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Last Update: 21 Jul 2000
5 Requirements on systems
5.1 System Model
5.2 Availability requirements
5.3 Sensitivity requirements
5.3.1 Class Requirements: Public / non classified data
5.3.2 Class Requirements: Internal data
5.3.3 Class Requirements: Confidential data
5.3.4 Class Requirements: Top Secret data
5.3.5 Requirements: Summary of relationship to Orange Book Classes
5.4 Component security checklist
The security required of a system depends on what information it processes, for what
purposes. The information and information processing functions should have sensitivity and
availability labels (i.e. be classified, as in the preceding chapter), so that the
security needs to the system can be specified.
The security needs of the system may concentrate on availability, confidentiality or
integrity, for example:
- A system may not contain confidential data, but it must be available 24 hrs a day - so
it has low data sensitivity, but high availability requirements. High availability systems
always require better confidentiality to prevent "denial of service" attacks.
- For other systems confidentiality (i.e. non disclosure of information) is more important
than integrity (modification of information), for others the reverse is true.
In the following sections, a set of requirements are proposed, based on the sensitivity
and availability classes proposed in the previous chapter.
Systems need to be broken down into components and the security of each
component analysed. When a user access data, he passes through a variety of possible
security controls, or components. Some of these controls are physical, some organisational
and some are computer based. The following is one way of representing the components
For data to be secured each
of the different layers (security components) from Physical to Operating system need to be
correctly configured and monitored. It is not enough to simply secure the physical
access for example, because access via the network may also be possible.
In using a model of this kind, it is possible to break down the measures
needed to protect a system (or data) into subsystems which correspond to real-life
In this chapter requirements are defined, in part II & III of the
document, guidelines are proposed for securing each of the layers in the above model.
The following specifies requirements on systems and organisation for each availability
||A backup and restore policy must exist.
Test restore policy regularly!
Minimum frequency of regular backups.
Minimum frequency of off-site backups.
Electromagnetic protection (EMP hardening)
UPS (220V protection)
Clearly defined support organisation.
Documentation of availability measures, recovery
procedures, disaster plans (virus outbreak, security breach, fire, power failures etc.)
Change management (Updates/patches/new SW)
Effective (if possible automated) log monitoring and
Prevention of resource abuse
Service slots must be defined. Regular maintenance
24 Hour personnel
Help Desk (reaction times, escalation procedures).
Hardware spares or Vendor maintenance contract.
Database servers: RAID, Replication, transaction
Naming servers (NT, NIS+, DNS, NIS...): backup/replica
File Servers: Mirroring, RAID 5, file replication.
Complexity / ease of maintenance is also important for availability. Complex, difficult
to maintain systems will have a high probability of misconfiguration (unless expert system
administrators are available) and hence increase the security risk.
The requirements proposed here for each of the sensitivity classes to are based on the
American DoD (Department of Defense) Orange Book [tcsec] (see Appendix E) classes
C1,C2 & B1 and the European Orange Book (or ITSEC) [itsec] class F-DX for
secure communications requirements. Basing requirements on international standards such as
these is preferred, since inter-company security policy may be more easily defined and
these standards have been already tried & tested by other companies.
- The Orange Book specifications for Classes C1 and C2 is included in appendix E. A brief
summary of the relevant Orange Books classes is included below.
- The Orange Book classes use the notion of a Trusted Computing Base (or TCB)
extensively. This is the central part of the system (e.g. the kernel) which is
trusted to carry out security functions.
are not based on any standard, but "common sense". These requirements stipulate
a minimum to be achieved by all systems in a company.
Even systems non-sensitive data should have a minimum security level (especially if
they are networked), otherwise they could be used as a point of entry for attacks on more
- Network sniffing software should not be installed.
- A virus scanner should be installed (DOS/Windows).
- Accounts should only exist for authorised persons and must always have a password.
- Screen locking with password protection should be activated automatically after 15
minutes idle time.
- Write access to network filesystems should be restricted to groups of users or machines.
- Communications software (NFS, LanManager, RAS, PPP, UUCP, Workgroups..) should be
correctly installed with security options enabled.
Summary: Orange book C1 (Discretionary Security Protection).
C1 is used for co-operating users working with data of the same sensitivity level.
- Documentation: test, security design philosophy, security features user guide
(description of security mechanisms from users point of view), trusted facility manual
(i.e. security administration guide).
- Assurance: System Architecture: does the TCB run in protected mode?. Functions
should exist for checking hardware & firmware integrity. Have the security mechanisms
been successfully tested?
- User identification and authorisation is required, along with protection of
- Discretionary access control: access is controlled between named users (or user
groups) and named objects.
Summary: Orange book C2 + secure data transmission.
- C2 (Controlled Access Protection):
- As C1 plus additional requirements for: trusted facility manual
(describe C2 mechanisms), identification & authorisation (no group accounts may
exist), discretionary access control (control assignment of privileges) and security
testing (test C2 mechanisms).
- User accountability: Users are accountable for their actions. Audit trails
should be available with monitoring and alert functions. Audit logs should be protected.
- Object Re-use: Objects used by a subject should be reinitialised before use by an
other subject. i.e. should not be possible to compromise security by reuse of objects.
