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Guidelines for Writing Trusted Facility Manuals
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Table of Contents
FOREWORD
ACKNOWLEDGMENTS
PREFACE
1 INTRODUCTION
1.1 Purpose
1.2 Scope and Contents
1.3 Control Objectives
1.4 TFM Introduction
2 SYSTEM SECURITY OVERVIEW
2.1 Threats
2.2 Countermeasures Based on Security and Accountability Policies
and Procedures
2.3 Explicit Physical Security Assumptions
2.4 Protection Mechanisms Available to Administrative Users
2.5 Security Vulnerabilities and Warnings
2.6 Separation of Administrative Roles
3 SECURITY POLICY
4 ACCOUNTABILITY
4.1 Identification and Authentication Functions of Administrative
Users
4.2 Audit
5 ROUTINE OPERATIONS
6 SECURITY OF THE TCB
7 SATISFYING THE TCSEC REQUlREMENTS
7.1 Requirements and Recommendations for Security Class C1
7.1.1 TFM Introduction
7.1.2 System Security Overview
7.1.3 Accountability
7.1.4 Routine Operations
7.1.5 Security of the TCB
7.2 Requirements and Recommendations for Security Class C2
7.2.1 TFM Introduction
7.2.2 System Security Overview
7.2.3 Security Policy
7.2.4 Accountability
7.2.4.1 Identification and Authentication
7.2.4.2 Audit
7.2.5 Routine Operations
7.2.6 Security of the TCB
7.3 Requirements and Recommendations for Security Class B1
7.3.1 TFM Introduction
7.3.2 System Security Overview
7.3.3 Security Policy
7.3.4 Accountability
7.3.4.1 Identification and Authentication
7.3.4.2 Audit
7.3.5 Routine Operations
7.3.6 Security of the TCB
7.4 Requirements and Recommendations for Security Class B2
7.4.1 Introduction
7.4.2 System Security Overview
7.4.3 Security Policy
7.4.4 Accountability
7.4.4.1 Identification and Authentication
7.4.4.2 Audit
7.4.5 Routine Operations
7.4.6 Security of the TCIB
7.5 Requirements and Recommendations for Security Class B3
7.5.1 TFM Introduction
7.5.2 System Overview
7.5.3 Security Policy
7.5.4 Accountability
7.5.4.1 Identification and Authentication
7.5.4.2 Audit
7.5.5 Routine Operations
7.5.6 Security of the TCB
7.6 Requirements of Security Class A1
GLOSSARY
REFERENCES
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NATIONAL COMPUTER SECURITY CENTER
FORT GEORGE G. MEADE, MARYLAND 20755-6000
NCSC-TG-016
Library No. S239,639
Version 1
FOREWORD
Guidelines for Writing Trusted Facility Manuals provides a set of good
practices related to the documentation of trusted facility management
functions of systems employed for processing classified and other sensitive
information. A Trusted Facility Manual (TFM) is a document written by a
system vendor that describes how to configure and install a specific secure
system, operate the system in a secure manner, and make effective use of the
system privileges and protection mechanisms to control access to
administrative functions and databases.
Guidelines for Writing Trusted Facility Manuals is the latest addition to
the "Rainbow Series" of documents. These publications are the product of the
Technical Guidelines Program. The National Computer Security Center designed
these technical guidelines to provide insight to the Trusted Computer System
Evaluation Criteria requirements and guidance for meeting each requirement.
Recommendations for revision to this guideline are encouraged and will be
reviewed by the National Computer Security Center through a formal review
process.
Patrick R. Gallagher, Jr. October 1992
Director
National Computer Security Center
ACKNOWLEDGMENTS
The National Computer Security Center wishes to extend special recognition
and acknowledgement for their contributions to this document to Infosystems
Technology, Inc., and to Dr. Virgil D. Gligor of the University of Maryland
as primary author and preparer of this document. Special thanks also go to
the many computer vendor representatives, and members of the National
Computer Security Center (NCSC) community who enthusiastically gave of their
time and technical expertise in reviewing the material and providing
valuable comments and suggestions.
Special recognition goes to Leon Neufeld, NCSC, who served as project
manager for the preparation and production of this document.
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 are prefaced by
the word "should," whereas all requirements are prefaced by the word
"shall." It is hoped that this will help to avoid any confusion.
Examples in this document are not to be construed as the only implementation
that will satisfy the TCSEC requirement. The examples and literature
citations provided herein are merely suggestions of appropriate designs and,
possibly, 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.
1 INTRODUCTION
The Department of Defense Computer Security Center (DoDCSC), established in
January 1981, expands on the work started by the DoD Security Initiative. In
1985, the DoDCSC became the National Computer Security Center (NCSC) to
reflect its responsibility for computer security throughout the Federal
Government. The Director, NCSC, has the responsibility for establishing and
publishing criteria and guidelines for all areas of computer security.
The principal goal of the NCSC is to encourage the widespread availability
of trusted computer systems. In support of that goal, the NCSC created a
metric, known as the DoD Trusted Computer System Evaluation Criteria
(TCSEC), against which computer systems could be evaluated for security. The
DoDCSC originally published the TCSEC 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 Automated
Information Systems (AIS) requires the TCSEC to be used throughout the DoD.
The TCSEC is the standard used for evaluating the effectiveness of security
controls built into Automated Data Processing (ADP) systems. The TCSEC has
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, B, and A, a number of subdivisions,
known as classes, are also ordered in a hierarchical manner to represent
different levels of assurance in these classes.
1.1 Purpose
A Trusted Facility Manual (TFM) is one of the documents necessary to satisfy
the requirements of any class in the TCSEC. The TFM is directed towards the
administrators of an installation, and its goal is to provide detailed,
accurate information on how to (1) configure and install a specific secure
system, (2) operate the system in a secure manner, (3) make effective use of
the system privileges and protection mechanisms to control access to
administrative functions and databases, and (4) avoid pitfalls and improper
use of the administrative functions that would compromise the Trusted
Computing Base (TCB) and user security.
The importance of the TFM in supporting the operation of a secure computer
system cannot be over estimated. Even if one assumes, hypothetically, that
all users of a system and their applications are trusted, and that they will
use all of the available protection mechanisms correctly, the system may
still be administered and operated in an insecure manner. This may be
especially true when administrative users lack the skill, the care, or the
interest to use the system properly. Furthermore, the security damage that
administrative users can cause by careless use, or deliberate misuse, of
administrative authority is significantly larger than that caused by
ordinary users. Although use of a detailed, accurate TFM cannot address or
counter deliberate misuse of administrative authority, it can help minimize
chances of misuse due to lack of awareness of proper system use. To help
minimize these instances of system misuse, the TFM should include examples
of both proper uses and warnings about consequences of misuse of
administrative functions, procedures, privileges, and databases.
This guideline presents the issues involved in writing TFMs. Its objectives
are (1) to provide guidance to manufacturers on how to document functions of
trusted facility management implemented by their systems and (2) recommend a
TFM structure, format, and content that would satisfy the TCSEC
requirements. The recommendations made herein should not be considered as
the-only means to satisfy the TCSEC requirements. Additionally, this
document contains suggestions and recommendations derived from the TCSEC
objectives but which are not required by TCSEC in the TFM area. For example,
the TFM may include documentation required by the TCSEC in the areas of
System Architecture, Design Documentation, and Trusted Distribution. The
inclusion of this documentation in a TFM instead of other separate documents
is optional.
1.2 Scope and Contents
The TFM should give specific guidance to administrative users on how to
configure, install, and operate a secure computer system, and should clearly
illustrate the intended use of all security features, citing actual system
commands and procedures. Although a high level of detail in illustrating key
security concepts would benefit administrative users, the TFM cannot be
considered to be, nor can it be, a training manual in the area of computer
security in general, nor in the area of system administration in particular.
Instead, the TFM user is assumed to have some familiarity with the notion of
trusted systems within the realm of computer security. The TFM will provide
the user with detailed information on how to administer and operate a
specific trusted system in a secure manner.
