NIST SP 800-53 Rev. 5 AC-20 — Use Of External Systems

What this control requires

Establishes terms or identifies controls for external systems, allowing authorized access or data processing; or prohibits specific external systems to ensure security based on trust relationships. This control supports the System Connection Agreement capability within the Access Control family.

In plain English

Establishes terms or identifies controls for external systems, allowing authorized access or data processing; or prohibits specific external systems to ensure security based on trust relationships. In practice, this means you can prove you policy management-level controls on a defined cadence.

Why it matters

SP 800-53 is the catalogue every U.S. federal authorisation (FedRAMP, FISMA, CMMC) and most regulated industries point at. A missing control surfaces as a Plan of Action & Milestones (POA&M) item that an Authorizing Official must approve before the Authority To Operate is granted or renewed.

This control is rated high. Auditors expect a named owner, a remediation SLA, and a tooling artefact that evidences continuous operation.

What an auditor looks for

A 3PAO (FedRAMP) or Independent Assessor (NIST RMF) tests against NIST SP 800-53A assessment procedures. For each control they expect a documented implementation that addresses every assessment objective (Examine, Interview, Test) and an implementation summary in the System Security Plan.

Within Access Control, the assessor will trace your implementation back to the catalogue reference (Control AC-20) and look for objective evidence on the latest two assessment cycles.

How to implement

Policy Management

Docs. Keep each policy in version-controlled Markdown alongside the code that implements it; ship a signed PDF snapshot at each annual review and link the SHA into the SoA.

Operational checklist

• Policy. Document the trigger, the owner and the cadence.
• Access. Document the trigger, the owner and the cadence.
• Security. Document the trigger, the owner and the cadence.
• Data. Document the trigger, the owner and the cadence.
• Access Control. Document the trigger, the owner and the cadence.
• Scope: management, access control, system, external systems, controls, capability, authorized, policy.

Evidence to collect

• Policy Document. A signed and version-controlled policy / standard / procedure document.

Signed policy or standard documents the safeguard. Focus: Use Of External Systems.

Common pitfalls

• Policies that no engineer has read; the version in Confluence drifts from the version that's actually being enforced.

How it maps to other frameworks

Computed from shared tags, control component and key-operation subjects — handy when you are reconciling controls across an audit.

• NIST SP 800-171 Rev. 3 3.1.20 — Use of External Systems
• HIPAA (45 CFR Part 164) 164.310(a)(2)(iii) — Facility Access Controls - Access Control and Validation Procedures
• BSI C5:2020 SP-01 — Documentation, communication and provision of policies and instructions

Related controls in NIST SP 800-53 Rev. 5

• Control AC-20(1) — Limits On Authorized Use
• Control AC-6 — Least Privilege
• Control AC-20(3) — Non-Organizationally Owned Systems -- Restricted Use
• Control AC-20(5) — Portable Storage Devices -- Prohibited Use
• Control PS-3(1) — Classified Information
• Control AC-16(4) — Association Of Attributes By Authorized Individuals

References

• NIST Computer Security Resource Center — NIST SP 800-53 Rev. 5
• Catalogue identifier: Control AC-20

Topics: governance riskpolicy management