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CVE-2026-77762 – HTTP/2 Trailer Injection (Race Condition) – Apache Tomcat 8.5.59–8.5.100, 9.0.39–9.

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Summary

CVE-2026-77762 is a race condition (CWE-362) in Apache Tomcat’s HTTP/2 implementation in which a stale HPACK emitter can inject trailer fields into a different, recycled pooled Request object. Under concurrent HTTP/2 traffic, this improper synchronization can cause trailer header data from one request to be attributed to another request being processed on the same connection pool. The issue is rated High severity (CVSS 8.1) and affects Apache Tomcat 8.5.x, 9.0.x, 10.1.x, and 11.0.x.

Technical details

  • Root cause: Tomcat’s HTTP/2 HPACK trailer emitter does not properly synchronize access to pooled/recycled Request objects, allowing a "stale" emitter instance to write trailer header fields into a Request object that has since been reused for a different, unrelated HTTP/2 request.
  • Trigger conditions: Occurs under concurrent execution — multiple HTTP/2 streams/requests processed against the same connection where Tomcat recycles Request objects from a pool, with timing that allows a stale emitter to still be active.
  • Attack vector: Network-accessible; attack complexity is High (the race condition must be reliably triggered), no privileges or user interaction are required.
  • Impact: Successful exploitation can result in trailer field data being misattributed to another client’s request, with the CVSS vector indicating potential impact to confidentiality, integrity, and availability.

Affected software

  • Apache Tomcat 11.0.0-M1 through 11.0.25
  • Apache Tomcat 10.1.0-M1 through 10.1.59
  • Apache Tomcat 9.0.39 through 9.0.121
  • Apache Tomcat 8.5.59 through 8.5.100 (end-of-life at time of disclosure, confirmed affected)

Severity

  • CVSS v3.1 Base Score: 8.1 (High)
  • Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade to Apache Tomcat 11.0.26, 10.1.60, or 9.0.122, which contain the fix for this issue.
  • If no patch can be applied immediately: Apache Tomcat 8.5.x is end-of-life and does not have an official fixed release; organizations still running 8.5.59–8.5.100 should prioritize migration to a supported, patched branch (9.0.122, 10.1.60, or 11.0.26). Where upgrading is not immediately possible, restrict or monitor HTTP/2 usage on exposed Tomcat instances and ensure network-layer controls limit exposure until patched.

References

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