Information about Mandatory Access Control

In computer security, mandatory access control (MAC) refers to a kind of access control defined by the Trusted Computer System Evaluation Criteria^[1] as "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". In addition, the term 'mandatory' used with access controls has historically implied a very high degree of robustness that assures that the control mechanisms resist subversion, thereby enabling them to enforce an access control policy that is mandated by some regulation that must be absolutely enforced, such as the Executive Order 12958 for US classified information.

MAC Precludes Informal Access Decisions

MAC also implies that access rules or decisions cannot be casually or informally determined. Access authorization is contingent on a formalized process that documents prerequisite trust in the individual gaining access. An example of a such a document is a security clearance Letter of Consent. An example of such a process is a security clearance background check mandated by Executive Order 12958. [1] MAC is most commonly applicable to Classified National Security Information where best effort mechanisms are inadequate; absolute enforcement is mandated.

MAC's most important feature involves denying users full control over the access to resources that they create. The system security policy entirely determines the access rights granted, and a user may not grant less restrictive access to their resources than the administrator specifies. MAC can be contrasted to Discretionary access control, where systems permit users to entirely determine the access granted to their resources, which means that they can (through accident or malice) give access to unauthorized users.

Special Information Assurance Implications of the term 'Mandatory'

For MAC, the access control decision is contingent on verifying the compatibility of the security properties of the data and the clearance properties of the individual (or the process proxying for the individual). The decision depends on the integrity of the metadata (e.g. label) that defines the security properties of the data, as well as the security clearance of the individual or process requesting access. For example, if a security label can be changed by a user, a surprisingly common vulnerability in some self-proclaimed 'MAC capable' systems, then that user can corrupt the access controls. Security mechanisms that protect such metadata and the access control decision logic from corruption are MAC-critical objects and require appropriate robustness.

The term mandatory in MAC has acquired a special meaning derived from its use with military systems. MAC means access controls that are mandated by order of a government and so enforcement is supposed to be more imperative than for commercial applications. This precludes enforcement by best-effort mechanisms, only mechanisms that can provide absolute, or near-absolute enforcement of the mandate are acceptable for MAC. This is a tall order and sometimes assumed unrealistic by those unfamiliar with high assurance strategies, and very difficult for those who are.

Vendors claiming to enforce MAC are sometimes making claims beyond their capability, and sometimes making claims beyond their understanding. The claim that MAC is enforced implies a claim of very high robustness. Vendors claiming MAC capability do usually have functions that enable defining of MAC privileges and rules but their implementations can be woefully unable to enforce them under even the mildest of attack. Ordinary 'best practices' does not produce software that has this kind of assurance level; in fact, no successful software-only approach has ever been documented. The only approach that has succeeded at protecting MAC controls from subversion has been to design the kernel to maintain a domain for its own execution using highly specialized hardware designed into the microprocessor architecture. Besides its cost, this is often unpopular because it affects portability of the operating system.

Degrees of MAC System Strength

In some systems users have the authority to decide whether to grant access (called Discretionary Access Control) to any another user. To allow that, all users have clearances for all data. This is not true of a MAC system. If individuals or processes exist that may be denied access to any of the data in the system environment, then the system must be trusted to enforce MAC. Since there can be various levels of data classification and user clearances, this implies a quantified scale for robustness. For example, more robustness is indicated for system environments containing classified Top Secret information and uncleared users than for one with Secret information and users cleared to at least Confidential. To promote consistency and eliminate subjectivity in degrees of robustness, an extensive scientific analysis and risk assessment of the topic produced a landmark benchmark standardization quantifying security robustness capabilities of systems and mapping them to the degrees of trust warranted for various security environments. The result was documented in CSC-STD-004-85. [2] Two relatively independent components of robustness were defined: Assurance Level and Functionality. Both were specified with a degree of precision that warranted significant confidence in certifications based on these criteria.

