection mechanism is maintained.] Only in this way can
confidence be provided that the hardware and software
interpretation of the security policy is maintained
accurately and without distortion."[1]
2
4. ORGANIZATION
This document has been written to provide the reader with an
understanding of what configuration management is and how it may
be implemented in an ADP system.
For developers of trusted systems, this document also relates the
TCSEC requirements to the configuration management practices that
meet them. This document has been organized to illustrate the
connection between practices and requirements through the use of
a numbering convention for the TCSEC requirements. The
configuration management requirements have been broken down into
19 separate requirements in Section 6 of this document. The
requirement number(s) will be located in parenthesis following
its appropriate discussion, e.g., (Requirements 2, 15), signifies
that the previous discussion dealt with TCSEC requirements 2 and
15 as stated in Section 6.
3
5. OVERVIEW OF CONFIGURATION MANAGEMENT PRINCIPLES
Configuration management consists of four separate tasks:
identification, control, status accounting, and auditing. For
every change that is made to an automated data processing (ADP)
system, the design and requirements of the changed version of the
system should be identified. The control task of configuration
management is performed by subjecting every change to
documentation, hardware, and software/firmware to review and
approval by an authorized authority. Configuration status
accounting is responsible for recording and reporting on the
configuration of the product throughout the change. Finally,
through the process of a configuration audit, the completed
change can be verified to be functionally correct, and for
trusted systems, consistent with the security policy of the
system. Configuration management is a sound engineering practice
that provides assurance that the system in operation is the
system that is supposed to be in use. The assurance control
objective as it relates to configuration management of trusted
systems is to "guarantee that the trusted portion of the system
works only as intended."[1]
Procedures should be established and documented by a
configuration management plan to ensure that configuration
management is performed in a specified manner. Any deviation
from the configuration management plan could contribute to the
failure of the configuration management of a system entirely, as
well as the trust placed in a trusted system.
5.1 Purpose of Configuration Management
Configuration management exists because changes to an existing
ADP system are inevitable. The purpose of configuration
management is to ensure that these changes take place in an
identifiable and controlled environment and that they do not
adversely affect any properties of the system, or in the case of
trusted systems, do not adversely affect the implementation of
the security policy of the TCB. Configuration management
provides assurance that additions, deletions, or changes made to
the TCB do not compromise the trust of the originally evaluated
system. It accomplishes this by providing procedures to ensure
that the TCB and all documentation are updated properly.
4
6. MEETING THE CRITERIA REQUIREMENTS
This section lists the TCSEC requirements for configuration
management. Each requirement for each class has been listed
separately and numbered. Each number may be referenced to the
requirement discussions that follow in this document. This
section is designed to serve as a quick reference for TCSEC class
requirements.
6.1 The B2 Configuration Management Requirements
Requirement 1 - "During development and maintenance of the TCB, a
configuration management system shall be in place."[1]
Requirement 2 - The configuration management system shall
maintain "control of changes to the descriptive top-level
specification (DTLS)."[1]
Requirement 3 - The configuration management system shall
maintain control of changes to "other design data."[1]
Requirement 4 - The configuration management system shall
maintain control of changes to "implementation documentation"[1]
(e.g., user's manuals, operating procedures).
Requirement 5 - The configuration management system shall
maintain control of changes to the "source code."[1]
Requirement 6 - The configuration management system shall
maintain control of changes to "the running version of the object
code."[1]
Requirement 7 - The configuration management system shall
maintain control of changes to "test fixtures."[1]
Requirement 8 - The configuration management system shall
maintain control of changes to test "documentation."[1]
Requirement 9 - "The configuration management system shall assure
a consistent mapping among all documentation and code associated
with the current version of the TCB."[1]
Requirement 10 - The configuration management system shall
provide tools "for generation of a new version of the TCB from
the source code."[1]
Requirement 11 - The configuration management system shall
provide "tools for comparisons of a newly generated TCB version
5
with the previous version in order to ascertain that only the
intended changes have been made in the code that will actually be
used as the new version of the TCB."[1]
6.2 The B3 Configuration Management Requirements
The requirements for configuration management at TCSEC class B3
are the same as the requirements for TCSEC class B2. Although no
additional requirements have been added, the configuration
management system shall change to reflect changes in the design
documentation requirements at class B3. This means that the
additional documentation required for TCSEC class B3 shall also
be maintained under configuration management.
6.3 The A1 Configuration Management Requirements
Requirements 2 through 11 are the same as those described in
Section 6.1 for a class B2 rating. In addition the following
requirements are added for class A1:
Requirement 12 - "During the entire life-cycle, i.e., during the
design, development, and maintenance of the TCB, a configuration
management system shall be in place for all security-relevant
hardware, firmware, and software."[1]
Requirement 13 - The configuration management system shall
maintain control of changes to the TCB hardware.
Requirement 14 - The configuration management system shall
maintain control of changes to the TCB software.
Requirement 15 - The configuration management system shall
maintain control of changes to the TCB firmware.
Requirement 16 - The configuration management system shall
"maintain control of changes to the formal model."[1]
Requirement 17 - The configuration management system shall
maintain control of changes to the "formal top-level
specifications."[1]
Requirement 18 - The tools available for configuration management
shall be "maintained under strict configuration control."[1]
Requirement 19 - "A combination of technical, physical, and
procedural safeguards shall be used to protect from unauthorized
modification or destruction the master copy or copies of all
material used to generate the TCB."[1]
6
7. FUNCTIONS OF CONFIGURATION MANAGEMENT
7.1 Configuration Identification
Configuration management procedures should enable a person to
"identify the configuration of a system at discrete points in
time for the purpose of systematically controlling changes to the
configuration and maintaining the integrity and traceability
of this configuration throughout the system life cycle."[4] The
basic function of configuration identification is to identify the
components of the design and implementation of a system. When it
concerns trusted systems, this specifically means the design and
implementation of the TCB. This task may be accomplished through
the use of identifiers and baselines (see Section 9.1 The
Baseline Concept). By establishing configuration items and
baselines, the configuration of the system and its TCB can be
accurately identified throughout the system life-cycle.
At TCSEC class B2, the TCSEC requires that "changes to the
descriptive top-level specification, other design data,
implementation documentation, source code, the running version of
the object code, and test fixtures and documentation"[1] of the
TCB be controlled by configuration management (Requirements 2, 3,
4, 5, 6, 7, 8). Configuration identification helps achieve this
control. The TCSEC requires that each change to the TCB shall be
individually identifiable so that a history of the TCB may be
generated at any time. At TCSEC class A1, the requirements are
extended to include that the "formal model...and formal top-level
specifications" of the TCB shall also be maintained under the
configuration management system (Requirements 16, 17).
The following is a sample list of what shall be identified and
maintained under configuration management:
* the baseline TCB including hardware, software, and firmware
* any changes to the TCB hardware, software, and firmware
since the previous baseline
* design and user documentation
* software tests including functional and system integrity
tests
* tools used for generating current configuration items
(required at TCSEC class A1 only)
Configuration management procedures should make it possible to
accurately reproduce any past TCB configuration. In the event a
7
security vulnerability is discovered in a version of the TCB
other than the most current one, analysts will need to be able to
reconstruct the past environment. This reconstruction will be
possible to perform if proper configuration identification has
been performed throughout the system life-cycle.
The TCSEC also requires at class B2 and above, that tools shall
be provided "for generation of a new version of the TCB from the
source code" and that there "shall be tools for comparing a newly
generated version with the previous TCB version in order to
ascertain that only the intended changes have been made in the
code that will actually be used as the new version of the TCB"[1]
(Requirements 10, 11). These tools are responsible for providing
assurance that no additional changes have been inserted into the
TCB that were not intended by the system designer. Automated
tools are available that make it possible to identify changes to
a system online (see APPENDIX A: AUTOMATED TOOLS). Any changes,
or suggested changes to a system should be entered into an online
library. This data can later be used to compare any two versions
of a system. Such online configuration libraries may even
provide the capability for line-by-line comparison of software
modules and documentation. At Class A1, the tools used to
perform this function shall be "maintained under strict
configuration control"[1] (Requirement 18). These tools shall
not be changed without having to undergo a strict review process
by an authorized authority.
7.1.1 Configuration Items
A configuration item is an uniquely identifiable subset of the
system configuration that represents the smallest portion of the
system to be subject to independent configuration management
change control procedures. Configuration items need to be
individually controlled because any change to a configuration
item may have some effect upon the properties of the system or
the security policy of the TCB.
Configuration items as they relate to the TCB, are subsets of the
TCB's hardware, firmware, software, documentation, tests, and at
class A1, development tools. Each module of TCB software for
example, may constitute a separate configuration item.
Configuration items should be assigned unique identifiers (e.g.,
serial numbers, names) to make them easier to identify throughout
the system life-cycle. Proper identification plays a vital role
in meeting the TCSEC requirement for class B2 that requires the
configuration management system to "assure a consistent mapping
among all documentation and code associated with the current
version of the TCB"[1] (Requirement 9). Used in conjunction with
8
a configuration audit, a consistent labeling system helps tie
documentation to the code it describes. Not only does labeling
each configuration item make them easier to identify, but it also
increases the level of control that may be maintained over the
entire system by making these items more traceable.
Configuration items may be given an identifier through a random
distribution process, but, it is more useful for the
configuration identifier to describe the item it identifies.
Selecting different fields of the configuration identifier to
represent characteristics of the configuration item is one method
of accomplishing this. The United States Social Security number
is a "configuration identifier" we all have that uses such a
system. The different fields of the number identify where we
applied for the Social Security card, hence describing a little
bit about ourselves. As the configuration identifier relates to
computer systems, one field should identify the system version
the item belongs to, the version of software that it is, or its
interface with other configuration items. When using a
numbering scheme like this, a change to a configuration item
should result in the production of a new configuration
identifier. This new identifier should be produced by an
alteration or addition to the existing configuration identifier.
A new version of a software program should not be identified by
the same configuration item number as the original program. By
treating the two versions as distinct configuration items, line-
by-line comparisons are possible to perform.
Identifying configuration items is a task that should be
performed early in the development of the system, and once
something is designated as a configuration item, the design
of that item should not change without the knowledge and
permission of the party controlling the item. Early
identification of configuration items increases the level of
control that may be maintained over the item and allows the item
to be traced back through all stages of the system development.
In the event that a configuration item is not identified until
late in the development process, accountability for that item in
the early stages of the system development would be non-existent.
Configuration items may vary widely in complexity, size, and
type, and it is important to choose configuration items with
appropriate granularity. If the items are too large, the data
identifying each one will overwhelm anyone trying to audit the
system. If the items are too small, the amount of total
identification data will overwhelm the system auditors.[2] The
appropriate granularity for configuration items should be
identified by each vendor and documented in the configuration
management plan.
9
7.2 Configuration Control
"Configuration control involves the systematic evaluation,
coordination, approval, or disapproval of proposed changes to the
design and construction of a configuration item whose
configuration has been formally approved."[5] Configuration
control should begin in the earliest stages of the design and
development of the system and extend over the full life of the
configuration items included in the design and development
stages. Early initiation of configuration control procedures
provides increased accountability for the system by making its
development more traceable. The traceability function of
configuration control serves a dual purpose. It makes it
possible to evaluate the impact of a change to the system and
controls the change as it is being made. With configuration
control in place, there is less chance of making undesirable
changes to a system that may later adversely affect the security
of the system.
