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

AC-20 Use of External Systems

a. Selection (one or more): establish [organization-defined terms and conditions; identify organization-defined controls asserted to be implemented on external systems], consistent with the trust relationships established with other organizations owning, operating, and/or maintaining external systems, allowing authorized individuals to: 1. Access the system from external systems; and 2. Process, store, or transmit organization-controlled information using external systems; or b. Prohibit the use of [organizationally-defined types of external systems].

Source: NIST SP 800-53 Rev 5, AC-20. 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.

8
artefacts
0
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

none for this control

periodic reviewEvidence produced at each review

  • Test of Mechanisms implementing terms and conditions on the use of external systems · Identity provider / directory

governing documentDocuments that govern the control

  • Access control policy · Policy repository / GRC workspace
  • Procedures addressing the use of external systems · Policy repository / GRC workspace
  • External systems terms and conditions · Policy repository / GRC workspace
  • List of types of applications accessible from external systems · Policy repository / GRC workspace
  • Maximum security categorization for information processed, stored, or transmitted on external systems · Policy repository / GRC workspace
  • System configuration settings and associated documentation · Policy repository / GRC workspace
  • System security plan · Policy repository / GRC workspace

First move

No system holds this evidence for you: it is people and documents. Reminders and routing to the owner still save time. Put the document under version control with an owner and review date, and log each review as a record with reviewer and date.

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-19(5) Access Control for Mobile Devices | Full Device or Container-based Encryption · AC-20(1) Use of External Systems | Limits on Authorized Use