Many different organizations of the TFM are possible. For example, an
acceptable TFM format would provide a separate section describing specific
security responsibilities of any separate administrative roles, such as
those of the security administrator, auditor, system programmer, operator,
that are supported in the system; available commands for each role; use of
each command; parameter and default settings; specific warnings and advice
regarding the use of functions, privileges and databases of that role; and
the specific responsibilities of that role for TCB security. Use of this
format is advisable for manuals of systems in higher security classes,
namely B2, B3, and A1, where separation of administrative roles is required.
An equally acceptable TFM organization and section format would provide a
separate section for each functional requirement area of the TCSEC, namely,
for security policy (e.g., Discretionary Access Control (DAC), Mandatory
Access Control, (MAC)), accountability, and TCB protection. Each section
would include available commands, system calls, and procedures relevant to
that area; use of each command (including the effects of each command when
used by different administrative roles); parameter and default settings; and
specific warnings and advice regarding the use of functions, privileges, and
databases available to commands of that area. Use of this alternate format
is advisable for lower security classes, namely C1-B1, where the TCSEC does
not mandate any separation of administrative roles. Either of the two
alternate TFM formats mentioned above is equally acceptable for all TCSEC
security classes as long as the TFM satisfies the TCSEC requirements
Furthermore, other TFM formats would also be acceptable as long as they
satisfy the stated TCSEC requirements. The TCSEC neither requires nor
suggests a specific TFM format.
This guideline contains eight additional sections. Section 2 defines the
security and accountability policies and mechanisms of systems. Section 3
identifies and explains the security-relevant and security-irrelevant
functions of an administrator. Section 4 identifies and explains the use of
TCB commands and interfaces used by administrative users. Section 5 defines
day-to-day routine operations performed by administrative users and the
security vulnerabilities of these operations. Section 6 identifies all TCB
security and integrity responsibilities of administrative users.
Section 7 presents recommendations for writing the TFM that satisfy the
requirements of the TCSEC. Section 8 is a glossary. Section 9 lists the
references cited in the text. Each section consists of three parts: a
statement of purpose, an explanation of how that purpose can be achieved,
and an outline summarizing the recommendations made.
These guidelines apply to computer systems and products built or modified
with the intention of satisfying the TCSEC requirements.
1.3 Control Objectives
The control objectives for the TFM are similar to those of other
documentation areas of the TCSEC. They refer to what should be documented in
a particular area, such as the trusted facility management, and how this
documentation should be structured. Thus, the control objectives for writing
the TFM are:
(1) the TFM shall address all the requirements specified by the TCSEC
that are relevant to it; and
(2) the TFM shall provide detailed, accurate information on how to:
- configure and install a specific secure system;
- operate a system in a secure manner;
- avoid pitfalls and improper use of administrative functions that
would compromise system and user security.
1.4 TFM Introduction
The purpose of this section in the TFM is to explain the scope, use, and
contents of the TFM of a particular system. In general, the scope of the TFM
should include explanations of how to configure and maintain secure systems,
administer and operate them in a secure manner, make effective use of the
system's privileges and protection mechanisms for administrative use, and
avoid pitfalls and misuse of administrative authority. Depending on the
particular computer system, the complexity of trusted facility management
may differ and thus the scope of the TFM may differ accordingly. For
example, in large systems, system configuration and installation is a
complex activity described in a separate system administration manual that
may, or may not, include the other important areas of the TFM. In contrast,
system configuration and installation is a relatively simple activity
defined in a single chapter of a TFM for a small system, such as a
multi-user workstation.
The introduction to the TFM should also discuss the recommended use of the
manual. In particular, this section should define the skills and general
computer systems and security background assumed for administrative
personnel. This is necessary because different administrative functions
require different levels of skill. For example, an individual in the system
programming staff that configures, installs, and maintains the TCB code
often needs considerably more technical skills than an individual in the
accounts management staff. Similarly, a security administrator needs more
detailed knowledge of the system security policy and accountability than
individuals assigned to operator's roles. The definition of required skills
and background is important in aiding the management of a particular
organization in assigning appropriately trained individuals to various
administrative tasks.
In defining the use of the TFM, the introductory section should also include
a list of other system manuals that may be consulted by the administrative
staff. For example, most administrators may benefit from an understanding of
the Security Features User's Guide (SFUG). Most system designs use the DAC
mechanisms described in the SFUG for protection of, at least, some
administrative files, and may use the trusted path mechanism to prevent
spoofing of administrative commands. Similarly, whenever manual sections
that logically belong in the TFM are in fact provided in other manuals ---
system configuration and installation manuals, and system reference manuals
containing descriptive top-level specifications (DTLSs) of commands and
interfaces used by administrative users---the TFM Introduction should
include references to these additional manuals. The TFM should place the
references to these manuals in context and provide a brief synopsis of the
relevant information from the specific manual citation. This citation would
help narrow the reader's focus to a few pages of the referenced manual.
Furthermore, references to documents, manuals, and standards that may be
beneficial to some administrative personnel, such as password management and
use guidelines and standards, should be made in this section. References to
educational and training documents that are helpful to administrative
personnel may also be included here.
The TFM writer may also want to define the limitations of the TFM in terms
of security scope. For example, some security issues such as personnel
background verification, assignment and maintenance of users' trust levels,
physical system and environmental security, proper use of cryptographic
techniques and devices, and procedures that assign individuals to
administrative roles, generally fall outside the scope of TFM definition.
Explicit recognition of such limitations enables the management of a secure
facility to plan countermeasures for areas of vulnerability not countered by
the trusted systems.
Finally, the introductory section of the TFM should include a "road map"
defining the contents of each TFM section and possibly the relationships
between various manual sections. This road map may also identify the
self-contained sections of the manual that can be read independently of
other sections.
In summary, the introductory section of the TFM should include:
(1) Scope of the manual
- guide the configuration and installation of secure systems;
- guide the operation of a system in a secure manner;
- enable administrative personnel to make effective use of the
system's privileges and protection mechanisms;
-issue warnings about possible misuse of administrative authority.
(2) Recommended use of the manual
- review skills and systems background necessary for
administrative personnel;
- suggest additional manuals, reference material, and standard,
needed by administrative personnel;
- specify the limitations of security scope;
(3) TFM contents
-contents of each section;
- section relationships.
2 SYSTEM SECURITY OVERVIEW
The purpose of this section of the TFM is to define the security and
accountability policies and mechanisms of the system that are designed to
counter a set of perceived threats. The focus of this section should be on
the administrative-user functions available to counter threats, the
privileges and protection mechanisms available to administrative users, and
the general vulnerabilities associated with actions of administrative users.
This section should also include a list of dependencies on other security
measures, such as those for the maintenance of physical security, which,
although not required by the TCSEC, should be taken into account by the
management of the system installation and by system accreditors.
2.1 Threats
Examples of the general security threat handled by systems built to satisfy
a TCSEC class is that of unauthorized disclosure of information through
either unauthorized direct or indirect access to system and user objects
through system failures, subversion, and TCB tampering or through use of
covert channels. The manual should describe some of the common attacks that
cause unauthorized disclosure of information, in the context of the specific
system. These examples might include the use of Trojan horses in untrusted
shared programs, the use of covert channels by untrusted users and
applications, the use of known penetration methods that cause unauthorized
disclosure of sensitive or proprietary information, and the misuse of access
authorization to retrieve and disclose sensitive information (e.g., insider
attacks).
2.2 Countermeasures Based on Security and Accountability Policies and
Procedures
This section of the TFM should include a brief discussion of the protection
mechanisms available in the system that help counter the threats defined in
the above section. This discussion should serve as a summary of the
protection philosophy used in the design and implementation of the
protection mechanisms and should include a presentation of the role of
security policy (both discretionary and mandatory policy, if any),
accountability, and assurance (both operational and life-cycle assurance).