Initial phases of configuration control are directed towards
control of the system configuration as defined primarily in
design documents. For these, the Configuration Management plan
shall specify procedures to ensure that all documentation is
updated properly and presents an accurate description of the
system and TCB configuration. Often a change to one area of a
system may necessitate a change to another area. It is not
acceptable to only write documentation for new code or newly
modified code, but rather documentation for all parts of the TCB
that were affected by the addition or change shall be updated
accordingly. Although documentation may be available, unless it
is kept under configuration management and updated properly it
will be of little, if any use. In the event that the system is
found to be deficient in documentation, efforts should be made to
create new documentation for areas of the system where it is
presently inadequate or non-existent.
To meet the TCSEC requirements though, configuration control
shall cover a broader area than just documentation, and at Class
B2 shall also maintain control of "design data, source code, the
running version of the object code, and test fixtures"[1] of the
TCB (Requirements 3, 5, 6, 7). A change to any of these shall be
subject to review and approval by an authorized authority.
For TCB configuration items, those items shall not be able to
change without the permission of the controlling party. At
TCSEC class A1, this requirement is strengthened to require
"procedural safeguards"[1] to protect against unauthorized
modification of the materials used in the TCB (Requirement 19).
These procedures should require that not only does the
10
controlling party need to give permission to have a change
performed, but that the controlling party performs the change on
the master copy of the TCB that will be released. This ensures
against changes being made to the master copy that are different
than the approved changes.
The degree of configuration control that is exercised over the
TCB will affect whether or not it meets the TCSEC requirements
for configuration management. The configuration management
requirements in the TCSEC require that a configuration management
system be in place during the "development and maintenance of the
TCB" at Class B2 (Requirement 1), and at Class A1, "during the
entire life-cycle"[1] of the TCB (Requirement 12). A minimal
configuration control system that would not be sufficient in
meeting the TCSEC requirements, may only provide for review after
a change has been made to the system. A system such as this may
ensure that the change is complete and acceptable and may control
the release of the change, but for the most part, the control
exercised is little more than an after-the-fact quality assurance
check. This system is certainly better than having no control
system in place, but it would not meet the TCSEC requirements for
configuration management. What is missing from this system that
would bring it closer to the B2 requirements is control over the
change as it is being made. The configuration control required
by the TCSEC should provide for constant checking and approval of
a change from its inception, through implementation and testing,
to release. The level of control exercised over the TCB may
exceed that of the rest of the system, but it is recommended that
all parts of the system be under configuration control.
In the case of a change to hardware or software/firmware that
will be used at multiple sites, configuration control is also
responsible for ensuring that each site receives the appropriate
version of the system.
The point behind configuration control of the TCB is that all
changes to the TCB shall be approved, monitored, and evaluated to
provide assurance that the TCB functions properly and that all
security policies are maintained.
7.3 Configuration Status Accounting
Configuration status accounting is charged with reporting on the
progress of the development in very specific ways. It
accomplishes this task through the processes of data recording,
data storing, and data reporting. The main objective of
configuration status accounting is to record and report all
information that is of significance to the configuration
11
management process. What is of significance should be outlined
in the Configuration Management Plan. The establishment of a new
baseline (see Section 9.1 THE BASELINE CONCEPT) or the meeting of
a milestone is an example of what should be recorded as
configuration status accounting information. The requirements in
the configuration management plan should be viewed as the minimum
and any events that seem relevant to configuration management
should be captured and recorded in that they may prove to be
useful in the future.
The configuration accounting system may consist of tracing
through documentation manually to find the status of a change or
it may consist of a database that can automatically track a
change. As long as the information exists accurately in some
form though, it will serve its purpose. The benefit of an online
status accounting system is that the information may be kept in a
more structured fashion, which would facilitate keeping it up to
date. Being able to query a database for information concerning
the status of a configuration change or configuration item would
also be less cumbersome than sorting through notebook pages.
Finally, the durability of a diskette or hard disk for storage
outweighs that of a spiral notebook or folder, provided that it
is properly backed up to avoid data loss in the event of a system
failure.
Whichever system is used, it should be possible to quickly locate
all authorized versions of a configuration item, add together all
authorized changes with comments about the reason for the change,
and arrive at either the current status of that configuration
item, or some intermediate status of the requested item. The
status of all authorized changes being performed should be
formulated into a System Status Report that will be presented at
a Configuration Control Board meeting (see Section 9.3 THE
CONFIGURATION CONTROL BOARD).
Configuration status accounting "establishes records and reports
which enable proper logistics support, i.e., the supplying of
spares, instruction manuals, training and maintenance facilities,
etc. to be established."[5] The records and reports produced
through configuration status accounting should include a current
configuration list, an historical change list, the original
designs, the status of change requests and their implementation,
and should provide the ability to trace all changes.
7.4 Configuration Audit
Configuration auditing involves checking for top to bottom
completeness of the configuration accounting information "to
12
ascertain that only the [authorized] changes have been made in
the code that will actually be used as the new version of the
TCB."[1] (Requirement 11) When a change has been made to a
system, it should be reviewed and audited for its effect on the
rest of the system. This should include reviewing and testing all
software to ensure that the change has been performed correctly.
Configuration auditing is concerned with examining the control
process of the system and ensuring that it actually occurs the
way it should. Configuration auditing for trusted systems
verifies that after a change has been made to the TCB, the
security features and assurances are maintained. Configuration
audits should be performed periodically to verify the
configuration status accounting information. The configuration
audit minimizes the likelihood that unapproved changes have been
inserted without going unnoticed and that the status accounting
information adequately demonstrates that the configuration
management assurance is valid.
"A complete audit should include tracing each requirement down
through all functions that implement it to see if that
requirement is met."[2] Furthermore, the configuration audit
should also ensure that no additions were made that were not
required. For the audit to provide a useful form of technical
review, it should be predictable and as foolproof as possible,
i.e., there should be specific desired results.
The configuration audit should verify that:
* the architectural design satisfies the requirements
* the detailed design satisfies the architectural design
* the code implements the detailed design
* the item/product performs per the requirements
* the configuration documentation and the item/product match
The main emphasis of configuration auditing is on providing the
user with reasonable assurance that the version of a system in
use is the same version that the user expects to be in use.
Configuration audits ensure that the configuration control
procedures of the configuration management system are being
followed. The assurance feature of configuration auditing is
provided through reasonable and consistent accountability
procedures. All code audits should follow roughly the same
procedures and perform the same set of checks for every change to
the system.
13
8. THE CONFIGURATION MANAGEMENT PLAN
Effective configuration management should include a well-thought-
out plan that should be prepared immediately after project
initiation. This plan should describe, in simple, positive
statements, what is to be done to implement configuration
management in the system and TCB. A minimal configuration
management plan may be limited to simply defining how
configuration management will be implemented as it relates to the
identification, control, accounting, and auditing tasks. The
configuration management plan described in the following
paragraphs is an example of a plan that goes into more detail and
contains documentation on all aspects of configuration
management, such as examples of documents to be used for
configuration management, procedures for any automated tools
available, or a Configuration Control Board roster (see Section
9.3 THE CONFIGURATION CONTROL BOARD). The configuration
management plan should contain documentation that describes how
the configuration management "tasks are to be carried out in
sufficient detail that anyone involved with the project can
consult them to determine how each specific development task
relates to CM."[2]
One portion of the configuration management plan should define
the roles played by designers, developers, management, the
Configuration Control Board, and all of the personnel involved
with any part of the life-cycle of the system. The
responsibilities required by all those involved with the system
should be established and documented in the configuration
management plan to ensure that the human element functions
properly during configuration management. A list of
Configuration Control Board members, or the titles of the members
should also be included in this section.
Any tools that will be available and used for configuration
management should be documented in the configuration management
plan. At TCSEC class A1, it is required that these tools shall
be "maintained under strict configuration control"[1]
(Requirement 18). These tools may include forms used for change
control, conventions for labeling configuration items, software
libraries, as well as any automated tools that may be available
to support the configuration management process. Samples of any
documents to be used for reporting should also be contained in
the configuration management plan with a description of each.
A section of the Configuration Management Plan should deal with
procedures. Since the main thrust of configuration management
consists of the following of procedures, there needs to be
thorough documentation on what procedures one should follow
14
during configuration management. The configuration management
plan should provide the procedures to take to ensure that both
user and design documentation are updated in synchrony with all
changes to the system. It should include the guidelines for
creating and maintaining functional tests and documentation
throughout the life of the system. The configuration management
plan should describe the procedures for how the design and
implementation of changes are proposed, evaluated, coordinated,
and approved or disapproved. The configuration management plan
should also include the steps to take to ensure that only those
approved changes are actually included and that the changes are
included in all of the necessary areas.
Another portion of the configuration management plan should
define any existing "emergency" procedures, e.g., procedures for
performing a time sensitive change without going through a full
review process, that may override the standard procedure. These
procedures should define the steps for retroactively implementing
configuration management after the emergency change has been
completed.
The configuration management plan is a living document and should
remain flexible during design and development phases. Although
the configuration management plan is in place to impose control
on a project, it should still be open to additions and changes as
designers and developers see fit. This is not to say that the
configuration management plan is only a guide and need not be
followed, but that modifications should be able to occur. If the
plan is not followed, there is no way it will be able to provide
the appropriate assurances. In the event that a change is needed
to the configuration management plan, the change should be
carefully evaluated and approved. In changes to the
configuration management plan of a trusted system this evaluation
shall ensure that the security features and assurances supported
by the plan are still maintained after the change has been
implemented.
15
9. IMPLEMENTATION METHODS
This section discusses implementation methods for configuration
management that may be used to meet some of the requirements of
the TCSEC. Section 9.1 discusses the baseline concept as a
method of configuration identification. The baseline concept
utilizes the features of configuration management spoken of
previously, but divides the life-cycle of the system into
different baselines.
Section 9.2 illustrates how a fictitious company, MER, Inc.,
conducts configuration management. They are attempting to meet
the TCSEC requirements for a B2 system.
Section 9.3 discusses the concept of a Configuration Control
Board (CCB) for carrying out configuration control. A CCB is a
body of people responsible for configuration control. This
concept is widely used by many computer vendors.
9.1 The Baseline Concept
Baselines are established at pre-selected design points in the
system life-cycle. One baseline may be used to describe a
specific version of a system, or in some configuration management
systems a single baseline may be defined at each of several major
milestones. Baselines should be established at the discretion
of the Configuration Control Board and outlined in the
configuration management plan. In cases where several baselines
are established, each baseline serves as a cutoff point for one
segment of development, while simultaneously acting as the step
off point for another segment. The characteristics common to
all baselines are that the design of the system will be approved
at the point of their establishment and it is believed that any
changes to this design will have some impact on the future
development of the system.
Baseline management is one technique for performing configuration
identification. It identifies the system and TCB design and
development as a series of phases or baselines that are subject
to configuration control. Used in conjunction with configuration
items, this is another effective way to identify the system and
its TCB configuration throughout its life-cycle.