Rating Evaluation of MAC System Strength

The Common Criteria [3] is based on this science and it intended to preserve the Assurance Level as EAL levels and the functionality specifications as Protection Profiles.[4] Of these two essential components of objective robustness benchmarks, only EAL levels were faithfully preserved. In one case, Orange Book level C2 [5] (not a MAC capable category) was fairly faithfully preserved in the Common Criteria, as the Controlled Access Protection Profile (CAPP) [6]. MLS Protection Profiles (such as MLSOSPP similar to B2) [7] is more general than B2. They are pursuant to MLS, but lack the detailed implementation requirements of their Orange Book predecessors, focusing more on objectives. This gives certifiers more subjective flexibility in deciding whether the evaluated product’s technical features adequately achieve the objective, potentially eroding consistency of evaluated products and making it easier to attain certification for less trustworthy products. For these reasons, the importance of the technical details of the Protection Profile is critical to determining the suitability of a product.

Such an architecture prevents an authenticated user or process at a specific classification or trust-level from accessing information, processes, or devices in a different level. This provides a containment mechanism of users and processes, both known and unknown (an unknown program (for example) might comprise an untrusted application where the system should monitor and/or control accesses to devices and files).

Implementations

A few MAC implementations were certified robust enough to separate Top Secret from Unclassified late in the last millennium, such as Blacker. Their underlying technology became obsolete and they were not refreshed. Today there are no current implementations certified to that level of robust implementation. However, some less robust products exist.

• An NSA research project called SELinux (Security-Enhanced Linux) added a Mandatory Access Control architecture to the Linux Kernel, which was merged into the mainline version of Linux in August 2003. Red Hat Enterprise Linux version 4 (and later versions) come with an SELinux-enabled kernel. Although SELinux is capable of restricting all processes in the system, the supported policy in RHEL only targets the most vulnerable programs (thus the name, the Targeted Policy). SELinux utilizes a Linux 2.6 kernel feature called LSM (Linux Security Modules interface).
• SUSE Linux (now supported by Novell) and Ubuntu 7.10 have added a MAC implementation called AppArmor. AppArmor utilizes a Linux 2.6 kernel feature called LSM (Linux Security Modules interface). LSM provides a kernel API that allows modules of kernel code to govern access control. AppArmor is not capable of restricting all programs and is not yet included in the kernel.org kernel source tree. In most Linux distributions MAC is not installed.
• Beginning with version 5.0, the work of the TrustedBSD project has been incorporated into releases of the FreeBSD operating system. Development is a work in progress, and the implementation models as well as the capabilities are constantly improving. MAC on FreeBSD comes with pre-built structures for implementing MAC models such as Biba and Multi-Level Security.
• Sun's Trusted Solaris uses a mandatory and system-enforced access control mechanism (MAC), where clearances and labels are used to enforce a security policy. However note that the capability to manage labels does not imply the kernel strength to operate in Multi-Level Security mode. Access to the labels and control mechanisms are not robustly protected from corruption in protected domain maintained by a kernel. The applications a user runs are combined with the security label at which the user works in the session. Access to information, programs and devices are only weakly controlled.

See also

• Role-based access control - RBAC
• Capability-based security
• Security-related security classification
• Security-related type enforcement
• GWVr2 - Least Privilege Infrastructure and Information Flow Security Policy
• Rule-Set-Based Access Control (RSBAC)
• Security Modes of Operation
• Bell-La Padula security model
• Multiple Single-Level (MSL)
• Organisation-Based Access Control (Or-BAC)
• Take-Grant Model
• The Clark-Wilson Integrity Model
• Graham-Denning Model
• Systrace

References

• P. A. Loscocco, S. D. Smalley, P. A. Muckelbauer, R. C. Taylor, S. J. Turner, and J. F. Farrell. The Inevitability of Failure: The Flawed Assumption of Security in Modern Computing Environments. In Proceedings of the 21st National Information Systems Security Conference, pages 303–314, Oct. 1998. http://csrc.nist.gov/nissc/1998/proceedings/paperF1.pdf.

External links

• Weblog post on the how virtualization can be used to implement Mandatory Access Control.
• Weblog post from a Microsoft employee detailing Mandatory Integrity Control and how it differs from MAC implementations.
• GWV Formal Security Policy Model A Separation Kernel Formal Security Policy, David Greve, Matthew Wilding, and W. Mark Vanfleet.