NIST 800-171 3.1.21 (AC.L2-3.1.21): Portable Storage Use

Limit use of portable storage devices on external systems.

NIST SP 800-171 Rev. 2, requirement 3.1.21

CMMC Level 2 practice AC.L2-3.1.21Published 2026-11-11

What 3.1.21 means in practice

USB drives and other portable storage holding CUI should not be plugged into systems you do not control. Organizations usually either ban portable storage outright or allow only encrypted, company-issued drives under written conditions, and enforce the rule with device control on their own computers.

SPRS points and the CMMC POA&M

CMMC Level 2 practice: AC.L2-3.1.21
SPRS points: 1 point. When it is not met, the DoD Assessment Methodology subtracts 1 from the score of 110.
CMMC POA&M: Allowed. It is worth 1 point, and 32 CFR 170.21(a)(2)(ii) admits requirements worth 1 point. Conditional status also needs a score of at least 88, and the item must be closed within 180 days.

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 3.1.21

NIST SP 800-171A gives 3 determination statements for 3.1.21, each found satisfied or other than satisfied. Determine if:

3.1.21[a] the use of portable storage devices containing CUI on external systems is identified and documented.
3.1.21[b] limits on the use of portable storage devices containing CUI on external systems are defined.
3.1.21[c] the use of portable storage devices containing CUI on external systems is limited as defined.

NIST SP 800-171A (June 2018), assessment objectives for 3.1.21

Evidence that shows 3.1.21 is met

The written limits on using portable storage with external systems.
Device control settings that block or restrict removable storage.
A register of issued encrypted drives and their holders, where drives are allowed.

How PolicyCortex gathers the evidence

PolicyCortex reads this evidence with the collectors below, read only, and grades 3.1.21 from it. Each line says what the product checks.

Microsoft Intune: Device configurations · Compliance policies. Checks that removable storage is blocked on every laptop and desktop Intune manages.

Related pages

Calculator: SPRS score calculator: mark 3.1.21 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: 3.1.20: External Connections
Next: 3.1.22: Public Information
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.