"For each different type of baseline, the individual components
to be controlled should be identified, and any changes that
update the current configuration should be approved and
documented. For each intermediate product in the development
[life-cycle] there is only one baseline. The current
16
configuration can be found by applying all approved changes to
the baseline."[2]
In a system defining several baselines for different stages of
development, these baselines or milestones should be established
at the system inception to serve as guides throughout the
development process. Although specific baselines are
established in this case, alternatives may be recommended to
promote greater design flexibility or efficiency. The number of
baselines that may be established for a system will vary
depending upon the size and complexity of the system and the
methods supported by the designers and developers. It is
possible to establish multiple baselines existing at the same
time so long as configuration management practices are applied
properly to each baseline. The following example will discuss
the baseline concept using three common baseline categories:
functional, allocated, and product. It should be emphasized that
these are simply basic milestones and baselines should be
established depending upon the decisions of the designers and
developers.
The first baseline, the functional baseline, is established at
the system inception. It is derived from the performance and
objectives criteria documentation that consists of specifications
defining the system requirements. Once these specifications have
been established, any changes to them should be approved.
The requirements produced in the functional baseline may be
divided and subdivided into various configuration items. Once it
has been decided what the configuration items will be, each of
the items should be given a configuration identifier. From the
analysis of the system requirements the allocated baseline will
be established. This baseline identifies all of the required
functions with a specific configuration item that is responsible
for the function. In this baseline, an individual should be
charged with the responsibility for each configuration item.
All changes affecting specifications defining design requirements
for the system or its configuration items as stated in the
allocated baseline should require approval of the responsible
individual.
The final baseline, the product baseline, should contain that
version of the system that will be turned over for integration
testing. This baseline signifies the end of the development
phase and should contain a releasable version of the system.
The baseline example mention earlier in which one baseline is
established for a single version of a system entails the same
reasoning as the functional, allocated, and product baseline
17
example. The system established as a baseline in the single
baseline example will need to have an approved design before
being placed under configuration control. Prior to the design
approval, the system design will have to have undergone some type
of functional review and a process that would allocate these
functions to various configuration items. Although the early
processes of the design will not be as formal in the single
baseline example as they are when the early tasks are
individually defined, the system will still benefit from being
under the control of configuration management as a baseline. The
main point of establishing any baseline is controlling changes to
that baseline by requiring any changes to it to have to undergo
an established change control process.
9.2 Configuration Management at MER, Inc.
MER, Inc., is a manufacturer of computer systems. Their latest
project consists of building a system that will meet the B2
requirements of the TCSEC. In the past, their configuration
management has only consisted of quality assurance checks, but to
meet the B2 requirements they realize that they will need to have
specific configuration management procedures in place during the
development and maintenance of the system.
The project manager was assigned the task of writing the
configuration management procedures and elected to present them
in a configuration management plan. After doing some research on
what should be contained in the configuration management plan, he
proceeded to write a plan for MER, Inc. The configuration
management plan that was written listed all of the steps to be
followed when carrying out configuration management for the
system. It described the procedures to be followed by the
development team and described the automated tools that were
going to be used at MER, Inc. for configuration management.
These tools consisted of an online tracking data base to be used
for status accounting, an online data base that contained a
listing of all of the items under configuration control, and
automated libraries used for storing software. Before
development began, all of the development team was responsible
for reading the configuration management plan to ensure that they
were aware of the procedures to be followed for configuration
management.
As the system was developed, the TCB hardware, software, and
firmware were labeled using a configuration item numbering scheme
that had been explained in the configuration management plan. In
addition, the documentation and tests accompanying these items
were also given configuration item numbers to assure a consistent
18
mapping between TCB code and these items. All of the
configuration item numbers and a description of the items were
stored in a data base that could be queried at any time to derive
the configuration of the entire system. Software and
documentation were stored in a software library where they could
be retrieved and worked on without affecting the master versions.
The master copies of all software were stored in a master library
that contained the releasable versions of the software. Both of
these libraries are protected by a discretionary access control
mechanism to prevent any unauthorized personnel from tampering
with the software.
During the development of the system, changes were required. The
procedures for performing a change under configuration control
are described in the configuration management plan. These are
the same procedures that will remain in effect throughout the
life-cycle of the system. For each proposed change, a decision
has to be made by management whether or not the change is
feasible and necessary. MER, Inc. has an online forum for
reviewing suggested changes. This forum makes it possible for
all of the members of the development team to comment on how the
proposed change may affect their work. Management would often
consult this forum to help arrive at their final decision.
After a decision was made, a programmer was assigned to perform
the change. The programmer would retrieve the most recent
version of the software from the software library and proceed to
change it. As the change was being performed, the changes were
entered into the online tracking data base. This made it
possible for members of the development team to query this data
base to find the current status of the change at any time. After
the change had been performed it was tested and documented, and
upon successful completion it was forwarded to a reviewer. This
reviewer was the software manager, who was the only person
authorized to approve a changed version for release. After the
change was approved for release, the changed version was stored
in the master library and a second copy was stored in the
software library. Each change stored in these libraries was
given a new configuration identification number. A tool was
available at MER, Inc. that made it possible to identify changes
made to software. It compared any two versions of the software
and provided a line-by-line listing of the differences between
the two.
It was realized at the beginning of the development process that
there would be times when critical changes would need to be
performed that would not be able to undergo this review process.
For these changes, emergency procedures had been listed in the
configuration management plan and a critical fix library was
19
available to record critical changes that had occurred since a
release.
A control process for changes to the TCB hardware was also
provided for in the configuration management plan. The
procedures ensured that changes to the TCB hardware were
traceable and did not violate the security assumptions made by
the TCB software. Similar to software changes, all hardware
changes were reviewed by the project manager before being
implemented.
After a change is made to the TCB software, MER, Inc. performs a
configuration audit to verify the information that exists in the
tracking data base. Whether or not a change is performed, the
configuration management plan at MER, Inc. specifies that a
configuration audit be performed at least once a month. This
audit compares the current master version with the status
accounting information to verify that no changes have been
inserted that were not approved.
This configuration management plan encompasses the descriptive
top-level specification (DTLS), implementation documentation,
source code, object code, test fixtures, and test documentation,
and has been found to satisfy the TCSEC requirements for
configuration management at class B2.
9.3 The Configuration Control Board (CCB)
Configuration control may be performed in different ways. One
method of configuration control that is in use by systems already
evaluated at TCSEC Class B2 and above is to have the control
carried out by a body of qualified individuals known as the
Configuration Control Board (CCB), also known as the
Configuration Change Board. The Board is headed by a
chairperson, who is responsible for scheduling meetings and for
giving the final approval on any proposed changes. The
membership of the CCB may vary in size and composition from
organization to organization, but it should include members from
any or all of the following areas of the system team:
* Program Management
* System Engineering
* Quality Assurance
* Technical Support
20
* Integration and Test
* System Installation
* Technical Documentation
* Hardware and Software/Firmware Acquisition
* Program Development
* Security Engineering
* User Groups
The members of the CCB should interact periodically, either
through formal meetings, electronic forums, or any other
available means, to discuss configuration management topics such
as proposed changes, configuration status accounting reports, and
other topics that may be of interest to the different areas of
the system development. These interactions should be held at
periodic intervals to keep the entire system team up-to-date with
all advancements or alterations in the system. The Board serves
to control changes to the system and ensures that only approved
changes are implemented into the system. The CCB carries out
this function by considering all proposals for modifications and
new acquisitions and by making decisions regarding them.
An important part of having cross representation in the CCB from
various groups involved in the system development is to prevent
"unnecessary and contradictory changes to the system while
allowing changes that are responsive to new requirements, changed
functional allocations, and failed tests."[2] All of the members
of the Board should have a chance to voice their opinions on
proposed changes. For example, if system engineering proposes a
change that will affect security, both sides should be able to
present their case at a CCB meeting. If diversity did not exist
in the CCB, changes may be performed, and upon implementation may
be found to be incompatible with the rest of the system.
The configuration control process begins with the documentation
of a change request. This change request should include
justification for the proposed change, all of the affected items
and documents, and the proposed solution. The change request
should be recorded, either manually or online in order to provide
a way of tracking all proposed changes to the system and to
ensure against duplicate change requests being processed.
When the change request is recorded, it should be distributed for
analysis by the CCB who will review and approve or disapprove the
21
change request. An analysis of the total impact of the change
will decide whether or not the change should be performed. The
CCB will approve or disapprove the change request depending upon
whether or not the change is viewed as a necessary and feasible
change that will further the design goals of the system. In
situations where trusted systems are involved, the CCB shall also
ensure that the change will not affect the security policy of the
system.
Once a decision has been reached regarding any modifications, the
CCB is responsible for prioritizing the approved modifications to
ensure that those that are most important are developed first.
When prioritizing changes, an effort should be made to have the
changes performed in the most logical order whenever possible.
The CCB is also responsible for assigning an authority to perform
the change and for ensuring that the configuration documentation
is updated properly. The person assigned to do the change should
have the proper authorization to modify the system, and in
trusted systems processing sensitive information, this
authorization shall be required. During the development of any
enhancements and new developments, the CCB continues to exert
control over the system by determining the level of testing
required for all developments.
Upon completion of the change, the CCB is responsible for
verifying that the change has been properly incorporated and that
only the approved change has been incorporated. Tests should be
performed on the modified system or TCB to ensure that they
function properly after the change is completed. The CCB should
review the test results of any developments and should be the
final voice on release decisions.
The use of a CCB is one way of performing configuration control,
but not every vendor may have the desire or resources to
establish one. Whatever the preference, there should still be
some way of performing the control processes described
previously.
22
10. OTHER TOPICS
10.1 Trusted Distribution
Related to the configuration management requirements for trusted
systems is the TCSEC requirement for trusted distribution at
class A1 which states:
"A trusted ADP system control and distribution facility
shall be provided for maintaining the integrity of the
mapping between the master data describing the current
version of the TCB and the on-site master copy of the code
for the current version. Procedures (e.g., site security
acceptance testing) shall exist for assuring that the TCB
software, firmware, and hardware updates distributed to a
customer are exactly as specified by the master
copies."[1]
Two questions that the trusted distribution process should answer
are: (a) Did the product received come from the organization who
was supposed to have sent it? and (b) Did the recipient receive
exactly what the sender intended?
Configuration management assists trusted distribution by ensuring
that no alterations are made to the TCB from the time of approved
modification to the time of release. The additional
configuration management requirement at A1 that supports this is,
"A combination of technical, physical and procedural safeguards
shall be used to protect from unauthorized modification or
destruction the master copy or copies of all material used to
generate the TCB"[1] (Requirement 19). This requirement calls
for strict control over changes made to any versions of the TCB.
The possibility that a change may not be performed as specified,
or that a harmful modification may be inserted into the TCB
should be considered and the authority to perform changes to the
master copy should be restricted. A single master copy authority
should be made responsible for ensuring that only approved and
acceptable changes are implemented into the master copy.
