EvidenceSheet
NIST SP 800-53 Rev 5 · AC - Access Control

AC-25 Reference Monitor

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.

Source: NIST SP 800-53 Rev 5, AC-25. The statement above is our statement of the clause, read from the published text and cited so you can check it against the official text, not the instrument verbatim. Open NIST SP 800-53 Rev 5 in the compliance library.

Base controls and their enhancements. Which apply follows the baseline you are assessed against (for example FedRAMP Moderate or High, or the low, moderate and high baselines of SP 800-53B). Text in square brackets is a parameter your organisation sets.

7
artefacts
1
held by a system
1
at each review
hard
to go live
Policy repository / GRC workspace
where the evidence lives
teal = a system already holds it · olive = produced at each review. To go live: easy when half or more of the artefacts are already held by a system, moderate from a quarter to a half, hard below a quarter.

These are the artefacts auditors commonly ask for on this requirement, our guidance rather than text from the standard. Your assessor decides what is sufficient in your context, and asks for samples across the period when a report covers one.

system holds itEvidence a system already holds

  • Test of Mechanisms implementing access enforcement functions · Identity provider / directory

periodic reviewEvidence produced at each review

  • System audit records · Identity provider / directory

governing documentDocuments that govern the control

  • Access control policy · Policy repository / GRC workspace
  • Procedures addressing access enforcement · Policy repository / GRC workspace
  • System design documentation · Policy repository / GRC workspace
  • System configuration settings and associated documentation · Policy repository / GRC workspace
  • System security plan · Policy repository / GRC workspace

First move

Mostly documents and reviews. Pull the 1 system-held artefact from your Identity provider / directory on a schedule; put the documents under version control with an owner and review date, and log each review as a dated record with a named reviewer.

Common gaps auditors find

Do this for your whole sheet

Paste the rows you run your controls from and get this mapping for every control at once, with the periodic-review ones flagged and a first move per row. No account for the first run.

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AC-24(2) Access Control Decisions | No User or Process Identity · AT-1 Policy and Procedures