The dependency of the system security mechanisms on administrative-user
actions should be emphasized here.
This section should point out clearly the types of threats that can, or
cannot, be countered by a specific policy or mechanism. For example, this
section should state that DAC mechanisms cannot, and are not meant to,
prevent or contain threats posed by Trojan horses implementing time bombs,
trap doors, or viruses placed in shared, untrusted applications [2]. DAC
mechanisms cannot, nor are they meant to, detect or prevent access performed
by an authorized subject on behalf of an unauthorized subject (e.g., the
surrogate access problem [3]). Furthermore, DAC mechanisms are not, nor were
they ever claimed to be, capable of controlling information (as opposed to
access privilege) flows. Only MAC can handle these problems.
This section should discuss, in the context of the specific system, the role
of specific accountability mechanisms and policies in countering security
threats not handled by access control mechanisms. An example is the use of
audit mechanisms to complement access control mechanisms in the sense that
they can detect attacks initiated by authorized users (i.e., by "insiders"),
or that trusted-path mechanisms are required to prevent spoofing, a threat
not usually countered by access control mechanisms or policies.
The emphasis in describing the above-mentioned threats and countermeasures
should be on the identification of the TCB mechanisms and policies that
counter a specific threat. For example, the summary of the countermeasures
supported by the system should include the basic assertion (and in other
design documents, the justification) that the TCB itself is
non-circumventable and tamper proof. Additional points of emphasis may be
that all countermeasures supported in the system require the interaction of
both access control and accountability mechanisms, and that these mechanisms
should be employed by both ordinary and administrative users. This section
should provide examples of interaction between ordinary and administrative
user decisions to illustrate both the positive and negative consequences of
such interaction.
2.3 Explicit Physical Security Assumptions
The TCSEC does not include requirements for physical security of the system
installation. However, the TFM should include a section or a subsection that
states the physical security assumptions made by the system designers. These
assumptions should be satisfied by the management of the organization
responsible for deploying the system, as the evaluation of physical security
is the responsibility of the system's accreditors.
The explicit inclusion of the physical security assumptions made by
designers in the TFM will provide the accreditors with the necessary input
for the deployment of the system in different operational environments and
provide the administrative users an important input for the sound definition
of the system security profile. For example, systems that do not provide
trusted paths for administrative users usually assume that a set of terminal
ports is reserved for the connection of administrative consoles that are
physically separated from the rest of the user terminals for the entire
lifetime of the system. Also, a common assumption is that the system
definition of the security profile ensures that the level of trust
associated with the physical environment containing a system's peripheral
will always dominate the maximum sensitivity associated with that
peripheral. Similarly, this section should emphasize that systems allowing
legitimate users to access their components (e.g., removable media) should
be used only in environments where both administrative and ordinary users
are trusted to access all data in the system and are trusted not to misuse
their physical access permissions. (In such environments, the use of
untrusted applications may still require the use of trusted systems even
though all users are trusted to access all data.) In systems that do not
allow users to access the system components, or when the above level of user
trust cannot be guaranteed, the TFM should suggest the physical controls
necessary to counter, or deter, the potential threat of physical access to
system components. The presentation of the physical security assumptions
made by system designers should enable accreditors to determine the security
risks and exposures assumed by system use as well as the required
countermeasures.
2.4 Protection Mechanisms Available to Administrative Users
The security of any system depends directly on the security of the
administrative commands, interfaces, and databases. For this reason,
administrative commands, privileges, and databases shall be protected from
ordinary users, and in some TCSEC security classes, shall be separated on a
role basis. This section should identify the protection mechanisms available
to administrative users to ensure that these users are aware of the means
available to control access to their commands, privileges, and databases.
All protection mechanisms that can be manipulated by ordinary users are also
usually available to administrative users. For example, all user
identification and authentication, and DAC mechanisms are available to
administrative users. In addition to mentioning these mechanisms, which the
SFUG already defines, this TFM section should include the description of the
mechanisms available only to the administrative users and the mode of their
safe use. For example, the use of special trusted-path mechanisms based on
physically protected, hard-wired consoles, which may allow the invocation of
command processors available only to administrative users, and the use of
audit mechanisms to detect potential intrusion by authorized users, are only
a few of the protection mechanisms specific to administrative users [7].
2.5 Security Vulnerabilities and Warnings
This section should describe the security vulnerabilities of administrative
commands and procedures, and should suggest specific ways to counter them.
Reference [7] cites generic examples of common vulnerabilities of
administrative roles and role-specific vulnerabilities. In addition to
similar examples, this TFM section should include a discussion of
system-specific vulnerabilities and countermeasures required in the assumed
environments of system use.
In any system, design and implementation assumptions are made about
administrative actions and their sequence of use. For example, the loading
of a system during the installation phase, and the installation itself, may
require the use of special administrative commands in a specific sequence.
The definition of a user security profile may require that administrators do
not reuse user and group identifiers, and that the definition of the system
security profile prohibits the reuse of bit encodings of sensitivity levels
without careful analysis of consequences. Other potential vulnerabilities,
such as those resulting from mismanagement of audit logs and post processing
of files (in on-line, off-line, and hard-copy form) should also be explained
here. Design and implementation assumptions should be stated explicitly in
this section to ensure that administrative users are aware of the negative
consequences of not satisfying these assumptions.
2.6 Separation of Administrative Roles
Security classes B2-A1 of the TCSEC require that the roles of the
administrative users be separated. This requirement means that the commands,
procedures, privileges, and databases of the various administrative roles
shall be separated by system design and shall be documented as such. Role
separation of classes B3 and A1 also requires the separation of
security-relevant functions from the security irrelevant ones. Reference [7]
cites the rationale and the means of achieving role separation in trusted
systems.
The TFM shall define each separate role supported by the system. Each role
should be clearly defined in terms of the commands and TCB interfaces
available to the role, the use of each command, the command effects and
exceptions (whenever these are not defined in the DTLS of the TCB),
parameter and default settings, specific warnings for the command use, and
advice. The TFM should also define the specific security mechanisms used to
protect privileged commands and data used by administrators.
In summary, the TFM section presenting the system security overview for
administrative users should include the following subsections:
2.1 Threats to System Security
2.2 Countermeasures Based on Security Policy and Accountability
2.3 Explicit Physical Security Assumptions
2.4 Protection Mechanisms Available to Administrative Users
2.5 Security Vulnerabilities of Administrative Users and Warnings
2.6 Separation of Administrative Roles (for classes B2-A1)
3 SECURITY POLICY
The purpose of this section is to identify and explain the security-relevant
and security-irrelevant functions of the administrators. In particular, this
section should explain, in the area of security-relevant functions, the use
of the TCB commands and interfaces by administrative users to initialize
discretionary access privileges, to set default user accesses to system
objects after user registration, and to distribute, review, and revoke
access privileges on behalf of users in systems that implement DAC in a
centralized way (2]. In systems that support MAC, this section also
identifies and explains the use of TCB commands and interfaces by
administrators to define and change the system security profile (e.g., the
system-sensitivity map, sensitivity level limits for system devices, and
file systems), to define and change object sensitivity levels (e.g., label
imported, unlabeled data, and media), and to change the trust level of
active subjects, whenever such a function is supported. This section also
should define the administrator's interfaces for other functions related to
the support of DAC and MAC, such as changing object ownership, restoring
privileges deleted accidentally, destroying errant processes, running
consistency checks on system and user security profiles, and managing user
accounts.
Reference [7] outlines the role of the security administrators in support of
the security policy defined in a system. The TFM should specify the
commands, system calls, functions, their parameters and default settings
provided for each area of security policy and support, and should provide
examples of use, potential misuse, and security implications of command
misuse. For example, the TFM should explain how the administrator can change
the sensitivity label of an object or a subject, and cite the expected
security consequences Of such action and also how the administrator may
determine the consequences of such a change in the given system. Similarly,
the administrator may decide to reuse a binary representation of a
sensitivity level to define a new sensitivity level. For this process, the
manual shall state the circumstances in which this change is allowed, if
ever, and should explain the conditions under which this change is safe. All
commands, system calls, and functions should be defined in terms of their
effects, exceptions, and parameters. The use of commands should be
illustrated by examples showing the correct settings of various command
options. This section should describe the recommended reactions of the
administrator to such exceptions (unless these reactions are already
described in the call/command DTLS).
