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AWS Security Hub · NetworkFirewall

NetworkFirewall.5: Default stateless action (fragmented)

Written and reviewed by Emnode · Last reviewed

What does AWS Security Hub NetworkFirewall.5 check?

NetworkFirewall.5 checks a firewall policy's StatelessFragmentDefaultActions — the default applied to fragmented packets the stateless engine does not match. It passes when that default is aws:drop or aws:forward_to_sfe and fails when it is aws:pass.

Why does NetworkFirewall.5 matter?

IP fragmentation is a long-standing evasion technique: an attacker splits a payload across fragments so filters that only inspect the first fragment never see the malicious content, and the target reassembles it. A pass default on fragments is precisely the condition that makes this work against your network — a quiet hole in an otherwise tight policy.

How do I fix NetworkFirewall.5?

  1. Set the policy's fragmented-packet default action to aws:forward_to_sfe so fragments are inspected by the stateful engine.
  2. Use aws:drop if fragmented traffic has no legitimate role in your network.
  3. Re-check after editing — the control is change-triggered and re-evaluates on every policy edit.

Remediation script · bash

# Attach the AWS Managed Rules common baseline to an empty web ACL, in Count mode.
# update-web-acl REPLACES the entire Rules array, so supply the full desired set and the current LockToken.
aws wafv2 update-web-acl \
  --scope REGIONAL --name public-alb-waf --id a1b2c3d4-0000-1111-2222-3333 \
  --lock-token e4f5g6h7 --default-action Allow={} \
  --visibility-config SampledRequestsEnabled=true,CloudWatchMetricsEnabled=true,MetricName=public-alb-waf \
  --rules '[{"Name":"AWS-CommonRuleSet","Priority":0,"Statement":{"ManagedRuleGroupStatement":{"VendorName":"AWS","Name":"AWSManagedRulesCommonRuleSet"}},"OverrideAction":{"Count":{}},"VisibilityConfig":{"SampledRequestsEnabled":true,"CloudWatchMetricsEnabled":true,"MetricName":"AWS-CommonRuleSet"}}]'

# Associate a baseline web ACL with an unprotected API Gateway stage (the resource ARN is the stage, not the API).
aws wafv2 associate-web-acl \
  --web-acl-arn arn:aws:wafv2:us-east-1:111122223333:regional/webacl/prod-api-baseline/1a2b3c4d \
  --resource-arn arn:aws:apigateway:us-east-1::/restapis/a1b2c3d4e5/stages/prod

# Protect a Network Firewall from accidental deletion.
aws network-firewall update-firewall-delete-protection \
  --firewall-name prod-egress-inspection --delete-protection

Full walkthrough (console steps, edge cases and verification) in the lesson Protect APIs and edge with WAF.

Is NetworkFirewall.5 a false positive?

People tune the full-packet default (NetworkFirewall.4) carefully and leave the fragment default at pass because fragments feel like an edge case. The two defaults are independent and both must be non-pass.

Full lesson Protect APIs and edge with WAF Step-by-step remediation with CLI, code and persona-specific context.
AWS documentation for NetworkFirewall controls ↗ Check your own account for NetworkFirewall.5 →
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