Configuration status accounting records and auditing reports can
provide accountability for all TCB versions in use. In the event
of altered copies being distributed or "bogus" copies being
distributed that were not manufactured by the vendor,
configuration management records will be able to assess the
validity and accuracy of all TCB versions. Trusted distribution
displays the need for configuration control over all changes to
the TCB. Without configuration control there would be no
accountability for the TCB versions distributed to the customer.
23
10.2 Functional Testing
"The system developer shall provide to the evaluators a document
that describes the test plan, test procedures that show how the
security mechanisms were tested, and results of the security
mechanisms' functional testing."[1] The creation and maintenance
of these functional tests is required to be part of the
configuration management procedures. Test results and any
affected test documentation shall be maintained under
configuration management and updated wherever necessary
(Requirements 7, 8). The tests should be repeatable, and include
sufficient documentation so that any knowledgeable programmer
will be able to figure out how to run them. The test plan for
the system should be described in the functional specification
(or other design documentation) for the TCB, along with
descriptions of the test programs. The test plan and programs
should be reviewed and audited along with the programs they test,
although the coding standards need not be as strict as those of
the tested programs.
It is not acceptable to only generate tests for code that was
opened or replaced, but all of the portions of the TCB that were
affected by the change should also be tested. The NCSC
evaluators can provide a description of the security functional
tests required to meet the TCSEC testing requirements, including
the testing required as stated above for configuration
management.
10.3 Configuration Management Training
Each new technical employee should receive training in the
configuration management procedures that a particular
installation follows. Experienced programmers, although they may
be familiar with some form of configuration management, will also
require training in any new procedures, i.e., an automated
accounting system, that will be required to be followed.
Training should be conducted either "by holding formal classes or
by setting aside sufficient time for the reading of the company
wide configuration standards."[2] New programmers should become
familiar with the Configuration Management Plan before being
allowed to incorporate any changes into the design baseline. It
should be stressed that a failure to maintain the configuration
management standards resulting from untrained employees, could
prevent the system from receiving a rating.[2]
24
10.4 Configuration Management Supervision
A successful configuration management system requires the
following of many procedures. Considering the demands made on
the system staff, errors may occur and shortcuts may be sought
which will jeopardize the entire configuration management plan.
A review process should be present to ensure that no single
person can create a change to the system and implement it without
being subject to some type of approval process. Supervisors, who
are responsible for the personnel performing the change should be
required to sign an official record that the change is the
correct change.[2]
Proper supervision also provides assurance that whoever performs
the change has the proper authorization to do so. Changes should
not be performed by personnel that are not qualified to perform
the change. Also, in systems that process sensitive information,
the programmer performing the change shall possess the proper
security clearance to perform the change.
Management itself must directly support the configuration
management plan in order for it to work. It should not encourage
cutting configuration management corners under any circumstances,
e.g., due to scheduling or budgeting. Management should be
willing to support the expenditure of money, people, and time to
allow for proper configuration management.
25
11. RATINGS MAINTENANCE PROGRAM
The Ratings Maintenance Program (RAMP) has been developed by the
NCSC in an effort to keep the Evaluated Products List (EPL)
current. By training vendor personnel to recognize which changes
may adversely affect the implemetation of the security policy of
the system, and to track these changes to the evaluated product
through the use of configuration management, RAMP will permit a
vendor to maintain the rating of the evaluated product without
having to re-evaluate the new version. Because changes from one
version of an operating system to the next version may affect the
security features and assurances of that operating system,
configuration management is an integral part of RAMP. For a
system to maintain its rating under this program, the NCSC shall
be assured, through the vendor's configuration management
procedures, that the changes made have not adversely affected the
implementation of the security mechanisms and assurances of the
system.
Each RAMP participant shall develop an NCSC approved Rating
Maintenance Plan (RMPlan) which includes a detailed Configuration
Management Plan (CMP) to support the rating maintenance process.
This requirement applies to all systems participating in RAMP,
regardless of class. For further information about the RAMP
program and about configuration management requirements for RAMP,
contact:
National Computer Security Center
9800 Savage Road
Fort George G. Meade, MD 20755©6000
Attention: Chief, Requirements and Resources Division
26
12. CONFIGURATION MANAGEMENT SUMMARY
The assurance provided by configuration management is beneficial
to all systems. It is a requirement for trusted systems for
classes B2 and above that a configuration management system "be
in place that maintains control of changes to the descriptive
top-level specification, other design data, implementation
documentation, source code, the running version of the object
code, and test fixtures and documentation"[1] (Requirements 1, 2,
3, 4, 5, 6, 7, 8). Although configuration management is a
requirement for trusted systems for classes B2 and above, it
should be in place in all systems regardless of class rating, or
if the system has a rating at all.
Successful configuration management is built around four main
objectives: control, identification, accounting, and auditing.
Through the accomplishment of these objectives, configuration
management is able to maintain control over the TCB and protect
it against "unauthorized changes that could cause protection
mechanisms to malfunction or be bypassed completely."[1] Even
for those aspects of the system which are not security-relevant,
configuration management is still a valuable method of ensuring
that all of the properties of a system are maintained after a
change. It is very important to the success of configuration
management that a formal configuration management plan be adhered
to during the life-cycle of the system.
A successful configuration management plan should begin with
early and complete definition of configuration management goals,
scope, and procedures. The success of configuration management
is dependent upon accuracy. Changes should be identified and
accounted for accurately, and after the change is completed, the
change, and all affected parts of the system should be thoroughly
documented and tested.
Configuration management provides control and traceability for
all changes made to the system. Changes in progress are able to
be monitored through configuration status accounting information
in order to control the change and to evaluate its impact on
other parts of the system.
An important part of having a successful configuration management
plan is that the people involved with it must adhere to its
procedures in order to keep all documentation current and the
status of changes up-to-date.
With a firm and well documented configuration management plan in
place, the occurrence of any unnecessary or duplicate changes
will be reduced greatly and any necessary changes that are
27
required should be able to be identified with great ease. An
effective configuration management system should be able to show
what was supposed to have been built, what was built, and what is
presently being built.
28
APPENDIX A: AUTOMATED TOOLS
Automated tools may be used to perform some of the configuration
management functions that previously had to be performed
manually. A data base management system, even with just a
limited query system, may be used to perform the configuration
audit and status accounting functions of configuration
management. The principle behind using automated systems is that
text, both from source code and other documents involved in the
development of the system, can be entered into a Master Library
and modified only through the use of the automated system. This
prevents anyone from performing a change without having the
proper authorization to access the configuration data base. "In
general, only one program librarian, who should be the project
manager or someone directly responsible to the manager, should
have write access to the Master Library during development."[2]
A number of software developers have created software control
facilities that are currently available to be used for
configuration status accounting. A brief discussion of two of
these systems follows.
A.1 UNIX (1) SCCS
"Under the Unix (1) system, the make utility, and the elements
admin, get, prs, and delta, which comprise the Source Code
Control System, provide a basic configuration accounting system.
Initially a directory is created using the mkdir function. At
this point, it is possible to use the owner, group, world
protection scheme provided by Unix (1) to protect the directory.
In addition a list of login identifiers is created which
specifies who may update each element to be processed by SCCS."
[2]
Following directory initiation, each document is entered using
the admin -n function. Each entry that is made is referred to as
an element. As each update is made to a new element, a new
generation of that element, known as a delta, is created. The
name of each element that is stored in a file by SCCS is preceded
by "s.". If a file is added to the directory that does not
contain this prefix, it is ignored by the SCCS function calls.
When the admin function is called, a number of arguments may be
specified that "specify parameters that may affect the file, and
may be changed by a subsequent call to admin. The alogin
argument is used to create the equivalent of an access control
list by listing the login names of users who can apply the delta
function to the element, thus creating either a new generation
(1) UNIX is a registered trademark of AT&T Bell Laboratories
29
(delta) or variant branch."[2]
The initial release, or initial delta, of each code module is
entered into the SCCS directory through the admin -n function,
thus creating the Master Library. The programmer may update each
module in the Master Library by using the get -e function "which
indicates that the module will be edited and then the completed
document will be reentered into the directory using the delta
function. As long as the module being edited was extracted from
the SCCS directory using get -e, it can be returned to the
library using delta, and all necessary update information will be
entered with it. The get function can be used to extract a copy
of any document, but after it is edited it cannot be reentered
into the library."[2]
"SCCS provides the capability to specify a software build by the
way it assigns an SCCS Identification Number (SID) to each output
of the delta function."[2] One can get any version of a text or
source code by specifying the appropriate SID. "There are
straightforward rules regarding how to specify the particular SID
desired when get is called. If no SID is specified, the latest
release and level is provided." The SID of the resulting call to
delta is affected by the SID used when get -e is called.[2]
"The function prs allows for configuration accounting, since it
extracts information from the s. files in the SCCS directory and
prints them out for the user. Prs can be used to quickly create
reports, listing one or two important values such as the last
modified date for many SCCS files, or many values for one or two
file. Larger reports can also be processed and created using an
editor."[2]
A.2 VAX DEC/CMS
"VAX DEC/CMS [7] is also used to track a history of each text
file stored in a CMS directory, but CMS does significantly more
auditing and cross-checking than admin does. For example, if an
editor is used directly to modify a file in a CMS directory, any
further use by CMS of that file generates a warning meassage.
Any files entered into a CMS directory by other than the CMS
utility will cause CMS itself to issue a warning message when it
is invoked for that directory. Otherwise, the process of
configuration accounting is similar to SCCS.
The CMS CREATE LIBRARY function causes a directory to be set up,
and initial logging to start. The project manager enters each
element into the directory by using the CMS CREATE ELEMENT
function. One must RESERVE an element of a library to modify it,
30
and it can only be put back into the library using the REPLACE
function. If someone else has RESERVEd an element between the
original programmer's RESERVE and REPLACE calls, a warning is
issued to both programmers and the occurrence is logged. To get
a sample copy of the text, such as a program source, the FETCH
function will generate the latest generation or any specified
generation of an element, but will not allow an edited copy to be
reinserted into the library. The SHOW function can be used to
audit the information about each element in the library.
Differences between SCCS and DEC/CMS appear concerning software
builds. In Unix (1) a build must be either described in a
makefile, or else each element to be used in a build must be
retrieved from the SCCS directory using get, placed in another
directory, and the makefile then may refer to these source files
to create the executable build. In CMS, the process of selecting
only a subset of source files, including some which are not the
most current, is automated by the use of class and group
mechanisms. To explain how this works, one must understand the
CMS concepts of generations and variants. Each generation of a
file corresponds to a Unix (1) delta. Generations are normally
numbered in ascending order. CMS also has the capability of
creating a variant development line to any generation by
specifying in the REPLACE function a variant name. For example,
if one RESERVEs generation 3 of an element, then performs a
REPLACE/VARIANT = T, this will create generation 3T1 which may
then be developed separately from generation 3. The first time
this is used, the equivalent of an SCCS branch delta is created.
Branches themselves can have branches, a capability that SCCS
does not have.
A group can be defined within a CMS directory, using the CMS
CREATE GROUP, and CMS INSERT ELEMENT functions. A group is
composed of all generations, including variant generations, of
all elements inserted into the group. Groups can be included
within other groups. Groups can be defined with a non-empty
intersection so that they have overlapping membership.