The administrative functions and interfaces used in supporting the security
policy have potential vulnerabilities. Reference [7] outlines some of these
generic vulnerabilities. The TFM shall include warnings of all known
specific vulnerabilities in the given system and possibly suggest means of
reducing system risk associated with such vulnerabilities. Minimally, the
TFM should specify the dependencies of the administrative roles on external
policies and procedures that would help reduce system risk associated with
identified vulnerabilities.
In summary, the security policy section of the TFM should include the
following subsections (whose contents are discussed in more detail in
reference [7]):
3.1 Discretionary Access Control
-TCB commands and interfaces used to initialize DAC privileges and
defaults;
-TCB command interfaces to distribute, review, and revoke user
privileges in systems that support centralized DAC;
-group membership definition and impact on DAC.
-change of object ownership (if any), restoration of accidentally
deleted privileges, destruction of errant processes;
3.2 Mandatory Access Control
- TCB commands and interfaces to define and change system security
profile; classify, reclassify and import objects; and change trust
level of active subjects;
- consistency checking of system security and user profiles.
3.3 Management of User Accounts
-definition and deletion of user and group accounts and
identifiers.
3.4 Command System Call and Function Definitions
- effects and exceptions (if not defined in DTLSs);
- parameter and default settings;
- examples of command use and potential misuse.
3.5 Warnings of Specific Vulnerabilities of Administrative Procedures
and Activities Related to Security Policy.
4 ACCOUNTABILITY
4.1 Identification and Authentication Functions of Administrative Users
The purpose of this section is to identify and explain the use of TCB
commands and interfaces that should be used by administrative users to set
up user security profiles, and to determine authentication and authorization
parameters associated with the user identification and authentication
mechanism. Reference [7] defines the role of the security administrator in
the identification and authentication area. The TFM shall specify the
commands, system calls and functions, and their parameters and default
settings that are provided by the specific system, and should provide
examples of the use, or potential misuse of these commands, and the security
implications of command misuse. For example, the TFM should explain how the
administrator can initialize user passwords, can distribute special
passwords to other administrative users, and set up account restrictions
(e.g., restricted time intervals for login, account cutoff). The commands
that allow the definition of user and group identifiers shall include an
explanation of how these identifiers should be chosen, why they should not
be reused, and what the consequences of identifier reuse are.
In most systems, the setting of the user security profile also includes the
definition of some discretionary privileges associated with the user
account. For example, in systems that use groups to enforce DAC policies,
administrators define the group membership. The TFM shall explain the
consequences of adding or deleting a user identity to a group in terms of
the added or lost discretionary privileges, and provide appropriate
warnings. In systems where the user security profile also includes the
specification of the maximum level of trust for each user, the TFM shall
also discuss the security implications of incorrect definition or change of
these levels and the interactions between these levels and the sensitivity
levels of various system components (defined in the system security
profile). It should also include examples of and warnings about such
changes. The commands available to system administrators also include those
to define and change the parameters of the login/logout mechanism used by a
system. Consequently, the TFM should explain how to define these parameters,
which include the time-out period, multiple login attributes, maximum Iogin
time, and limits on unsuccessful logins from a terminal or into an account
[7] (e.g., specific commands, command options, formats, parameter ranges,
and default values). Whenever the trusted path mechanisms available to
administrative users require special procedures, such as use of specific
hard-wired consoles, the TFM shall specify how the administrative users can
use the trusted path mechanism in a secure manner.
The TFM shall also explain the implications of the system security profile
definition in providing authorization data for user log ins. For example, a
terminal's maximum and minimum sensitivity levels provide cutoff values for
whether a certain user login level can be used and whether a certain user
with a given user and group level clearance can log in at all from a given
terminal. The relationship between the terminals minimum and maximum
sensitivity levels and the user's clearance level shall be explained so that
consistent levels can be defined for both terminal sensitivity and user
level of trust.
Finally, administrator commands for temporarily terminating a user access to
the system and for permanently deleting the user account shall be defined,
and the implications of such actions defined. This section should also
include warnings about potential vulnerabilities, such as object ownership
set to the identity of an user or account that is no longer valid, or the
reuse of an old identifier, that persist when a user account is not deleted
correctly or completely, and examples of such vulnerabilities [7].
For all administrative commands defined in this and other system security
areas, this TFM section should include an explanation of all exceptions and,
possibly, a administrator's recommended response to these exceptions. (This
reaction may already be described in the system call/command DTLS). All
administrative data bases that are accessed by these commands should be
identified showing how they are, or can be, protected. All mechanisms
available for the protection of the identification and authentication data
shall be clearly explained. The use of these mechanisms should be
illustrated by examples.
4.2 Audit
The purpose of this section of the TFM is to familiarize administrative
users with the TCB commands and interfaces of the system's audit mechanism.
These commands include those that enable or disable the audit selectivity
mechanism (e.g., audit-event setup and change), those that help manage the
audit trails (logs), those that perform data compression and post processing
analysis, and in classes B2---A1, those that set correct channel delays and
randomize variables.
Some system includes a set of audit events that should always be selected
for audit to ensure the consistency of subsequent events selected by the
auditor and the proper functioning of the post processing tools. These
events should be explicitly highlighted for special discussion in the list
of auditable events supported by the system. The complete list of events
shall be defined in the TFM. The audit selection mechanism should also be
presented, and examples of use should be provided. Commands of the audit
selectivity mechanism include those that turn on and off events on a
per-user, per-process, per-terminal, per-sensitivity-level, or per-object
basis. In TCSEC classes B3 and A1, the commands that turn on and off events
representing accumulations of other auditable events and audit-system alarms
(if any) shall also be presented.
Systems that support audit mechanisms include commands that help manage the
audit files. These commands, which include those to create new and destroy
old audit logs, to change audit log size and warning points, to display,
format, and compress audit data, and to check the consistency of the audit
database after crashes, and when these changes take effect, shall also be
included in the TFM. The procedures that shall be used by auditors to ensure
that the audit files do not overflow shall also be presented. The format in
the audit log file of each record field and of each type of auditable event
shall be presented and explained. Commands for post processing of audit logs
(if any) shall also be included in the TFM. Systems designed to satisfy the
B2---A1 security requirements need to have covert channels restricted to
certain limits. One means of reducing covert channel bandwidths is by
placement of delays and by setting of randomization variables in system
kernels and trusted processes. Commands that accomplish this task should be
presented in the TFM of these systems along with a description of the covert
channel handling policy recommended for enforcement. These recommendations
should be derived from the covert-channel analysis guideline of the TCSEC
and are important because they affect not only the security policy and the
accountability areas of the system, but also system performance. Reference
[7] defines the administrative functions necessary to support audit
activities. As suggested in the covert channel guidelines of the TCSEC,
bandwidth reduction policy should be coordinated with audit policy. For this
reason, the TFM should present the bandwidth reduction policy in the same
section with that presenting the audit policy.
Recommendations on audit procedures should also be included in the TFM.
These procedures would suggest auditing groups of specific events that may
reveal misuse of access privileges, potential system-penetration attacks,
and covert channel usage. They may also suggest the frequency of audit
review and provide advice on how to manage audit files on-line and off-line.
For commands used by administrative users for audit, the TFM should include
a description of their effects and exceptions, and should provide examples
of use, potential misuse, and security implications of command misuses.
Recommendations for administrator's reactions to command exceptions should
also be made. Reference [7] provides examples of vulnerabilities caused by
misuse of audit command and authority. These examples include loss of audit
log consistency, loss of audit logs, loss of user privacy, and various forms
of denial of service. Specific instances of vulnerability in a given system
and possible suggestions for reducing the system's exposure to such
vulnerabilities should also be included in the audit section of the TFM.
