NIST 800-171 3.1.20 (AC.L2-3.1.20): External Connections

Verify and control/limit connections to and use of external systems.

NIST SP 800-171 Rev. 2, requirement 3.1.20

CMMC Level 2 practice AC.L2-3.1.20Published 2026-11-11

What 3.1.20 means in practice

External systems are ones you do not control, such as personal devices, a partner's network or a cloud service outside your boundary. List the external systems that connect to or are used with your system, verify their security before allowing the connection, and set terms that limit what may be done with them.

SPRS points and the CMMC POA&M

CMMC Level 2 practice: AC.L2-3.1.20
SPRS points: 1 point. When it is not met, the DoD Assessment Methodology subtracts 1 from the score of 110.
CMMC POA&M: Never. 32 CFR 170.21(a)(2)(iii) names 3.1.20 among the 6 requirements a CMMC POA&M may never include, whatever their point value.

Sources: NIST SP 800-171 DoD Assessment Methodology, version 1.2.1 (June 24, 2020), Annex A; 32 CFR 170.21, Plan of Action and Milestones requirements.

What an assessor checks for 3.1.20

NIST SP 800-171A gives 6 determination statements for 3.1.20, each found satisfied or other than satisfied. Determine if:

3.1.20[a] connections to external systems are identified.
3.1.20[b] the use of external systems is identified.
3.1.20[c] connections to external systems are verified.
3.1.20[d] the use of external systems is verified.
3.1.20[e] connections to external systems are controlled/limited.
3.1.20[f] the use of external systems is controlled/limited.

NIST SP 800-171A (June 2018), assessment objectives for 3.1.20

Evidence that shows 3.1.20 is met

An inventory of the external systems and cloud services that connect to or are used with the system.
Agreements or terms of use for each, with the security review or FedRAMP authorization relied on.
Configuration that blocks unapproved external connections, such as personal devices or unsanctioned cloud applications.

How PolicyCortex gathers the evidence

No PolicyCortex collector reads the evidence for 3.1.20. Your team provides the evidence listed above.

Related pages

Calculator: SPRS score calculator: mark 3.1.20 and the other 109 requirements and see the score.
Sample package: Sample package: an SSP, a POA&M and an evidence chain the product generated from demonstration data.
Family: All 22 Access Control requirements (3.1).
Previous: 3.1.19: Encrypt CUI on Mobile
Next: 3.1.21: Portable Storage Use
All requirements: All 110 NIST SP 800-171 requirements, by family.

Related records

Guides and articles describe the work. The evidence that work produces is described in three proof pages, the architecture page, and the two pages on authorization.

proof.state: Proof of State. What the environment was, as of a date someone else picks: point-in-time records, content hashed and chained.
proof.change: Proof of Change. Who or what altered the environment, under what authority, with before and after state hashes.
proof.agency: Proof of Agency. What a machine was permitted to do before it acted, what it did, and what would have stopped it.
architecture: Architecture. How the chain is built and where the evidence is stored in each delivery mode.
authorization: Continuous ATO. How an authorizing official's decision is recorded with its package, and how the authorized system is checked against it every hour.
baseline: NIST SP 800-53. The Low, Moderate and High baselines as evidence, with the SSP, SAR and POA&M generated from one record.