Network Communication By Exception
Deny network communications traffic by default and allow network communications traffic by exception (i.e., deny all, permit by exception).
What an assessor scores, the objectives
SC.L2-3.13.6 is met only when every one of these 2 objectives, from NIST SP 800-171A, is satisfied. A single missed objective makes the whole requirement not met.
- a.network communications traffic is denied by default
- b.network communications traffic is allowed by exception
How a C3PAO checks it
NIST SP 800-171A defines three assessment methods. For SC.L2-3.13.6, an assessor uses these:
System and communications protection policy; procedures addressing boundary protection; system security plan; system design documentation; system configuration settings and associated documentation; system audit logs and records; other relevant documents or records
System or network administrators; personnel with information security responsibilities; system developer; personnel with boundary protection responsibilities
Mechanisms implementing traffic management at managed interfaces
What it means, in context
This requirement applies to inbound and outbound network communications traffic at the system boundary and at identified points within the system. A deny-all, permit-by-exception network communications traffic policy ensures that only those connections which are essential and approved are allowed.
Block all traffic entering and leaving the network , but permit specific traffic based on organizational policies, exceptions, or criteria. This process of permitting only authorized traffic to the network is called whitelisting and limits the number of unintentional connections to the network. This requirement, SC.L2-3.13.6, requires a deny -all permit by exception approach for all network communications. In doing so, it adds specifics for SC.L2-3.13.1, which only requires monitoring, control, and protection of communication channels. Example You are setting up a new environment to house CUI. To properly isolate the CUI network, you install a firewall between it and other networks and set the firewall rules to deny all traffic [a]. You review each service and application that runs in the new environment and determine that you only need to allow http and https traffic outbound [b]. You test the functionality of the required services and make some needed adjustments, then comment each firewall rule so there is documentation of why it is required. You review the firewall rules on a regular basis to make sure no unauthorized changes were made. Potential Assessment Considerations • Are network communications traffic on relevant system components (e.g., host and network firewalls, routers, gateways) denied by default (e.g., configured with an implicit deny rule that takes effect in the absence of any other matching traffic rules) [a]? • Are network communications traffic on relevant system components (e.g., host and network firewalls, routers, gateways) allowed by exception (e.g., configured with explicit allow rules that takes effect only when network traffic matches one or more rules) [b]?
What passing evidence looks like
Deny by default at the boundary: the firewall's outbound and inbound posture denying except allowed exceptions, from the rule table itself.
Common ways contractors fail SC.L2-3.13.6
- !Five points. Inbound deny by default is nearly universal; OUTBOUND deny by exception is the half shops miss. If full egress filtering is impractical, document the risk decision and the compensating monitoring honestly.
The step by step walkthrough for Microsoft 365 GCC High, Google Workspace, and on premises setups, plus the exact evidence to capture, lives inside the Level 2 Accelerator.
Prove SC.L2-3.13.6, and the other 109
The Level 2 Accelerator walks all 110 requirements with you, generates your SSP, POA&M, and Audit Room from real evidence, includes the full Level 1 platform, and puts a credentialed officer alongside you for 180 days. Filed in 180 days, or we work free until you are.
No credit card. Phase 2 begins Nov 10, 2026, when applicable DoD solicitations start requiring a current Level 2 status to win the award.
SC.L2-3.13.6 questions, answered
How many points is CMMC requirement SC.L2-3.13.6 worth?+
SC.L2-3.13.6 is worth 5 points in the CMMC Level 2 score under 32 CFR 170.24. If it is not met, you lose 5 from your total of 110.
Can SC.L2-3.13.6 be placed on a POA&M?+
No. SC.L2-3.13.6 must be fully met before you can file. It cannot be deferred to a POA&M, so it is a gate on your assessment.
What family does SC.L2-3.13.6 belong to?+
SC.L2-3.13.6 is in the System & Communications Protection (SC) family, one of the 14 families of NIST SP 800-171 that make up CMMC Level 2.
- NIST SP 800-171 Rev. 2 3.13.6