The CMS CREATE CLASS function, together with the CMS INSERT
GENERATION function, can be used to specify the exact elements of
a software build, and the DESCRIPTION file can then refer to the
entire class by using the /GENERATION=classname qualifier on
either the source or action line of a dependency rule. The
makefile required by Unix (1) SCCS can be much more complex when
it is required to describe a software build for intermediate
testing."[2]
(1) Unix is a registered trade mark of Bell Laboratories
31
GLOSSARY
Automatic Data Processing (ADP) System - An assembly of computer
hardware, firmware, and software configured for the purpose of
classifying, sorting, calculating, computing, summarizing,
transmitting and receiving, storing, and retrieving data with a
minimum of human intervention.[1]
Baseline - A set of critical observations or data used for a
comparison or a control. A baseline indicates a cutoff point in
the design and development of a configuration item beyond which
configuration does not evolve without undergoing strict
configuration control policies and procedures.
Configuration Accounting - The recording and reporting of
configuration item descriptions and all departures from the
baseline during design and production.[2]
Configuration Audit - An independent review of computer software
for the purpose of assessing compliance with established
requirements, standards, and baselines.[2]
Configuration Control - The process of controlling modifications
to the system's design, hardware, firmware, software, and
documentation which provides sufficient assurance the system is
protected against the introduction of improper modification prior
to, during, and after system implementation.
Configuration Control Board (CCB) - An established committee that
is the final authority on all proposed changes to the ADP system.
Configuration Identification - The identifying of the system
configuration throughout the design, development, test, and
production tasks.
Configuration Item - The smallest component of hardware,
software, firmware, documentation, or any of its discrete
portions, which is tracked by the configuration management
system.
Configuration Management - The management of changes made to a
system's hardware, software, firmware, documentation, tests, test
fixtures, and test documentation throughout the development and
operational life of the system.
Descriptive Top-Level Specification (DTLS) - A top-level
specification that is written in a natural language (e.g.,
English), an informal program design notation, or a combination
of the two.[1]
32
Firmware - Equipments or devices within which computer
programming instructions necessary to the performance of the
device's discrete functions are electrically embedded in such a
manner that they cannot be electrically altered during normal
device operations.[3]
Formal Security Policy Model - An accurate and precise
description, in a formal, mathematical language, of the security
policy supported by the system.
Formal Top-Level Specification - A top-level specification that
is written in a formal mathematical language to allow theorems
showing the correspondence of the system specifications to its
formal requirements to be hypothesized and formally proven.[1]
Granularity - The relative fineness or courseness by which a
mechanism can be adjusted. The phrase "the granularity of a
single user" means the access control mechanism can be adjusted
to include or exclude any single user.[1]
Hardware - The electric, electronic, and mechanical equipment
used for processing data.[3]
Informal Security Policy Model - An accurate and precise
description, in a natural language (e.g., English), of the
security policy supported by the system.
Software - Various programming aids that are frequently supplied
by the manufacturers to facilitate the purchaser's efficient
operation of the equipment. Such software items include various
assemblers, generators, subroutine libraries, compilers,
operating systems, and industry application programs.[6]
Tools - The means for achieving an end result. The tools
referred to in this guideline are documentation, procedures, and
the organizational body, i.e., the CCB, which all contribute to
achieving the control objective of configuration management.
Trusted Computing Base (TCB) - The totality of protection
mechanisms within a computer system -- including hardware,
firmware, and software -- the combination of which is responsible
for enforcing a security policy. A TCB consists of one or more
components that together enforce a unified security policy over a
product or system. The ability of a TCB to correctly enforce a
security policy depends solely on the mechanisms within the TCB
and on the correct input by system administrative personnel of
parameters (e.g., a user's clearance) related to the security
policy.[1]
33
REFERENCES
1. National Computer Security Center, DOD Trusted Computer
System Evaluation Criteria, DOD, DOD 5200.28-STD, 1985.
2. Brown, R. Leonard, "Configuration Management for Development
of a Secure Computer System", ATR-88(3777-12)-1, The
Aerospace Corporation, 1987.
3. Subcommittee on Automated Information System Security,
Working Group #3, "Dictionary of Computer Security
Terminology", 23 November 1986.
4. Bersoff, Edward H., Henderson, Vilas D., Siegal, Stanley G.,
Software Configuration Management, Prentice Hall, Inc.,
1980.
5. Samaras, Thomas T., Czerwinski, Frank L., Fundamentals of
Configuration Management, Wiley-Interscience, 1971.
6. Sipple, Charles J., Computer Dictionary, Fourth Edition,
Howard W. Sams & Co., 1985.
7. Digital Equipment Corporation, VAX DEC/CMS Reference Manual,
AA-L372B-TE, Digital Equipment Corporation, 1984.
34
NCSC-TG-001
Library No. S-228,470
FOREWORD
This publication, "A Guide to Understanding Audit in Trusted
Systems," is being issued by the National Computer Security
Center (NCSC) under the authority of and in accordance with
Department of Defense (DoD) Directive 5215.1. The guidelines
described in this document provide a set of good practices
related to the use of auditing in automatic data processing
systems employed for processing classified and other sensitive
information. Recommendations for revision to this guideline are
encouraged and will be reviewed biannually by the National
Computer Security Center through a formal review process.
Address all proposals for revision through appropriate channels
to:
National Computer Security Center
9800 Savage Road
Fort George G. Meade, MD 20755-6000
Attention: Chief, Computer Security Technical Guidelines
_________________________________
Patrick R. Gallagher, Jr. 28 July 1987
Director
National Computer Security Center
i
ACKNOWLEDGEMENTS
Special recognition is extended to James N. Menendez, National
Computer Security Center (NCSC), as project manager of the
preparation and production of this document.
Acknowledgement is also given to the NCSC Product Evaluations
Team who provided the technical guidance that helped form this
document and to those members of the computer security community
who contributed their time and expertise by actively
participating in the review of this document.
ii
CONTENTS
FOREWORD ................................................... i
ACKNOWLEDGEMENTS ........................................... ii
CONTENTS ................................................... iii
PREFACE ..................................................... v
1. INTRODUCTION ............................................. 1
1.1 HISTORY OF THE NATIONAL COMPUTER SECURITY CENTER .... 1
1.2 GOAL OF THE NATIONAL COMPUTER SECURITY CENTER ....... 1
2. PURPOSE .................................................. 2
3. SCOPE .................................................... 3
4. CONTROL OBJECTIVES ....................................... 4
5. OVERVIEW OF AUDITING PRINCIPLES .......................... 8
5.1 PURPOSE OF THE AUDIT MECHANISM....................... 8
5.2 USERS OF THE AUDIT MECHANISM......................... 8
5.3 ASPECTS OF EFFECTIVE AUDITING ....................... 9
5.3.1 Identification/Authentication ................ 9
5.3.2 Administrative ............................... 10
5.3.3 System Design ................................ 10
5.4 SECURITY OF THE AUDIT ............................... 10
6. MEETING THE CRITERIA REQUIREMENTS ........................ 12
6.1 THE C2 AUDIT REQUIREMENT ............................ 12
6.1.1 Auditable Events ............................. 12
6.1.2 Auditable Information ........................ 12
6.1.3 Audit Basis .................................. 13
6.2 THE B1 AUDIT REQUIREMENT ............................ 13
6.2.1 Auditable Events ............................. 13
6.2.2 Auditable Information ........................ 13
6.2.3 Audit Basis .................................. 14
iii
CONTENTS (Continued)
6.3 THE B2 AUDIT REQUIREMENT ............................ 14
6.3.1 Auditable Events ............................. 14
6.3.2 Auditable Information ........................ 14
6.3.3 Audit Basis .................................. 14
6.4 THE B3 AUDIT REQUIREMENT ............................ 15
6.4.1 Auditable Events ............................. 15
6.4.2 Auditable Information ........................ 15
6.4.3 Audit Basis .................................. 15
6.5 THE A1 AUDIT REQUIREMENT ............................ 16
6.5.1 Auditable Events ............................. 16
6.5.2 Auditable Information ........................ 16
6.5.3 Audit Basis .................................. 16
7. POSSIBLE IMPLEMENTATION METHODS .......................... 17
7.1 PRE/POST SELECTION OF AUDITABLE EVENTS .............. 17
7.1.1 Pre-Selection ................................ 17
7.1.2 Post-Selection ............................... 18
7.2 DATA COMPRESSION .................................... 18
7.3 MULTIPLE AUDIT TRAILS ............................... 19
7.4 PHYSICAL STORAGE .................................... 19
7.5 WRITE-ONCE DEVICE ................................... 20
7.6 FORWARDING AUDIT DATA ............................... 21
8. OTHER TOPICS ............................................. 22
8.1 AUDIT DATA REDUCTION ................................ 22
8.2 AVAILABILITY OF AUDIT DATA .......................... 22
8.3 AUDIT DATA RETENTION ................................ 22
8.4 TESTING ............................................. 23
8.5 DOCUMENTATION ....................................... 23
8.6 UNAVOIDABLE SECURITY RISKS .......................... 24
8.6.1 Auditing Administrators/Insider Threat ....... 24
8.6.2 Data Loss .................................... 25
9. AUDIT SUMMARY ........................................... 26
GLOSSARY
REFERENCES .............................................. 27
PREFACE
Throughout this guideline there will be recommendations made that
are not included in the Trusted Computer System Evaluation
Criteria (the Criteria) as requirements. Any recommendations
that are not in the Criteria will be prefaced by the word
"should," whereas all requirements will be prefaced by the word
"shall." It is hoped that this will help to avoid any confusion.
v
1
1. INTRODUCTION
1.1 History of the National Computer Security Center
The DoD Computer Security Center (DoDCSC) was established in
January 1981 for the purpose of expanding on the work started by
the DoD Security Initiative. Accordingly, the Director, National
Computer Security Center, has the responsibility for establishing
and publishing standards and guidelines for all areas of computer
security. In 1985, DoDCSC's name was changed to the National
Computer Security Center to reflect its responsibility for
computer security throughout the federal government.
1.2 Goal of the National Computer Security Center
The main goal of the National Computer Security Center is to
encourage the widespread availability of trusted computer
systems. In support of that goal a metric was created, the DoD
Trusted Computer System Evaluation Criteria (the Criteria),
against which computer systems could be evaluated for security.
The Criteria was originally published on 15 August 1983 as CSC-
STD-001-83. In December 1985 the DoD adopted it, with a few
changes, as a DoD Standard, DoD 5200.28-STD. DoD Directive
5200.28, "Security Requirements for Automatic Data Processing
(ADP) Systems" has been written to, among other things, require
the Department of Defense Trusted Computer System Evaluation
Criteria to be used throughout the DoD. The Criteria is the
standard used for evaluating the effectiveness of security
controls built into ADP systems. The Criteria is divided into
four divisions: D, C, B, and A, ordered in a hierarchical manner
with the highest division (A) being reserved for systems
providing the best available level of assurance. Within
divisions C and B there are a number of subdivisions known as
classes, which are also ordered in a hierarchical manner to
represent different levels of security in these classes.