In summary, the accountability section of the TFM should include the
following subsections:
4.1 Identification and Authentication
- TCB commands and interfaces for setting up user security
profiles and authentication and authorization parameters of the
login mechanism;
- password distribution to ordinary and administrative users,
management of password generation, and protection of passwords;
- account restrictions (e.g., restricted time intervals for log
in, and account cutoffs);
- choice of user and group identifiers;
- maximum levels of trust for users and groups;
- computation of the current level of trust for subjects (e.g.,
subject's clearance).
4.2 Definition and Change of System Parameters of the Log in Mechanism
and when they take effect
- timeout interval;
- multiple login attributes;
- maximum login time;
- limits on unsuccessful logins from a terminal or to an account;
- use of special trusted path mechanisms for administrative users.
4.3 Audit Mechanisms
- audit-event selection mechanisms (e.g., audit-event setup and
change);
- management of audit logs (e.g., protections of audit logs);
- functions for formatting, compressioning, and postprocessing of
audit files;
- interfaces for setting of covert channel delays and
randomization of variables;
- description of audit log and event formats.
4.4 Commands, System Calls and Function Definition
- effects and exceptions of each command of the accountability
area (if not defined in DTLSs);
- parameter and default settings;
- examples of use and potential misuse.
4.5 Warnings of Specific Security Vulnerabilities of Administrative
Activities and Procedures Related to Identification, Authentication,
Trusted Path and Audit
5 ROUTINE OPERATIONS
The purpose of this section of the TFM is to define the routine operations
performed by administrative users, describe the operation's security,
describe the vulnerabilites associated with these operations, and provide
appropriate warnings. These operations are carried out, in most cases, by
execution of appropriate commands from a system console. However, in some
instances, these operations involve manipulation of physical devices, such
as printers, storage devices, removable media, communication switches, and
modems. For this reason, this TFM section may differ from the rest of the
TFM. It should contain not only definitions of specific commands and TCB
interfaces, but also procedures and policies for secure use and manipulation
of hardware devices.
Routine operations of administrative personnel include both
security-relevant and security-irrelevant operations. Security-relevant
functions include those that boot and shut down the system, set system
clocks, identify damaged user volumes and files, perform TCB backups and
on-line device tests, run system integrity tests, and respond to user
requests to mount/unmount volumes. Routine security- irrelevant operations
include those that perform system metering, and that require operator
response to various user requests [7]
This section the TFM should include a description of each command used for
routine operations, including its effects and exceptions, and should provide
examples of use, potential misuse, and security implications of command
misuse. Examples of vulnerabilities of security-relevant, routine operations
include the booting of an old version of the TCB, causing inconsistency
problems for users; system shutdown while still in normal operation causing
loss of files and file system inconsistencies; and inadequate use of devices
and device interfaces (e.g., printers).
This section the TFM should also include descriptions of administrative
commands that perform security-irrelevant routine operations. These commands
include those traditionally performed by account administrators, such as
commands used for maintenance of accounting files, for turning on and off
accounting, for running accounting tools, for collecting statistics of
system and resource usage, and billing information.
Administrative policies and procedures that define security-relevant
handling of devices shall also be included in the TFM. For example,
procedures to install, activate, and set the current sensitivity level of a
printer within the pre-defined range should be defined, and examples of the
installation procedure should be given. In summary, the TFM section defining
the routine administrative operations and procedure in the following
subsections:
5.1 Security-Relevant Procedures and Operations
- running of system diagnostics;
- system boot and shutdown;
- setting of system clocks;
- identification of damaged user files and volumes;
- routine backup of TCB files;
- on-line device testing;
- response to user requests to mount/unmount volumes;
- handling of peripheral devices, removable storage, and output
(e.g., printers, printer output, diskpacks, tape reels).
5.2 Security-Irrelevant Procedures and Operations
- back-up of user volumes;
- system metering;
- response to various user requests;
- user account administration;
5.3 Commands, System Calls and Function Definitions
- effects and exceptions of each command of the routine operations
area
(unless defined in the DTLSs);
- parameter and default settings;
- examples of use and potential misuse.
5.4 Warning of Specific Security Vulnerabilities of Routine Operations
6 SECURITY OF THE TCB
The two purposes of this TFM section are to identify and explain all aspects
of TCB security and integrity that become the responsibility of
administrative users. Because the security of all user programs, data, and
application subsystems is provided by the TCB, the maintenance of TCB
security and integrity is one of the most sensitive administrative
functions.
Maintenance of TCB security spans the entire system life cycle. It includes
procedures for strict configuration management during system development and
use, and for secure system distribution, installation, and local
maintenance. In some cases, administrative users are allowed and required to
generate another evaluated version of the TCB from source code (e.g., make
changes to the TCB source code and regenerate the TCB on site). In such
cases, the TFM shall include detailed descriptions of procedures that
generate a new TCB version from source code, the necessary system commands,
the list of approved tools (e.g., compilers, linkers, loaders) for TCB
generation, examples of command use, warnings of possible problems in
generating a new TCB, vulnerabilities that may affect TCB security, and
configuration management.
The TFM shall also provide, or reference a separate document that provides,
a description of command exceptions, appropriate warnings, and possible
exception handling advice. The TFM should also provide, or reference a
separate document that describes, the configuration management tools. The
TFM shall include descriptions of the procedures that must be followed by
site administrators to install new releases of the TCB.
TCB security may be violated during installation and maintenance (see [7]).
For this reason, the TFM shall provide a description of the TCB installation
procedures, including the required commands, exceptions, parameter settings,
required system configuration, warnings, and advice. The installation
procedures should contain descriptions of the TCB data structures that must
be initialized by the user, and of the TCB loading. Also, the installation
procedures should include a list of tools (e.g., editors, loaders) approved
for TCB installation and an appropriate description of secure installation
assumptions (e.g., administrative procedures, such as those that require
physical audit of the installation procedure by independent personnel).
All TCB maintenance procedures shall be defined in the TFM. These procedures
should include analyzing system "dumps" after crashes, conducting
crash-recovery and restart actions, performing consistency checking of TCB
files and directories, changing system configuration parameters (e.g., table
sizes, devices, and device drivers), running periodic system integrity
checks, and repairing damaged labels. A list of the approved tools for TCB
maintenance, relevant commands, exceptions, warnings, and advice should also
be included in this section.
The ability to install and maintain a system's TCB in a secure manner
requires that administrative users be cognizant of all TCB modules.
Administrators should especially be cognizant of those hardware modules
containing the reference monitor mechanism, and of all the of default file
protections for TCB files or objects. If available, the command needed to
run a tool that checks the correct privilege and sensitivity-level
initialization for TCB files or objects shall be identified and its use
illustrated. Thus, either the TFM itself shall provide a list of all TCB
modules, including their interfaces, and shall specify the TCB file or
object privileges necessary to protect the TCB or the TFM shall list a
separate document that does.
The TFM shall include warnings and advice on how to handle both generic and
system-specific vulnerabilities (if any) of TCB installation and
maintenance. For example, administrative users should be warned that
interchanges of dedicated- console and user-terminal communication lines can
`cause potential loss of trusted path for administrative users, that
placement of extraneous code in the TCB configuration may result from using
an unapproved tool, and that running a borrowed untrusted program under
administrative identity may cause an untold number of TCB security problems
[7].
Finally, the TFM shall include a description of policies and procedures that
define the distribution procedures for a trusted system (i.e., a class A1
requirement). These policies and procedures shall be used to maintain the
integrity of the mapping between the master copy defining the current
version of the TCB and the on-site installed copy.
In summary, the TFM section that defines the security measures necessary for
protection of the TCB should include the following subsections:
6.1 The Generation of the TCB Source Code
- list of TCB code modules, module interface and data (including
modules of the reference monitor);
- list of approved tools for TCB generation
- procedures for TCB generation;
- vulnerabilities.
