NIST SP 800-171 Rev. 3 3.1.20 — Use of External Systems

What this control requires

This control prohibits the use of external systems unless specifically authorized. It establishes security requirements for external systems to be satisfied before allowing access to organizational systems or CUI processing. Authorized users can only access external systems once these requirements are met, and the use of organization-controlled portable storage devices on external systems is restricted. This control supports the External Systems Access Control capability within the Access Control (3.1) family.

In plain English

This control prohibits the use of external systems unless specifically authorized. In practice, this means you can prove you access management-level controls on a defined cadence.

Why it matters

DoD contractors and their subcontractors must implement all 100 SP 800-171 requirements to keep CUI handling contracts. Gaps are scored against the SPRS scoring methodology; below a 110-point starting score, each unmet requirement deducts points and a below-threshold score can disqualify a bid.

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 C3PAO conducting a CMMC Level 2 assessment uses NIST SP 800-171A objectives. They expect a System Security Plan (SSP), Plan of Action & Milestones (POA&M) for any gaps, and objective evidence (configurations, ticket records, training records) for every requirement.

Within Access Control, the assessor will trace your implementation back to the catalogue reference (Requirement 3.1.20) 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

• Access. Document the trigger, the owner and the cadence.
• Access Control. Document the trigger, the owner and the cadence.
• Authorized. Document the trigger, the owner and the cadence.
• Risk. Document the trigger, the owner and the cadence.
• Security. Document the trigger, the owner and the cadence.
• Scope: management, cui, external systems, security, storage, access control.

Evidence to collect

• Policy Document. A signed and version-controlled policy / standard / procedure document.
• Infra Scan. Continuous infrastructure-posture telemetry — AWS Config, Azure Policy compliance, GCP Organisation Policy, Wiz / Prisma findings.

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 800-53 Rev. 5 AC-20(5) — Portable Storage Devices -- Prohibited Use
• HIPAA (45 CFR Part 164) 164.308(a)(4)(ii)(A) — Information Access Management - Isolating Healthcare Clearinghouse Functions
• ISO/IEC 27001:2022 A.5.1 — Policies for information security
• BSI C5:2020 SP-01 — Documentation, communication and provision of policies and instructions
• SOC 2 (2017 TSC) CC2.3 — External Communication on Internal Control Matters

Related controls in NIST SP 800-171 Rev. 3

• Requirement 3.1.5 — Least Privilege
• Requirement 3.1.2 — Access Enforcement
• Requirement 3.1.9 — System Use Notification
• Requirement 3.10.7 — Physical Access Control
• Requirement 3.13.10 — Cryptographic Key Establishment and Management
• Requirement 3.1.10 — Device Lock

References

• NIST Computer Security Resource Center — NIST SP 800-171 Rev. 3
• Catalogue identifier: Requirement 3.1.20

Topics: governance riskpolicy management