# NIST SP 800-53 Rev. 5 AC-20 — Use Of External Systems - Source: https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-20-use-of-external-systems - Section: [Compliance](https://support.icompaas.com/compliance) › [NIST SP 800-53 Rev. 5 Controls](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5) › AC — Access Control - Last updated: 2026-06-05 - Publisher: iCompaas, Inc. (https://www.icompaas.com) **NIST SP 800-53 Rev. 5**·Control `AC-20`·high·Component: **Access Control** ## 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. > > — NIST SP 800-53 Rev. 5, Control AC-20 ### 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](https://support.icompaas.com/compliance/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](https://support.icompaas.com/compliance/hipaa/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](https://support.icompaas.com/compliance/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](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-20-1-limits-on-authorized-use) - [**Control AC-6** — Least Privilege](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-6-least-privilege) - [**Control AC-20(3)** — Non-Organizationally Owned Systems -- Restricted Use](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-20-3-non-organizationally-owned-systems-restricted-use) - [**Control AC-20(5)** — Portable Storage Devices -- Prohibited Use](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-20-5-portable-storage-devices-prohibited-use) - [**Control PS-3(1)** — Classified Information](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ps-3-1-classified-information) - [**Control AC-16(4)** — Association Of Attributes By Authorized Individuals](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5/ac-16-4-association-of-attributes-by-authorized-individuals) ## References - [NIST Computer Security Resource Center — NIST SP 800-53 Rev. 5](https://csrc.nist.gov/Projects/risk-management/sp800-53-controls) - Catalogue identifier: `Control AC-20` Topics: governance riskpolicy management ## Related compliance guidance - [NIST SP 800-53 Rev. 5: a practical implementation guide for cloud teams](https://support.icompaas.com/compliance/nist-sp-800-53-rev-5): Start with the full NIST SP 800-53 implementation guide — scope, roadmap, evidence and how the assessment runs. - [NIST SP 800-171 and CMMC: a practical guide to protecting CUI in the cloud](https://support.icompaas.com/compliance/nist-sp-800-171-rev-3): If your obligation is protecting CUI in your own systems for a federal or defence contract, this derived subset — not the full catalogue — is the requirement. - [SOC 2 compliance: a practical implementation guide for cloud-native SaaS teams](https://support.icompaas.com/compliance/soc-2-trust-services-criteria): Most commercial SaaS already hold a SOC 2 Type 2, and FedRAMP 20x Class A is designed for providers that have one.