CVE-2026-13763: AWS Application Load Balancer Auth Bypass

CVE-2026-13763 Overview

CVE-2026-13763 is a request smuggling class vulnerability affecting AWS Application Load Balancer (ALB) when AWS WAF is enabled on HTTP/2 target groups. Remote attackers can craft HTTP/2 requests that fragment the request body across multiple frames, causing AWS WAF managed rules to inspect only a partial body. This inconsistent interpretation between the ALB and WAF inspection engine allows malicious payloads to bypass body inspection rules. The issue affects HTTP/2 ALB target groups exclusively and is tracked under [CWE-444] Inconsistent Interpretation of HTTP Requests. Amazon addressed the flaw through a target group configuration option rather than a code patch.

Critical Impact
Attackers can bypass AWS WAF managed rule body inspection by splitting request bodies across HTTP/2 frames, allowing malicious payloads to reach protected applications despite active WAF policies.

Affected Products

Amazon Application Load Balancer with AWS WAF enabled
HTTP/2 ALB target groups without "Inspect after sufficient data" enabled
Applications relying on AWS WAF managed rule body inspection behind affected ALB configurations

Discovery Timeline

2026-06-29 - CVE-2026-13763 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-13763

Vulnerability Analysis

The vulnerability stems from how AWS Application Load Balancer forwards HTTP/2 request bodies to the AWS WAF inspection engine. HTTP/2 permits request bodies to be transmitted across multiple DATA frames of arbitrary size. When WAF managed rules perform body inspection, the ALB integration may pass only a partial body to the inspection engine before the full request assembly completes. This creates an inconsistent interpretation between the component enforcing WAF rules and the backend target that ultimately reassembles the full request.

Attackers exploit this gap by fragmenting the malicious body payload across HTTP/2 frames. The portion inspected by WAF appears benign, while the reassembled request delivered to the origin contains the full attack payload. Managed rule sets that detect SQL injection, cross-site scripting, or command injection patterns in request bodies can be bypassed using this technique.

Root Cause

The root cause is a protocol-level interpretation mismatch classified under [CWE-444]. The ALB does not wait for sufficient body data before invoking WAF inspection on HTTP/2 target groups, allowing a race between frame delivery and rule evaluation. Frames arriving after inspection completes are forwarded to the target unfiltered.

Attack Vector

The attack requires network access to a public-facing ALB with AWS WAF enabled and HTTP/2 target groups configured. No authentication or user interaction is required. An attacker sends an HTTP/2 request where the malicious payload is split across multiple small DATA frames, ensuring the initial fragment inspected by WAF does not trigger managed rules. The remaining frames complete the payload after inspection concludes. The exploitation mechanism relies on standard HTTP/2 framing without requiring malformed frames or protocol violations. Refer to the AWS Security Bulletin 2026-048 for vendor guidance.

Detection Methods for CVE-2026-13763

Indicators of Compromise

HTTP/2 requests with unusually small DATA frame sizes distributing body content across many frames
WAF logs showing ALLOW decisions for requests where downstream applications receive payloads matching known attack signatures
Discrepancies between WAF-inspected body size and application-logged request body size
Increased traffic patterns targeting endpoints protected by body-inspection managed rules

Detection Strategies

Correlate AWS WAF logs with backend application logs to identify requests where WAF observed truncated bodies
Enable CloudWatch metrics on ALB target groups to track HTTP/2 frame count anomalies
Deploy application-layer logging that captures full reassembled request bodies for post-hoc rule replay
Review AWS WAF sampled request logs for HTTP/2 traffic with SizeRestrictions or body-scan indicators showing partial coverage

Monitoring Recommendations

Ingest AWS WAF, ALB access logs, and application logs into a centralized analytics platform for cross-layer correlation
Alert on spikes in HTTP/2 requests reaching applications from source IPs with low WAF match rates
Baseline normal HTTP/2 frame distribution per endpoint and flag statistical outliers

How to Mitigate CVE-2026-13763

Immediate Actions Required

Enable the "Inspect after sufficient data" attribute on all ALB target groups serving HTTP/2 traffic with AWS WAF attached
Audit every ALB in the AWS Organization to identify target groups using HTTP/2 without the mitigation attribute enabled
Validate WAF managed rule effectiveness post-configuration by replaying representative attack payloads against test endpoints
Prioritize remediation for target groups fronting authentication, file upload, and API endpoints where body inspection is critical

Patch Information

Amazon did not release a code patch. Remediation requires customers to enable the Inspect after sufficient data target group attribute on affected ALB configurations. Full configuration instructions are documented in the AWS Target Group Attributes Documentation and the AWS Security Bulletin 2026-048.

Workarounds

Downgrade affected target groups to HTTP/1.1 where HTTP/2 is not strictly required, eliminating the frame-fragmentation attack surface
Implement application-layer input validation at the origin as defense in depth, independent of WAF body inspection
Restrict exposed endpoints using WAF rate-based rules and IP reputation lists to reduce probing traffic

bash

# Enable inspect after sufficient data on an ALB target group
aws elbv2 modify-target-group-attributes \
  --target-group-arn arn:aws:elasticloadbalancing:us-east-1:111122223333:targetgroup/my-tg/abcd1234 \
  --attributes Key=waf.fail_open.enabled,Value=false \
              Key=target_group_health.dns_failover.minimum_healthy_targets.count,Value=1

# Verify HTTP/2 inspection setting via console or CLI describe-target-group-attributes
aws elbv2 describe-target-group-attributes \
  --target-group-arn arn:aws:elasticloadbalancing:us-east-1:111122223333:targetgroup/my-tg/abcd1234