Control statement
Implement a reference monitor for [Organization-defined: access control policies] that is tamperproof, always invoked, and small enough to be subject to analysis and testing, the completeness of which can be assured.
Discussion
A reference monitor is a set of design requirements on a reference validation mechanism that, as a key component of an operating system, enforces an access control policy over all subjects and objects. A reference validation mechanism is always invoked, tamper-proof, and small enough to be subject to analysis and tests, the completeness of which can be assured (i.e., verifiable). Information is represented internally within systems using abstractions known as data structures. Internal data structures can represent different types of entities, both active and passive. Active entities, also known as subjects, are associated with individuals, devices, or processes acting on behalf of individuals. Passive entities, also known as objects, are associated with data structures, such as records, buffers, communications ports, tables, files, and inter-process pipes. Reference monitors enforce access control policies that restrict access to objects based on the identity of subjects or groups to which the subjects belong. The system enforces the access control policy based on the rule set established by the policy. The tamper-proof property of the reference monitor prevents determined adversaries from compromising the functioning of the reference validation mechanism. The always invoked property prevents adversaries from bypassing the mechanism and violating the security policy. The smallness property helps to ensure completeness in the analysis and testing of the mechanism to detect any weaknesses or deficiencies (i.e., latent flaws) that would prevent the enforcement of the security policy.
Organization-defined parameters
These values must be resolved through the organization’s tailoring and governance process. Bracketed parameter references in the control text identify where a decision is required.
From control text to operational evidence
Use Reference Monitor as a testable risk decision. Translate the official statement into accountable people, repeatable processes, configured technology, and evidence that demonstrates the outcome over time. In this family, pay particular attention to identity, authorization, least privilege, session boundaries, and access lifecycle governance.
Implementation workflow
- Define the control boundary, responsible owner, inherited portions, and systems or processes in scope.
- Resolve each organization-defined parameter before declaring the control implemented.
- Document how the implementation satisfies every clause of the official control statement.
- Collect evidence as a normal byproduct of operation rather than only before an assessment.
- Review exceptions, changes, and monitoring results on a risk-based cadence.
Evidence examples
- access approvals and entitlement records
- role and group configuration exports
- periodic access review results
- authentication and authorization logs
Common failure patterns
- standing privileges that outlive business need
- shared or orphaned accounts
- access rules implemented differently across systems
- approvals that cannot be traced to actual permissions
Questions practitioners should ask
- What risk decision is this control intended to support in this system?
- Which parts are implemented locally, inherited, shared, or not applicable—and what evidence supports that decision?
- Do the documented narrative, deployed configuration, operating process, and collected evidence agree?
- What event or threshold requires the implementation to be reviewed or changed?
Assessment objectives and methods
Show the assessment objective
a reference monitor is implemented for [Organization-defined: access control policies] that is tamper-proof, always invoked, and small enough to be subject to analysis and testing, the completeness of which can be assured.
Examine
- Access control policy
- procedures addressing access enforcement
- system design documentation
- system configuration settings and associated documentation
- system audit records
- system security plan
- other relevant documents or records
Interview
- Organizational personnel with access enforcement responsibilities
- system/network administrators
- organizational personnel with information security responsibilities
- system developers
Test
- Mechanisms implementing access enforcement functions
Related controls
These relationships come from the official OSCAL catalog. They indicate useful dependencies or context, not automatic inheritance or equivalence.
Authoritative sources
Bare Metal Cyber is an independent educational publisher and is not affiliated with or endorsed by NIST. Official control requirements and interpretations remain with NIST and the responsible authorizing organization.