NIST 800-171 3.13.6 (SC.L2-3.13.6): Deny by Default

Deny network communications traffic by default and allow network communications traffic by exception (i.e., deny all, permit by exception).

NIST SP 800-171 Rev. 2, requirement 3.13.6

CMMC Level 2 practice SC.L2-3.13.6Published 2026-11-11

What 3.13.6 means in practice

Network traffic is blocked unless a rule explicitly allows it, inbound and outbound, at the boundary and at key points inside the system. Each allow rule names specific sources, destinations and ports, with a recorded reason.

SPRS points and the CMMC POA&M

CMMC Level 2 practice: SC.L2-3.13.6
SPRS points: 5 points. When it is not met, the DoD Assessment Methodology subtracts 5 from the score of 110.
CMMC POA&M: Not allowed. It is worth 5 points, and 32 CFR 170.21(a)(2)(ii) admits only requirements worth 1 point, with one exception: 3.13.11 when encryption is used but not FIPS validated.

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 3.13.6

NIST SP 800-171A gives 2 determination statements for 3.13.6, each found satisfied or other than satisfied. Determine if:

3.13.6[a] network communications traffic is denied by default.
3.13.6[b] network communications traffic is allowed by exception.

NIST SP 800-171A (June 2018), assessment objectives for 3.13.6

Evidence that shows 3.13.6 is met

Firewall and network rule sets that end in a deny-all rule.
A review of the allow rules, with the business reason for each.
Outbound filtering rules, not only inbound ones.

How PolicyCortex gathers the evidence

PolicyCortex reads this evidence with the collectors below, read only, and grades 3.13.6 from it. Each line says what the product checks.

Azure: Network security groups. Checks that network security groups deny inbound and outbound traffic by default, carry an explicit deny-all rule, and allow only specific sources and destinations.

Related pages

Calculator: SPRS score calculator: mark 3.13.6 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 16 System and Communications Protection requirements (3.13).
Previous: 3.13.5: Public-Facing Subnets
Next: 3.13.7: Split Tunneling
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.