2. PURPOSE
For Criteria classes C2 through A1 the Criteria requires that a
user's actions be open to scrutiny by means of an audit. The
audit process of a secure system is the process of recording,
examining, and reviewing any or all security-relevant activities
on the system. This guideline is intended to discuss issues
involved in implementing and evaluating an audit mechanism. The
purpose of this document is twofold. It provides guidance to
manufacturers on how to design and incorporate an effective audit
mechanism into their system, and it provides guidance to
implementors on how to make effective use of the audit
1
capabilities provided by trusted systems. This document contains
suggestions as to what information should be recorded on the
audit trail, how the audit should be conducted, and what
protective measures should be accorded to the audit resources.
Any examples in this document are not to be construed as the only
implementations that will satisfy the Criteria requirement. The
examples are merely suggestions of appropriate implementations.
The recommendations in this document are also not to be construed
as supplementary requirements to the Criteria. The Criteria is
the only metric against which systems are to be evaluated.
This guideline is part of an on-going program to provide helpful
guidance on Criteria issues and the features they address.
3. SCOPE
An important security feature of Criteria classes C2 through A1
is the ability of the ADP system to audit any or all of the
activities on the system. This guideline will discuss auditing
and the features of audit facilities as they apply to computer
systems and products that are being built with the intention of
meeting the requirements of the Criteria.
2
4. CONTROL OBJECTIVES
The Trusted Computer System Evaluation Criteria gives the
following as the Accountability Control Objective:
"Systems that are used to process or handle classified or
other sensitive information must assure individual
accountability whenever either a mandatory or
discretionary security policy is invoked. Furthermore, to
assure accountability the capability must exist for an
authorized and competent agent to access and evaluate
accountability information by a secure means, within a
reasonable amount of time and without undue difficulty."(1)
The Accountability Control Objective as it relates to auditing
leads to the following control objective for auditing:
"A trusted computer system must provide authorized personnel
with the ability to audit any action that can potentially
cause access to, generation of, or effect the release
of classified or sensitive information. The audit
data will be selectively acquired based on the auditing
needs of a particular installation and/or application.
However, there must be sufficient granularity in the audit
data to support tracing the auditable events to a specific
individual (or process) who has taken the actions or on
whose behalf the actions were taken."(1)
3
5. OVERVIEW OF AUDITING PRINCIPLES
Audit trails are used to detect and deter penetration of a
computer system and to reveal usage that identifies misuse. At
the discretion of the auditor, audit trails may be limited to
specific events or may encompass all of the activities on a
system. Although not required by the TCSEC, it should be
possible for the target of the audit mechanism to be either a
subject or an object. That is to say, the audit mechanism should
be capable of monitoring every time John accessed the system as
well as every time the nuclear reactor file was accessed; and
likewise every time John accessed the nuclear reactor file.
5.1 Purpose of the Audit Mechanism
The audit mechanism of a computer system has five important
security goals. First, the audit mechanism must "allow the
review of patterns of access to individual objects, access
histories of specific processes and individuals, and the use of
the various protection mechanisms supported by the system and
their effectiveness."(2) Second, the audit mechanism must allow
discovery of both users' and outsiders' repeated attempts to
bypass the protection mechanisms. Third, the audit mechanism
must allow discovery of any use of privileges that may occur when
a user assumes a functionality with privileges greater than his
or her own, i.e., programmer to administrator. In this case
there may be no bypass of security controls but nevertheless a
violation is made possible. Fourth, the audit mechanism must act
as a deterrent against perpetrators' habitual attempts to bypass
the system protection mechanisms. However, to act as a
deterrent, the perpetrator must be aware of the audit mechanism's
existence and its active use to detect any attempts to bypass
system protection mechanisms. The fifth goal of the audit
mechanism is to supply "an additional form of user assurance that
attempts to bypass the protection mechanisms are recorded and
discovered."(2) Even if the attempt to bypass the protection
mechanism is successful, the audit trail will still provide
assurance by its ability to aid in assessing the damage done by
the violation, thus improving the system's ability to control the
damage.
5.2. Users of the Audit Mechanism
"The users of the audit mechanism can be divided into two groups.
The first group consists of the auditor, who is an individual
with administrative duties, who selects the events to be audited
on the system, sets up the audit flags which enable the recording
4
of those events, and analyzes the trail of audit events."(2) In
some systems the duties of the auditor may be encompassed in the
duties of the system security administrator. Also, at the lower
classes, the auditor role may be performed by the system
administrator. This document will refer to the person
responsible for auditing as the system security administrator,
although it is understood that the auditing guidelines may apply
to system administrators and/or system security administrators
and/or a separate auditor in some ADP systems.
"The second group of users of the audit mechanism consists of the
system users themselves; this group includes the administrators,
the operators, the system programmers, and all other users. They
are considered users of the audit mechanism not only because
they, and their programs, generate audit events,"(2) but because
they must understand that the audit mechanism exists and what
impact it has on them. This is important because otherwise the
user deterrence and user assurance goals of the audit mechanism
cannot be achieved.
5.3 Aspects of Effective Auditing
5.3.1. Identification/Authentication
Logging in on a system normally requires that a user enter the
specified form of identification (e.g., login ID, magnetic strip)
and a password (or some other mechanism) for authentication.
Whether this information is valid or invalid, the execution of
the login procedure is an auditable event and the identification
entered may be considered to be auditable information. It is
recommended that authentication information, such as passwords,
not be forwarded to the audit trail. In the event that the
identification entered is not recognized as being valid, the
system should also omit this information from the audit trail.
The reason for this is that a user may have entered a password
when the system expected a login ID. If the information had been
written to the audit trail, it would compromise the password and
the security of the user.
There are, however, environments where the risk involved in
recording invalid identification information is reduced. In
systems that support formatted terminals, the likelihood of
password entry in the identification field is markedly reduced,
hence the recording of identification information would pose no
major threat. The benefit of recording the identification
information is that break-in attempts would be easier to detect
and identifying the perpetrator would also be assisted. The
5
information gathered here may be necessary for any legal
prosecution that may follow a security violation.
5.3.2 Administrative
All systems rated at class C2 or higher shall have audit
capabilities and personnel designated as responsible for the
audit procedures. For the C2 and B1 classes, the duties of the
system operators could encompass all functions including those of
the auditor. Starting at the B2 class, there is a requirement
for the TCB to support separate operator and administrator
functions. In addition, at the B3 class and above, there is a
requirement to identify the system security administrator
functions. When one assumes the system security administrator
role on the system, it shall be after taking distinct auditable
action, e.g., login procedure. When one with the privilege of
assuming the role is on the system, the act of assuming that role
shall also be an auditable event.
5.3.3 System Design
The system design should include a mechanism to invoke the audit
function at the request of the system security administrator. A
mechanism should also be included to determine if the event is to
be selected for inclusion as an audit trail entry. If
pre-selection of events is not implemented, then all auditable
events should be forwarded to the audit trail. The Criteria
requirement for the administrator to be able to select events
based on user identity and/or object security classification must
still be able to be satisfied. This requirement can be met by
allowing post-selection of events through the use of queries.
Whatever reduction tool is used to analyze the audit trail shall
be provided by the vendor.
5.4 Security of the Audit
Audit trail software, as well as the audit trail itself, should
be protected by the Trusted Computing Base and should be subject
to strict access controls. The security requirements of the
audit mechanism are the following:
(1) The event recording mechanism shall be part of the TCB and
shall be protected from unauthorized modification or
circumvention.
(2) The audit trail itself shall be protected by the TCB from
6
unauthorized access (i.e., only the audit personnel may
access the audit trail). The audit trail shall also be
protected from unauthorized modification.
(3) The audit-event enabling/disabling mechanism shall be part
of the TCB and shall remain inaccessible to the unauthorized
users.(2)
At a minimum, the data on the audit trail should be considered to
be sensitive, and the audit trail itself shall be considered to
be as sensitive as the most sensitive data contained in the
system.
When the medium containing the audit trail is physically removed
from the ADP system, the medium should be accorded the physical
protection required for the highest sensitivity level of data
contained in the system.
7
6. MEETING THE CRITERIA REQUIREMENTS
This section of the guideline will discuss the audit requirements
in the Criteria and will present a number of additional
recommendations. There are four levels of audit requirements.
The first level is at the C2 Criteria class and the requirements
continue evolving through the B3 Criteria class. At each of
these levels, the guideline will list some of the events which
should be auditable, what information should be on the audit
trail, and on what basis events may be selected to be audited.
All of the requirements will be prefaced by the word "shall," and
any additional recommendations will be prefaced by the word
"should."
6.1 The C2 Audit Requirement
6.1.1 Auditable Events
The following events shall be subject to audit at the C2 class:
* Use of identification and authentication mechanisms
* Introduction of objects into a user's address space
* Deletion of objects from a user's address space
* Actions taken by computer operators and system
administrators and/or system security administrators
* All security-relevant events (as defined in Section 5 of
this guideline)
* Production of printed output
6.1.2 Auditable Information
The following information shall be recorded on the audit trail at
the C2 class:
* Date and time of the event
* The unique identifier on whose behalf the subject generating
the event was operating
* Type of event
* Success or failure of the event
8
* Origin of the request (e.g., terminal ID) for
identification/authentication events
* Name of object introduced, accessed, or deleted from a
user's address space
* Description of modifications made by the system
administrator to the user/system security databases
6.1.3 Audit Basis
At the C2 level, the ADP System Administrator shall be able to
audit based on individual identity.
The ADP System Administrator should also be able to audit based
on object identity.
6.2 The B1 Audit Requirement
6.2.1 Auditable Events
The Criteria specifically adds the following to the list of
events that shall be auditable at the B1 class:
* Any override of human readable output markings (including
overwrite of sensitivity label markings and the turning off
of labelling capabilities) on paged, hard-copy output
devices
* Change of designation (single-level to/from multi-level) of
any communication channel or I/O device
* Change of sensitivity level(s) associated with a
single-level communication channel or I/O device
* Change of range designation of any multi-level communication
channel or I/O device
6.2.2 Auditable Information
The Criteria specifically adds the following to the list of
information that shall be recorded on the audit trail at the B1
class:
* Security level of the object
9
The following information should also be recorded on the audit
trail at the B1 class:
* Subject sensitivity level
6.2.3 Audit Basis
In addition to previous selection criteria, at the B1 level the
Criteria specifically requires that the ADP System Administrator
shall be able to audit based on individual identity and/or object
security level.
6.3 The B2 Audit Requirement
6.3.1 Auditable Events
The Criteria specifically adds the following to the list of
events that shall be auditable at the B2 class:
* Events that may exercise covert storage channels
6.3.2 Auditable Information
No new requirements have been added at the B2 class.
6.3.3 Audit Basis
In addition to previous selection criteria, at the B2 level the
Criteria specifically requires that "the TCB shall be able to
audit the identified events that may be used in the exploitation
of covert storage channels." The Trusted Computing Base shall
audit covert storage channels that exceed ten bits per second.(1)
The Trusted Computing Base should also provide the capability to
audit the use of covert storage mechanisms with bandwidths that
may exceed a rate of one bit in ten seconds.