- Secure data transmission : When sending messages or when programs communicate
with each other, privacy and completeness (i.e. confidentiality and integrity) must be
For certain applications it may also be necessary that the receiver be absolutely sure
that the information comes from the sender and not someone else. This is called non
repudiation of origin. It may also be required that the sender must be sure that the
message was received by the intended receiver - non repudiation of receipt.
Summary: Orange book B1 + secure data transmission.
- B1 (Labelled Security Protection):
- As in C2 plus additional requirements for identification & authentication (maintain
security compartment information), trusted facility manual (B1 mechanisms & how to
change security compartment), design manual (description of the security model &
mechanisms), assurance (system architecture: process isolation, integrity checking,
security testing: try penetration attacks & remove flaws) and auditing (log security
levels of objects).
- Labels : Maintain sensitivity labels under control of the TCB, Input/output of
labelled information, label integrity (linked to objects), label human readable output,
single & multi-level I/O.
- Verification of specification & design: Does the system behave according to
the Design Manual?
- Exporting of labelled information, exporting to multilevel and single level devices.
- Mandatory access control: access control for objects & subjects is specified
by the TCB (i.e. not the user).
- Not part of B1 is Covert channels and trusted path analysis. They may be necessary for
some systems. Class B2 includes these an other further requirements.
- Secure data transmission: as .
The following shows roughly how the data sensitivity classes to devised in this
document relate to orange book classes D-B1.
||Detail Orange Book Reference
||Data Sensitivity Class
Security features user manual,
Trusted facility manual
||System architecture verification
Hardware/firmware integrity checking
Security testing (test for loopholes)
||User identification /authorisation
||Audit Trail (Beweissicherung)
||Discretionary access control
||Object reuse :Reinitialisation of objects.
||Labels, integrity, human readable output.
||Specification and design verification
||of labelled information to multilevel & single level devices.
|Requirements in addition to the Orange book:
|Secure data exchange
||Peer entity authentication
||Data origin authentication / Non repudiation of origin
||Non repudiation of receipt
+ means as previous class with additional requirements.
= means same requirements as previous class.
It is suggested that individual components of a system and systems as a whole be
analysed according to the following checklist.
I. Documentation: Are the system security features well documented? Security
Features User's Guide, System Administrators Guide to Security, Test documentation, Design documentation.
II. Assurance : How can one be sure that
the systems does what it should do? :- System Architecture, System Integrity, Security
Testing. Has the system been certified to meet known standards?
III. Accountability : Users shall be
accountable for their actions.
- Identification / authentication: Users must be uniquely identified (e.g. usernames +
login) and be authenticated (e.g. via passwords). Authentication data must be protected.
- Audit Trail :
- A record of who did what, from which terminal, on what machine, when, with what object
and whether successful or not should be maintained.
- Events for logging: Identification/Authorisation, Access rights administration,
creation/deletion of sensitive objects, actions affecting security of the system.
- Logs must be protected (confidentiality, integrity e.g. file permissions) and should be
regularly monitored and when necessary (automatically) security alerts raised.
- Tools should exist for manipulating audit trails and should allow actions of a
particular user to be identified (in an understandable fashion).
IV. Access Control :
- Discretionary access control: The system shall distinguish and administer access rights
between users, groups of users, objects (e.g. permissions on filesystem, shared memory,
floppy drives, printers, devices, network services, menu options, application options).
- Mandatory access control : access control for
objects & subjects is specified by the TCB (i.e. not the user).
- Secure system startup - components should startup securely.
- Object reuse : Objects used by a subject must
be reinitialised before being used by another subject.
V. Secure data exchange / network communications
- Network Peer entity authentication: Both sides (users & processes) must identify
& authenticate themselves (have their identity verified), prior to the exchange of
- Network Data integrity: Data must remain complete during transmission. Unauthorised
manipulation of user data, audit trail data and replay of transmissions should be
reliably identified as errors.
- Network Data confidentiality: Only authorised persons should be able to access the data.
(e.g. end-to-end data encryption)
- Data origin authentication : Does the receiving
process know who the data is coming from? For class systems, non repudiation of origin may be required: On receipt of data,
it should be possible to uniquely identify and authenticate the sender of the data. Has
the receiver proof of where information came from? This can be achieved by the use of digital
- Non repudiation of receipt : Has the sender
proof that the information sent was received by the intended receiver?
- Access control : All information previously
transmitted which could be used for unauthorised decryption should only be accessible to
- Backup: Backup and restore policies shall exist and be tested regularly.
- Prevention of resource abuse : e.g. Quotas, CPU, memory, process limits per user.
- Patches/change management : careful, precise
procedures are required for updates, or configuration changes to production systems. A
"change log" should be kept up to date.
- Environment : air conditioning, cabling, norms.
- Disaster Recovery : Plan for Power failures
(UPS), fire and flooding, security breach.
- Organisation : Defined support procedures, roles and responsibilities with service level
agreements, documentation of procedures, service slots, 7x24H ().
Disposal of equipment.
- Redundancy : Redundancy is possible on many
levels. CPU, disks, disk drivers, network, transport (e.g. OLTP), application (e.g. NIS+,
Lan Manager, DNS) and complete system (e.g. HACMP). Redundancy should be regularly tested.
 On UNIX systems, this means that shadow password
files are required.
IT Security Cookbook, 21 July, 2000