---
title: "NIST 800-171 3.13.6 (SC.L2-3.13.6): Deny by Default"
description: "NIST SP 800-171 3.13.6 Deny by Default: 5 SPRS points, not allowed on a CMMC POA&M. The text, what assessors check and the evidence."
url: https://policycortex.com/nist-800-171/system-and-communications-protection/3.13.6
sealed: 2026-11-11
amended: 2026-11-11
register: nist-800-171/system-and-communications-protection/3.13.6
records: 7
digest: sha3:2d34f8b87d609adb27f453e8577f3f97a6f73318f9b1a841f5e50b845a8bf181
---

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

NIST 800-171 / System and Communications Protection / 3.13.6

"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](https://doi.org/10.6028/NIST.SP.800-171r2))

CMMC Level 2 practice SC.L2-3.13.6.

## 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](https://www.acq.osd.mil/asda/dpc/cp/cyber/docs/safeguarding/NIST-SP-800-171-Assessment-Methodology-Version-1.2.1-6.24.2020.pdf); [32 CFR 170.21, Plan of Action and Milestones requirements](https://www.ecfr.gov/current/title-32/section-170.21).

## 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](https://doi.org/10.6028/NIST.SP.800-171A)

## 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.](https://policycortex.com/sprs-score-calculator#family-SC)
- **Sample package**: [Sample package: an SSP, a POA&M and an evidence chain the product generated from demonstration data.](https://policycortex.com/sample-package)
- **Family**: [All 16 System and Communications Protection requirements (3.13).](https://policycortex.com/nist-800-171/system-and-communications-protection)
- **Previous**: [3.13.5: Public-Facing Subnets](https://policycortex.com/nist-800-171/system-and-communications-protection/3.13.5)
- **Next**: [3.13.7: Split Tunneling](https://policycortex.com/nist-800-171/system-and-communications-protection/3.13.7)
- **All requirements**: [All 110 NIST SP 800-171 requirements, by family.](https://policycortex.com/nist-800-171)

Meet the requirement once. Keep the evidence current. [Request Access](https://app.policycortex.com/auth?mode=request-access) · [Book a call](https://policycortex.com/book)

## Register colophon

| Seq | Label | SHA3-256 | Prev |
|---|---|---|---|
| REC 0000 | REQUIREMENT | fddf66ef91c4 | c0121ad13d2f |
| REC 0001 | MEANING | d181ac59c63a | fddf66ef91c4 |
| REC 0002 | POINTS AND POA&M | c924195f4dc9 | d181ac59c63a |
| REC 0003 | ASSESSMENT OBJECTIVES | 398efca467a0 | c924195f4dc9 |
| REC 0004 | EVIDENCE | c3922975a977 | 398efca467a0 |
| REC 0005 | COLLECTORS | e4f4d04a7adb | c3922975a977 |
| REC 0006 | RELATED | 2d34f8b87d60 | e4f4d04a7adb |

Head sha3:2d34f8b87d609adb27f453e8577f3f97a6f73318f9b1a841f5e50b845a8bf181. Each digest is SHA3-256 over the previous digest, the register key, the record label, and the record copy; a second party can recompute it from this document.