6.4 The B3 Audit Requirement
6.4.1 Auditable Events
The Criteria specifically adds the following to the list of
events that shall be auditable at the B3 class:
* Events that may indicate an imminent violation of the
10
system's security policy (e.g., exercise covert timing
channels)
6.4.2 Auditable Information
No new requirements have been added at the B3 class.
6.4.3 Audit Basis
In addition to previous selection criteria, at the B3 level the
Criteria specifically requires that "the TCB shall contain a
mechanism that is able to monitor the occurrence or accumulation
of security auditable events that may indicate an imminent
violation of security policy. This mechanism shall be able to
immediately notify the system security administrator when
thresholds are exceeded and, if the occurrence or accumulation of
these security-relevant events continues, the system shall take
the least disruptive action to terminate the event."(1)
Events that would indicate an imminent security violation would
include events that utilize covert timing channels that may
exceed a rate of ten bits per second and any repeated
unsuccessful login attempts.
Being able to immediately notify the system security
administrator when thresholds are exceeded means that the
mechanism shall be able to recognize, report, and respond to a
violation of the security policy more rapidly than required at
lower levels of the Criteria, which usually only requires the
System Security Administrator to review an audit trail at some
time after the event. Notification of the violation "should be
at the same priority as any other TCB message to an operator."(5)
"If the occurrence or accumulation of these security-relevant
events continues, the system shall take the least disruptive
action to terminate the event."(1) These actions may include
locking the terminal of the user who is causing the event or
terminating the suspect's process(es). In general, the least
disruptive action is application dependent and there is no
requirement to demonstrate that the action is the least
disruptive of all possible actions. Any action which terminates
the event is acceptable, but halting the system should be the
last resort.
11
7.5 The A1 Audit Requirement
7.5.1 Auditable Events
No new requirements have been added at the A1 class.
7.5.2 Auditable Information
No new requirements have been added at the A1 class.
7.5.3 Audit Basis
No new requirements have been added at the A1 class.
12
7. POSSIBLE IMPLEMENTATION METHODS
The techniques for implementing the audit requirements will vary
from system to system depending upon the characteristics of the
software, firmware, and hardware involved and any optional
features that are to be available. Technologically advanced
techniques that are available should be used to the best
advantage in the system design to provide the requisite security
as well as cost-effectiveness and performance.
7.1 Pre/Post Selection of Auditable Events
There is a requirement at classes C2 and above that all security-
relevant events be auditable. However, these events may or may
not always be recorded on the audit trail. Options that may be
exercised in selecting which events should be audited include a
pre-selection feature and a post-selection feature. A system may
choose to implement both options, a pre-selection option only, or
a post-selection option only.
If a system developer chooses not to implement a general pre/post
selection option, there is still a requirement to allow the
administrator to selectively audit the actions of specified users
for all Criteria classes. Starting at the B1 class, the
administrator shall also be able to audit based on object
security level.
There should be options to allow selection by either individuals
or groups of users. For example, the administrator may select
events related to a specified individual or select events related
to individuals included in a specified group. Also, the
administrator may specify that events related to the audit file
be selected or, at classes B1 and above, that accesses to objects
with a given sensitivity level, such as Top Secret, be selected.
7.1.1 Pre-Selection
For each auditable event the TCB should contain a mechanism to
indicate if the event is to be recorded on the audit trail. The
system security administrator or designee shall be the only
person authorized to select the events to be recorded.
Pre-selection may be by user(s) identity, and at the B1 class and
above, pre-selection may also be possible by object security
level. Although the system security administrator shall be
authorized to select which events are to be recorded, the system
security administrator should not be able to exclude himself from
being audited.
13
Although it would not be recommended, the system security
administrator may have the capability to select that no events be
recorded regardless of the Criteria requirements. The intention
here is to provide flexibility. The purpose of designing audit
features into a system is not to impose the Criteria on users
that may not want it, but merely to provide the capability to
implement the requirements.
A disadvantage of pre-selection is that it is very hard to
predict what events may be of security-relevant interest at a
future date. There is always the possibility that events not
pre-selected could one day become security-relevant, and the
potential loss from not auditing these events would be impossible
to determine.
The advantage of pre-selection could possibly be better
performance as a result of not auditing all the events on the
system.
7.1.2 Post-Selection
If the post-selection option to select only specified events from
an existing audit trail is implemented, again, only authorized
personnel shall be able to make this selection. Inclusion of
this option requires that the system should have trusted
facilities (as described in section 9.1) to accept
query/retrieval requests, to expand any compressed data, and to
output the requested data.
The main advantage of post-selection is that information that may
prove useful in the future is already recorded on an audit trail
and may be queried at any time.
The disadvantage involved in post-selection could possibly be
degraded performance due to the writing and storing of what could
possibly be a very large audit trail.
7.2 Data Compression
"Since a system that selects all events to be audited may
generate a large amount of data, it may be necessary to encode
the data to conserve space and minimize the processor time
required" to record the audit records.(3) If the audit trail is
encoded, a complementary mechanism must be included to decode the
data when required. The decoding of the audit trail may be done
as a preprocess before the audit records are accessed by the
database or as a postprocess after a relevant record has been
14
found. Such decoding is necessary to present the data in an
understandable form both at the administrators terminal and on
batch reports. The cost of compressing the audit trail would be
the time required for the compression and expansion processes.
The benefit of compressing data is the savings in storage and the
savings in time to write the records to the audit trail.
7.3 Multiple Audit Trails
All events included on the audit trail may be written as part of
the same audit trail, but some systems may prefer to have several
distinct audit trails, e.g., one would be for "user" events, one
for "operator" events, and one for "system security
administrator" events. This would result in several smaller
trails for subsequent analysis. In some cases, however, it may
be necessary to combine the information from the trails when
questionable events occur in order to obtain a composite of the
sequence of events as they occurred. In cases where there are
multiple audit trails, it is preferred that there be some
accurate, or at least synchronized, time stamps across the
multiple logs.
Although the preference for several distinct audit trails may be
present, it is important to note that it is often more useful
that the TCB be able to present all audit data as one
comprehensive audit trail.
7.4 Physical Storage
A factor to consider in the selection of the medium to be used
for the audit trail would be the expected usage of the system.
The I/O volume for a system with few users executing few
applications would be quite different from that of a large system
with a multitude of users performing a variety of applications.
In any case, however, the system should notify the system
operator or administrator when the audit trail medium is
approaching its storage capacity. Adequate advance notification
to the operator is especially necessary if human intervention is
required.
If the audit trail storage medium is saturated before it is
replaced, the operating system shall detect this and take some
appropriate action such as:
1. Notifying the operator that the medium is "full" and action
is necessary. The system should then stop and require
rebooting. Although a valid option, this action creates a
15
severe threat of denial-of-service attacks.
2. Storing the current audit records on a temporary medium with
the intention of later migration to the normal operational
medium, thus allowing auditing to continue. This temporary
storage medium should be afforded the same protection as the
regular audit storage medium in order to prevent any attempts
to tamper with it.
3. Delaying input of new actions and/or slowing down current
operations to prevent any action that requires use of the
audit mechanism.
4. Stopping until the administrative personnel make more space
available for writing audit records.
5. Stopping auditing entirely as a result of a decision by the
system security administrator.
Any action that is taken in response to storage overflow shall be
audited. There is, however, a case in which the action taken may
not be audited that deserves mention. It is possible to have the
system security administrator's decisions embedded in the system
logic. Such pre-programmed choices, embedded in the system
logic, may be triggered automatically and this action may not be
audited.
Still another consideration is the speed at which the medium
operates. It should be able to accommodate the "worst case"
condition such as when there are a large number of users on the
system and all auditable events are to be recorded. This worst
case rate should be estimated during the system design phase and
(when possible) suitable hardware should be selected for this
purpose.
Regardless of how the system handles audit trail overflow, there
must be a way to archive all of the audit data.
7.5 Write-Once Device
For the lower Criteria classes (e.g., C2, B1) the audit trail may
be the major tool used in detecting security compromises.
Implicit in this is that the audit resources should provide the
maximum protection possible. One technique that may be employed
to protect the audit trail is to record it on a mechanism
designed to be a write-only device. Other choices would be to
set the designated device to write-once mode by disabling the
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read mechanism. This method could prevent an attacker from
erasing or modifying the data already written on the audit trail
because the attacker will not be able to go back and read or find
the data that he or she wishes to modify.
If a hardware device is available that permits only the writing
of data on a medium, modification of data already recorded would
be quite difficult. Spurious messages could be written, but to
locate and modify an already recorded message would be difficult.
Use of a write-once device does not prevent a penetrator from
modifying audit resources in memory, including any buffers, in
the current audit trail.
If a write-once device is used to record the audit trail, the
medium can later be switched to a compatible read device to allow
authorized personnel to analyze the information on the audit
trail in order to detect any attempts to penetrate the system.
If a penetrator modified the audit software to prevent writing
records on the audit trail, the absence of data during an
extended period of time would indicate a possible security
compromise. The disadvantage of using a write-once device is
that it necessitates a delay before the audit trail is available
for analysis by the administrator. This may be offset by
allowing the system security administrator to review the audit
trail in real-time by getting copies of all audit records on
their way to the device.
7.6 Forwarding Audit Data
If the facilities are available, another method of protecting the
audit trail would be to forward it to a dedicated processor. The
audit trail should then be more readily available for analysis by
the system security administrator.
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8. OTHER TOPICS
8.1 Audit Data Reduction
Depending upon the amount of activity on a system and the audit
selection process used, the audit trail size may vary. It is a
safe assumption though, that the audit trail would grow to sizes
that would necessitate some form of audit data reduction. The
data reduction tool would most likely be a batch program that
would interface to the system security administrator. This batch
run could be a combination of database query language and a
report generator with the input being a standardized audit file.
Although they are not necessarily part of the TCB, the audit
reduction tools should be maintained under the same configuration
control system as the remainder of the system.
8.2 Availability of Audit Data
In standard data processing, audit information is recorded as it
occurs. Although most information is not required to be
immediately available for real-time analysis, the system security
administrator should have the capability to retreive audit
information within minutes of its recording. The delay between
recording audit information and making it available for analysis
should be minimal, in the range of several minutes.
For events which do require immediate attention, at the B3 class
and above, an alert shall be sent out to the system security
administrator. In systems that store the audit trail in a
buffer, the system security administrator should have the
capability to cause the buffer to be written out. Regarding
real-time alarms, where they are sent is system dependent.
8.3 Audit Data Retention
The exact period of time required for retaining the audit trail
is site dependent and should be documented in the site's
operating procedures manual. When trying to arrive at the
optimum time for audit trail retention, any time restrictions on
the storage medium should be considered. The storage medium used
must be able to reliably retain the audit data for the amount of
time required by the site.
The audit trail should be reviewed at least once a week. It is
very possible that once a week may be too long to wait to review
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the audit trail. Depending on the amount of audit data expected
by the system, this parameter should be adjusted accordingly.
The recommended time in between audit trail reviews should be
documented in the Trusted Facility Manual.