6.2 Configuration Management Policy (if required, reference to a
separate configuration management document)
6.3 Ratings-maintenance Plan (if applicable, reference to a separate
rating maintenance document)
6.4 TCB Installation Procedure
- TCB generation from source code (whenever allowed by the system
manufacturer);
- TCB hardware installation;
- TCB data structure initialization;
- TCB loading;
- setting of TCB file protection;
- list of approved tools.
6.5 TCB Maintenance Procedures
- analysis of system dumps;
- crash recovery and restart;
- changes of configuration parameters;
- repair of damaged TCB data structures;
- consistency-checking procedures;
- running of periodic system-integrity checks
6.6 Trusted Distribution of the TCB
- policies and procedures;
- correspondence between master copy and installed copy
6.7 Commands, System Calls, and Function Definitions for TCB Generation
from Source Code, Installation, Maintenance, and Trusted Distribution
- effects and exceptions (unless defined in DTLSs);
- parameter and default settings;
- examples of use and potential misuse.
6.8 Warnings of Specific Security Vulnerabilities of TCB Generation,
Installation, Maintenance, and Distribution
7 SATISFYING THE TCSEC REQUlREMENTS
This section of the TFM should contain the definition of the TFM
requirements on a TCSEC class basis. All of the requirements listed below
derive from corresponding documentation requirements and objectives of the
TCSEC. Although similar TFM requirements appear in multiple classes, the
contents of TFM sections shall reflect the complexity of policy,
accountability, assurance, and documentation of the evaluation class.
Consequently, this section should contain suggestions and recommendations
that may not be found in the TFM requirements area but that derive from
other TCSEC areas. These suggestions and recommendations illustrate the
added complexity of various TCSEC classes.
7.1 Requirements and Recommendations for Security Class C1
The TFM of a C1 class system may have the following structure:
7.1.1 TFM Introduction
The TFM introduction may include the following topics:
Scope of the TFM
- guide to configure and install secure systems;
- guide to operate a system in a secure manner;
- enable administrative personnel to make effective use of the
system's privileges and protection mechanisms;
- issue warnings about possible misuse of administrative
authority.
Recommended use of the TFM
- review skills and systems background necessary for
administrative personnel, suggest additional manuals, reference
material, and standards needed by administrative personnel;
- specify the limitations of security scope;
Contents of the TFM
- contents of each section;
- section relationships.
(For specific DAC requirements, the reader should refer to [2].)
7.1.2 System Security Overview
This section of the TFM shall include a brief description of the system
administration vulnerabilites specific to the given system, warnings, and
advice on how to counter these vulnerabilities.
"A manual addressed to the ADP administration shall present cautions
about the function and privileges that should be controlled when
runninga sucure facility [6]."
The above TCSEC requirement sugguest that the administrative functions and
privileges that need to be controlled when running a sucure facility shall
be identifies, and the vulnerabilities associated with those functgions and
privileges shall be determined. Warnings relatiing to thes vulnerabilites
shall be presented.
The administrative functions and privileges that need to be controlled when
running a class C1 secure facility include those supporting security olicy
(i.e., DAC), accountability (i.e., identification and authentication), and
operational assurance (i.e., system integrity).
Security Policy
This section of the TFM shall include descriptions of the TCB commands,
interfaces, and procedures to:
- initialize discretionary access privileges and defaults for
individual users and groups;
- distribute, review, and revoke privileges on an individual user or
group basis;
- change object ownership (if any), restore accidentally deleted
privileges, and kill errant processes;
- define and change group membership (whenever groups are supported),
and explain the effect of such action on DAC;
- explain the implications of creating and deleting user and group
accounts on DAC.
7.1.3 Accountability
Identification and Authentication
This section of the TFM shall include descriptions of the TCB commands,
interfaces and procedures to perform the following functions:
- conduct setup of user/group security profiles, and authentication and
authorization parameters of the login mechanism;
- conduct password management distribution to ordinary and
administrative users or groups (see [4]);
- define account restrictions (e.g., time intervals for login, account
cutoff time).
This section shall also include descriptions of the definition and change of
log in mechanism parameters. These parameters include:
- types of terminals supported and terminal; interface initialization;
- time---out interval;
- multiple log in attributes (if supported);
- maximum login time;
- limits on unsuccessful logins from a terminal or to an account.
7.1.4 Routine Operations
Although the TCSEC does not cite specific requirements in this area, the TFM
should include commands and procedures for the following activities:
- perform system boot and shut down;
- set system clocks;
- conduct on-line device testing;
- perform backup of user volumes;
- perform system metering;
- response to various user requests.
7.1.5 Security of the TCB
This section of the TFM shall include descriptions of the TCB command
procedures that are provided "to validate periodically the correct operation
of the on-site hardware and firmware elements of the TCB."[6]
In all areas of the TFM, and for all security classes where TCB commands and
interface descriptions are required, the TFM shall include:
- effects and exceptions of each command (if not already defined in the
DTLS);
- parameter and default setting;
- examples of potential use and misuse.
- In all areas of the TFM, and for all security classes, warnings
(i.e., cautions) shall be provided for specific security
vulnerabilities of the relevant administrative commands, interfaces,
and procedures. Any modification to the TCB, for all security classes,
may invalidate the systems rating [5].
7.2 Requirements and Recommendations for Security Class C2
Security class C2 includes all the TFM requirements of security class Cl. In
addition, the following documentation requirements are added.
7.2.1 TFM Introduction
No Additional Requirements/Recommendations (NAR)
7.2.2 System Security Overview
The first design documentation requirement of TCSEC is that:
"Documentation shall be available that provides a description of the
manufacturer's philosophy of protection and an explanation of how this
philosophy is translated into the TCB."[6]
The above requirement suggests that the system security overview section
should include an additional subsection on security philosophy. This section
should contain a discussion of the security threats that could be countered
by the use of this system, and of specific countermeasures based on security
policy and accountability.
7.2.3 Security Policy
(NAR)
7.2.4 Accountability
The second documentation requirement is: "The procedures for examining and
maintaining the audit files as well as the detailed audit record structure
for each type of audit event shall be given [6]. This requirements implies
that the following sections should be added to the accountability area:
7.2.4.1 Identification and Authentication
(NAR)
7.2.4.2 Audit
The TFM should include a section describing the audit mechanisms, TCB
commands, interfaces, and procedures for the following activities:
- determine audit selection mechanisms; these mechanisms include the
commands and procedures necessary to display all security-relevant
auditable events, to select the required and the optional audit events,
and to turn on and off events selectively on a per-user and per-process
basis;
- conduct audit log management; this activity includes commands and
procedures to create, save, and destroy saved audit logs; to change
audit log size and warning point for audit log overflow; to format,
compress and display audit logs;
- protect audit commands and databases;
- ensure maintenance of audit consistency;
- perform post processing of audit data; this is an optional
feature of a system and of the TFM, and includes mostly
application-specific commands and procedures for intrusion
detection. (However, all of these commands and procedures, and
also the available tools and their protection from unauthorized
user access, should be described whenever they are provided);
The audit section of the TFM shall include a detailed description
of the audit record structure for each type of audit event and of
the entire audit log. (For specific details of audit requirements,
the reader should refer to [1]).
7.2.5 Routine Operations
(NAR)
7.2.6 Security of the TCB
Additional requirement that is relevant to TCB protection is included here.
7.3 Requirements and Recommendations for Security Class B1
All TFM requirements of a class C2 system are included here. The
documentation requirements of class B1 suggest significant additions to the
TFM contents beyond those implied by the TCSEC requirements of security
policy and accountability.
The TFM of a class B1 system should include the following additional
documentation:
7.3.1 TFM Introduction
(NAR)
7.3.2 System Security Overview
This section should include any additional requirement referring to the
system security overview. That is, this section of the TFM "shall provide
guidelines on the consistent and effective use of the protection features of
the system; [and] how they interact." [6] This suggests that the TFM should
include a discussion of the interaction between the protection mechanisms
and functions available to administrative users and those available to
ordinary users. As mentioned in Section 2 above, this interaction is
particularly important in the areas of security policy and accountability.
7.3.3 Security Policy
The additional security policy requirements of MAC and labeling suggest that
additional administrative responsibilities should be documented in the TFM.
The TFM requirement that the "manual shall describe the operator and
administrator functions related to security,"[6] suggests that the TFM
should include a description of all TCB commands, interfaces and procedures
to perform the following functions:
- define and change system security profiles;
- classify, reclassify, import, and export objects;
- perform consistency checks on system and user security profiles.
7.3.4 Accountability
7.3.4.1 Identification and Authentication
The B1 requirements mandate the identification and authentication
recommendations of classes Cl and C2 (i.e., they mandate the identification
and authentication on a per-individual-user basis). In addition, it requires
that the TFM includes TCB commands and procedures to define and change the
user (and, possibly, group) levels of trust. It also requires that the
computation of a subject's login level of trust be included in the TFM.
7.3.4.2 Audit
The additional B1 requirements that shall be included in the TFM
documentation include:
- a description of how the audit mechanism records any override of
output markings;
- a description of how the TCB commands, interfaces, and procedures
support audit on a per-object sensitivity level basis
7.3.5 Routine Operations
(NAR)
7.3.6 Security of the TCB
The additional TFM requirement in this area is that the TFM "shall provide
guidelines on [...] how to securely generate a new TCB" [6].
This requirement suggests that the TFM include:
- a list of approved tools for TCB generation;
- a description of procedures for TCB generation;
- a description of the vulnerabilities in generating a new TCB.
The B1 requirements of the TFM also state that the TFM "shall provide
guidelines on [...] privileges needed to be controlled in order to
operate the facility in a secure manner" [6]. This implies that the
settings and the defaults for the protection privileges of the TCB
files should be specified. Warnings about the improper setting of such
privileges should be included.
7.4 Requirements and Recommendations for Security Class B2
All TFM requirements of the class B1 are included here. The documentation
requirements of class B2 suggest additions to the TFM contents beyond those
implied by the TCSEC requirements of security policy, accountability, and
operational assurance.
The TFM of a B2 system should include the following additional
documentation.
7.4.1 Introduction
(NAR)
7.4.2 System Security Overview
The only additional requirement for inclusion in this section is the
separation of administrative functions into two roles, namely that of the
administrator and that of the operator. Section 3 discusses the
documentation requirements for B2 role separation.
7.4.3 Security Policy
The two additional security-policy requirements that should be documented in
the TFM address the areas of subject sensitivity and device labels. The TFM
shall include the TCB commands and procedures to:
- change the security label of an active subject (if this function is
provided);
- assign and change the device sensitivity levels.
This section of the TFM shall also include a discussion of the security
vulnerabilities associated with change of trust level of an active subject.
Also it shall include a discussion of the relationship between the device
sensitivity levels and the level of trust associated with the physical
environment in which the devices are located.
7.4.4 Accountability
7.4.4.1 Identification and Authentication
The only additional TFM requirement here is that of documenting the trusted
path mechanisms available to administrative users whenever this mechanism
differs from that available to ordinary users (and documented in the SFUG).
7.4.4.2 Audit
The only additional TFM requirement of the audit area is that of defining
the TCB commands and interfaces for auditing covert channels, for setting
delays in covert channels, and for randomizing covert-channel variables.
7.4.5 Routine Operations
The routine operations performed by administrative users should be presented
according to the separation of roles required by the trusted facility
management area of the TCSEC and suggested by [7].
7.4.6 Security of the TCIB
The additional TFM requirements for this section include:
- the list of TCB modules shall identify the modules of the reference
monitor mechanism;
- "[...] the procedures for secure generation of a new TCB from source
after modification of any modules in the TCB shall be described" [6].
(This requirement implies that configuration management shall be in
place. References to additional documents defining these procedures and
plans could be included in the TFM).
7.5 Requirements and Recommendations for Security Class B3
The only additional requirements of class B3 that shall be included in the
TFM are in the areas of system overview, audit, routine operations, and
security of the TCB.
7.5.1 TFM Introduction
(NAR)
7.5.2 System Overview
The TFM should include a discussion of the physical security assumptions
made by the system designers and implementators that must be satisfied by
the installed system. Also, this section shall include a discussion of the
separation between the security-relevant and security-irrelevant functions
of the administrators and operators (see [7]).
7.5.3 Security Policy
(NAR)
7.5.4 Accountability
7.5.4.1 Identification and Authentication
(NAR)
7.5.4.2 Audit
The TFM should describe the TCB commands and interfaces available to the
auditor that enable him or her to monitor the accumulation of auditable
events and to respond effectively to such event signals.
7.5.5 Routine Operations
The additional routine operations carried out by secure and ordinary
operators should be specified in the TFM. These should include:
- the identification of damaged user files and volumes;
- the routine backup of TCB files;
- the mounting and unmounting of volumes.
Security-irrelevant administrator and operator actions, such as handling
user requests and managing the accounting system, should also be documented
here.
7.5.6 Security of the TCB
Two additional TFM requirements are included here. The first is that "[The
TFM] shall include procedures to ensure that the system is initially started
in a secure manner" [6]. This requirement suggests that the TFM must
document procedures for:
- TCB hardware installation (using the list of approved hardware
modules);
- TCB loading; TCB data structure initialization;
- initialization of privileges for TCB file
- use of approved initialization tools.
The second requirement is that "procedures shall also be included to resume
secure system operation after any lapse in system operation" [6].
This requirement suggests that the TFM should document procedures for:
- analysis of system dumps;
- crash recovery and restart in a secure state;
- repair of damaged TCB data structures (e.g., labels);
- changes of configuration parameters (e.g., table sizes);
- consistency checking procedures.
7.6 Requirements of Security Class A1
Although no additional explicit TFM requirements beyond that required for B3
are included here, the TFM should define procedures for trusted distribution
consistent with the [6] requirements.
GLOSSARY
Access -
A specific type of interaction between a subject and an object that
results in the flow of information from one to the other.
Account Administrator -
An administrative role or user assigned to maintain accounting files,
tools, user accounts, and system statistics.
Administrative User -
A user assigned to supervise all or a portion of an AIS system.
Approval Accreditation -
The official authorization that is granted to an AIS system to process
sensitive information in its operational environment, based upon
comprehensive security evaluation of the system's hardware, firmware,
and software security design, configuration, and implementation and of
the other system procedural, administrative, physical, TEMPEST,
personnel, and communications security controls.
Audit -
To conduct the independent review and examination of system records and
activities.
Audit Event Selection -
Selection, by authorized personnel, of the auditable events that are to
be recorded on the audit trail.
Audit Mechanism -
The part of the TCB used to collect, review, and/or examine system
activities.
Audit Post Processing -
Processing, by authorized personnel, of specified events that had been
recorded on the audit trail.
Audit Trail -
A chronological record of system activities that is sufficient to
enable the reconstruction, reviewing, and examination of the sequence
of environments and activities surrounding or leading to an operation,
a procedure, or an event in transaction from its inception to final
results.
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.
Auditor -
An authorized individual, or role, with administrative duties, which
include selecting the events to be audited on the system, setting up
the audit flags that enable the recording of those events, and
analyzing the audit. trail
Authenticate -
(1) To verify the identity of a user, device, or other entity in a
computer system, often as a prerequisite to allowing access to
resources in a system. (2) To verify the integrity of data that has
been stored, transmitted, or otherwise exposed to possible unauthorized
modification.
Authenticated User -
A user who has accessed an AIS system with a valid identifier and
authenticator.
Automated Information System (AIS) -
An assembly of computer hardware, firmware, and software configured to
collect, create, communicate, compute, disseminate, process, store, and
/or control data or information.