8.4 Testing
The audit resources, along with all other resources protected by
the TCB, have increasing assurance requirements at each higher
Criteria class. For the lower classes, an audit trail would be a
major factor in detecting penetration attempts. Unfortunately,
at these lower classes, the audit resources are more susceptible
to penetration and corruption. "The TCB must provide some
assurance that the data will still be there when the
administrator tries to use it."(3) The testing requirement
recognizes the vulnerability of the audit trail, and starting
with the C2 class, shall include a search for obvious flaws that
would corrupt or destroy the audit trail. If the audit trail is
corrupted or destroyed, the existence of such flaws indicates
that the system can be penetrated. Testing should also be
performed to uncover any ways of circumventing the audit
mechanisms. The "flaws found in testing may be neutralized in
any of a number of ways. One way available to the system
designer is to audit all uses of the mechanism in which the flaw
is found and to log such events."(3) An attempt should be made
to remove the flaw.
At class B2 and above, it is required that all detected flaws
shall be corrected or else a lower rating will be given. If
during testing the audit trail appears valid, analysis of this
data can verify that it does or does not accurately reflect the
events that should be included on the audit trail. Even though
system assurances may increase at the higher classes, the audit
trail is still an effective tool during the testing phase as well
as operationally in detecting actual or potential security
compromises.
8.5 Documentation
Starting at the C2 class, documentation concerning the audit
requirements shall be contained in the Trusted Facility Manual.
The Trusted Facility Manual shall explain the procedures to
record, examine, and maintain audit files. It shall detail the
audit record structure for each type of audit event, and should
include what each field is and what the size of the field is.
The Trusted Facility Manual shall also include a complete
19
description of the audit mechanism interface, how it should be
used, its default settings, cautions about the trade-offs
involved in using various configurations and capabilities, and
how to set up and run the system such that the audit data is
afforded appropriate protection.
If audit events can be pre- or post-selected, the manual should
also describe the tools and mechanisms available and how they are
to be used.
8.6 Unavoidable Security Risks
There are certain risks contained in the audit process that exist
simply because there is no way to prevent these events from ever
occurring. Because there are certain unpredictable factors
involved in auditing, i.e., man, nature, etc., the audit
mechanism may never be one hundred per cent reliable. Preventive
measures may be taken to minimize the likelihood of any of these
factors adversely affecting the security provided by the audit
mechanism, but no audit mechanism will ever be risk free.
8.6.1 Auditing Administrators/Insider Threat
Even with auditing mechanisms in place to detect and deter
security violations, the threat of the perpetrator actually being
the system security administrator or someone involved with the
system security design will always be present. It is quite
possible that the system security administrator of a secure
system could stop the auditing of activities while entering the
system and corrupting files for personal benefit. These
authorized personnel, who may also have access to identification
and authentication information, could also choose to enter the
system disguised as another user in order to commit crimes under
a false identity.
Management should be aware of this risk and should be certain to
exercise discretion when selecting the system security
administrator. The person who is to be selected for a trusted
position, such as the system security administrator, should be
subject to a background check before being granted the privileges
that could one day be used against the employer.
The system security administrator could also be watched to ensure
that there are no unexplained variances in normal duties. Any
deviation from the norm of operations may indicate that a
violation of security has occurred or is about to occur.
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An additional security measure to control this insider threat is
to ensure that the system administrator and the person
responsible for the audit are two different people. "The
separation of the auditor's functions, databases, and access
privileges from those of the system administrator is an important
application of the separation of privilege and least privilege
principles. Should such a separation not be performed, and
should the administrator be allowed to undertake auditor
functions or vice-versa, the entire security function would
become the responsibility of a single, unaccountable
individual."(2)
Another alternative may be to employ separate auditor roles.
Such a situation may give one person the authority to turn off
the audit mechanism, while another person may have the authority
to turn it back on. In this case no individual would be able to
turn off the audit mechanism, compromise the system, and then
turn it back on.
8.6.2 Data Loss
Although the audit software and hardware are reliable security
mechanisms, they are not infallible. They, like the rest of the
system, are dependent upon constant supplies of power and are
readily subject to interruption due to mechanical or power
failures. Their failure can cause the loss or destruction of
valuable audit data. The system security administrator should be
aware of this risk and should establish some procedure that would
ensure that the audit trail is preserved somewhere. The system
security administrator should duplicate the audit trail on a
removable medium at certain points in time to minimize the data
loss in the event of a system failure. The Trusted Facility
Manual should include what the possibilities and nature of loss
exposure are, and how the data may be recovered in the event that
a catastrophe does occur.
If a mechanical or power failure occurs, the system security
administrator should ensure that audit mechanisms still function
properly after system recovery. For example, any auditing
mechanism options pre-selected before the system malfunction must
still be the ones in operation after the system recovery.
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9. AUDIT SUMMARY
For classes C2 and above, it is required that the TCB "be able to
create, maintain, and protect from modification or unauthorized
access or destruction an audit trail of accesses to the objects
it protects."(1) The audit trail plays a key role in performing
damage assessment in the case of a corrupted system.
The audit trail shall keep track of all security-relevant events
such as the use of identification and authentication mechanisms,
introduction of objects into a user's address space, deletion of
objects from the system, system administrator actions, and any
other events that attempt to violate the security policy of the
system. The option should exist that either all activities be
audited or that the system security administrator select the
events to be audited. If it is decided that all activities
should be audited, there are overhead factors to be considered.
The storage space needed for a total audit would generally
require more operator maintenance to prevent any loss of data and
to provide adequate protection. A requirement exists that
authorized personnel shall be able to read all events recorded on
the audit trail. Analysis of the total audit trail would be both
a difficult and time-consuming task for the administrator. Thus,
a selection option is required which may be either a
pre-selection or post-selection option.
The audit trail information should be sufficient to reconstruct a
complete sequence of security-relevant events and processes for a
system. To do this, the audit trail shall contain the following
information: date and time of the event, user, type of event,
success or failure of the event, the origin of the request, the
name of the object introduced into the user's address space,
accessed, or deleted from the storage system, and at the B1 class
and above, the sensitivity determination of the object.
It should be remembered that the audit trail shall be included in
the Trusted Computing Base and shall be accorded the same
protection as the TCB. The audit trail shall be subject to
strict access controls.
An effective audit trail is necessary in order to detect and
evaluate hostile attacks on a system.
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GLOSSARY
Administrator - Any one of a group of personnel assigned to
supervise all or a portion of an ADP system.
Archive - To file or store records off-line.
Audit - To conduct the independent review and examination of
system records and activities.
Auditor - An authorized individual with administrative duties,
whose duties include selecting the events to be audited on the
system, setting up the audit flags which enable the recording of
those events, and analyzing the trail of audit events.(2)
Audit Mechanism - The device used to collect, review, and/or
examine system activities.
Audit Trail - A set of records that collectively provide
documentary evidence of processing used to aid in tracing from
original transactions forward to related records and reports,
and/or backwards from records and reports to their component
source transactions.(1)
Auditable Event - Any event that can be selected for inclusion in
the audit trail. These events should include, in addition to
security-relevant events, events taken to recover the system
after failure and any events that might prove to be
security-relevant at a later time.
Authenticated User - A user who has accessed an ADP system with a
valid identifier and authentication combination.
Automatic Data Processing (ADP) System - An assembly of computer
hardware, firmware, and software configured for the purpose of
classifying, sorting, calculating, computing, summarizing,
transmitting and receiving, storing, and retrieving data with a
minimum of human intervention.(1)
Category - A grouping of classified or unclassified sensitive
information, to which an additional restrictive label is applied
(e.g., proprietary, compartmented information) to signify that
personnel are granted access to the information only if they have
formal approval or other appropriate authorization.(4)
Covert Channel - A communication channel that allows a process to
transfer information in a manner that violates the system's
security policy.(1)
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Covert Storage Channel - A covert channel that involves the
direct or indirect writing of a storage location by one process
and the direct or indirect reading of the storage location by
another process. Covert storage channels typically involve a
finite resource (e.g., sectors on a disk) that is shared by two
subjects at different security levels.(1)
Covert Timing Channel - A covert channel in which one process
signals information to another by modulating its own use of
system resources (e.g., CPU time) in such a way that this
manipulation affects the real response time observed by the
second process.(1)
Flaw - An error of commission, omission or oversight in a system
that allows protection mechanisms to be bypassed.(1)
Object - A passive entity that contains or receives information.
Access to an object potentially implies access to the information
it contains. Examples of objects are: records, blocks, pages,
segments, files, directories, directory trees and programs, as
well as bits, bytes, words, fields, processors, video displays,
keyboards, clocks, printers, network nodes, etc.(1)
Post-Selection - Selection, by authorized personnel, of specified
events that had been recorded on the audit trail.
Pre-Selection - Selection, by authorized personnel, of the
auditable events that are to be recorded on the audit trail.
Security Level - The combination of a hierarchical classification
and a set of non-hierarchical categories that represents the
sensitivity of information.(1)
Security Policy - The set of laws, rules, and practices that
regulate how an organization manages, protects, and distributes
sensitive information.(1)
Security-Relevant Event - Any event that attempts to change the
security state of the system, (e.g., change discretionary access
controls, change the security level of the subject, change user
password, etc.). Also, any event that attempts to violate the
security policy of the system, (e.g., too many attempts to login,
attempts to violate the mandatory access control limits of a
device, attempts to downgrade a file, etc.).(1)
Sensitive Information - Information that, as determined by a
competent authority, must be protected because its unauthorized
disclosure, alteration, loss, or destruction will at least cause
perceivable damage to someone or something.(1)
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Subject - An active entity, generally in the form of a person,
process, or device that causes information to flow among objects
or changes the system state. Technically, a process/domain
pair.(1)
Subject Sensitivity Level - The sensitivity level of the objects
to which the subject has both read and write access. A subject's
sensitivity level must always be less than or equal to the
clearance of the user the subject is associated with.(4)
System Security Administrator - The person responsible for the
security of an Automated Information System and having the
authority to enforce the security safeguards on all others who
have access to the Automated Information System.(4)
Trusted Computing Base (TCB) - The totality of protection
mechanisms within a computer system -- including hardware,
firmware, and software -- the combination of which is responsible
for enforcing a security policy. A TCB consists of one or more
components that together enforce a unified security policy over a
product or system. The ability of a TCB to correctly enforce a
security policy depends solely on the mechanisms within the TCB
and on the correct input by system administrative personnel of
parameters (e.g., a user's clearance) related to the security
policy.(1)
User - Any person who interacts directly with a computer
system.(1)
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REFERENCES
1. National Computer Security Center, DoD Trusted Computer
System Evaluation Criteria, DoD, DoD 5200.28-STD, 1985.
2. Gligor, Virgil D., "Guidelines for Trusted Facility
Management and Audit," University of Maryland, 1985.
3. Brown, Leonard R., "Guidelines for Audit Log Mechanisms in
Secure Computer Systems," Technical Report
TR-0086A(2770-29)-1, The Aerospace Corporation, 1986.
4. Subcommittee on Automated Information System Security,
Working Group #3, "Dictionary of Computer Security
Terminology," 23 November 1986.
5. National Computer Security Center, Criterion
Interpretation, Report No. C1-C1-02-87, 1987.
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