Bandwidth -
A characteristic of a communication channel that is the amount of
information that can be passed through it in a given amount of time,
usually expressed in bits per second.
Category -
A restrictive label that has been applied to classified or unclassified
data as a means of increasing the protection of the data and further
restricting access to the data.
Channel -
An information transfer path within a system. May also refer to the
mechanism by which the path is effected.
Covert Channel -
A communication channel that allows two cooperating processes to
transfer information in a manner that violates the system's security
policy. Synonymous with Confinement Channel.
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.
Covert Timing Channel -
A covert channel in which one process signals information to another by
modulating its own use of system resources (e.g., Central Processing
Unit time) in such a way that this manipulation affects the real
response time observed by these second process.
Data -
Information with a specific physical representation.
Data Integrity -
The state that exists when computerized data is the same as that in the
source documents and has not been exposed to accidental or malicious
alteration or destruction.
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.
Discretionary Access Control -
A means of restricting access to objects based on the identity and
need-to-know of the user, process, and/or groups to which they belong.
The controls are discretionary in the sense that a subject with a
certain access permission is capable of passing that permission
(perhaps indirectly) on to any other subject.
Formal Security Policy Model -
A mathematically precise statement of a security policy. To be
adequately precise, such a model shall represent the initial state of a
system, the way in which the system progresses from one state to
another, and a definition of a "secure" state of the system. To be
acceptable as a basis for a TCB, the model shall be supported by a
formal proof that if the initial state of the system satisfies the
definition of a "secure" state. If all assumptions required by the
model hold, then all future states of the system will be secure. Some
formal modeling techniques include state transition models, temporal
logic models, denotational semantics models, and algebraic
specification models.
Formal Top-Level Specification (FTLS) -
A top-level specification that is written in a formal mathematical
language to allow theorems showing the correspondence of the system
specification to its formal requirements to be hypothesized and
formally proven.
Functional Testing -
The portion of security testing in which the advertised features of a
system are tested, under operational conditions, for correct operation.
Least Privilege -
The principle that requires that each subject in a system be granted
the most restrictive set of privileges (or lowest clearance) needed for
the performance of authorized tasks. The application of this principle
limits the damage that can result from accident, error, or unauthorized
use.
Mandatory Access Control -
A means of restricting access to objects based on the sensitivity (as
represented by a label) of the information contained in the objects and
the formal authorization (i.e., clearance) of subjects to access
information of such sensitivity.
Multilevel Device -
A device that is used in a manner that permits simultaneous processing
of data of two or more security levels without risk of compromise. To
accomplish this, sensitivity labels are normally stored on the same
physical medium and in the same form (i.e., machine-readable or
human-readable) as the data being processed.
Multilevel Secure -
A class of system containing information with different sensitivities
that, simultaneously permits access by users with different security
clearances and need-to-know, but prevents users from obtaining access
to information for which they lack authorization.
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,
and network nodes.
Operator -
An administrative role or user assigned to perform routine maintenance
operations of the AIS system and to respond to routine user requests.
Output -
Information that has been exported by a TCB.
Password -
A private character string that is used to authenticate an identity.
Process -
A program in execution. It is completely characterized by a single
current execution point (represented by the machine state) and address
space.
Read -
A fundamental operation that results only in the flow of information
from an object to a subject.
Read Access (Privilege) -
Permission to read information.
Security Administrator -
An administrative role or user responsible for the security of an AS
and having the authority to enforce the security safeguards on all
others who have access to the AIS (with the possible exception of the
Auditor).
Security Level -
The combination of a hierarchical classification and a set of
non-hierarchical categories that represents the sensitivity of
information.
Security Policy -
The set of laws, rules, and practices that regulate how an organization
manages, protects, and distributes sensitive information.
Security Policy Model -
A formal (informal.in the case of B1) presentation of the security
policy enforced by the system. It must identify the set of rules and
practices that regulate how a system manages, protects, and distributes
sensitive information.
Security-Relevant Event -
Any event that attempts to change the security state of the system
(e.g., change the DAC, change the security level of the subject, change
user password). Also, any event that attempts to violate the security
policy of the system, (e.g., too many attempts to log in, attempts to
violate the MAC limits of a device, attempts to downgrade a file).
Security Testing -
A process used to determine that the security features of a system are
implemented as designed and that they are adequate for a proposed
application environment.
Sensitive Information -
Any information, the loss, misuse, modification of, or unauthorized
access to, that could affect the national interest or the conduct of
Federal programs, or the privacy to which individuals are entitled
under Section 552a of Title 5, U.S. Code, but that has not been
specifically authorized under criteria established by an Executive
order or act of Congress to be kept classified in the interest of
national defense or foreign policy.
Sensitivity Label -
A piece of information that represents the security level of an object
and that describes the sensitivity (e.g., classification) of the data
in the object. Sensitivity labels are used by the TCB as the basis for
MAC decisions.
Subject -
An active entity, generally in the form of a person, process, or device
that causes information to flow among objects or changes in the system
state. Technically, a process/domain pair.
Subject Security Level -
A subject's security level that is equal to the security level of the
objects to which it has both read and write access. A subject's
security level shall always be dominated by the clearance of the user
associated with the subject.
System Programmer -
An administrative role or user responsible for the trusted system
distribution, configuration, installation, and non-routine maintenance.
System Security Map -
A map defining the correspondence between the binary and ASCII formats
of security levels (e.g., between binary format of security levels and
sensitivity labels).
Top-Level Specification (TLS) -
A non-procedural description of system behavior at the most abstract
level; typically, a functional specification that omits all
implementation details.
Trap Door -
A hidden software or hardware mechanism that can be triggered to
permits system protection mechanisms to be circumvented. It is
activated in some innocent-appearing manner (e.g., special "random" key
sequence at a terminal).
Trojan Horse -
A computer program with an apparently or actually useful function that
contains additional (hidden) functions that surreptitiously exploit the
legitimate authorizations of the invoking process to the detriment of
security; for example, making a "blind copy" of a sensitive file for
the creator of the Trojan horse.
Trusted Computer System -
A system that employs sufficient hardware and software assurance
measures to allow its use for simultaneous processing a range of
sensitive or classified information.
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 enforce a security
policy correctly 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.
Trusted Path -
A mechanism by which a person at a terminal can communicate directly
with the TCB. This mechanism can only be activated by the person or the
TCB and cannot be imitated by untrusted software.
User -
Person or process accessing an AS either by direct connections (i.e.,
via terminals), or indirect connections (i.e., prepare input data or
receive output that is not reviewed for content or classification by a
responsible individual).
Verification -
The process of comparing two levels of system specification for proper
correspondence (e.g., security policy model with top-level
specification, TLS with source code, or source code with object code).
This process may or may not be automated.
Write -
A fundamental operation that results only in the flow of information
from a subject to an object.
Write Access (Privilege) -
Permission to write an object.
REFERENCES
[1]
National Computer Security Center, A Guide to Understanding Audit in
Trusted Systems, NCSC-TG-001, Version 2, June 1988.
[2]
National Computer Security Center, A Guide to Understanding
Discretionary Access Control in Trusted Systems, NCSC-TG-003,
version-I, September 1987.
[3]
Gligor V. D., J. C. Huskamp, S. R. Welke, C. J. Linn, W. T. Mayfield,
Traditional Capability-Based Systems: An Analysis of their Ability to
Meet the Trusted Computer Security Evaluation Criteria, Institute for
Defense Analyses, IDA Paper PI 935, February 1987.
[4]
Department of Defense, Password Management Guideline, CSC-STD-002-85,
April 1985.
[5]
National Computer Security Center, The Rating Maintenance Phase,
NCSCTG---013-89, 23 June 1989.
[6]
National Computer Security Center, Department of Defense Trusted
Computer System Evaluation Criteria, DoD 5200. 28-STD, 1985.
[7]
National Computer Security Center, Guidelines for Trusted Facility
Management, NCSC---TG---0I5-89, 18 October 1989.