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CVE-2026-86350 – HTTP/2 Request Smuggling / Header Mix-up – Apache Tomcat 9.0.118–9.0.121, 10.1.55–1

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Summary

CVE-2026-86350 is a critical HTTP request smuggling vulnerability (CWE-444: Inconsistent Interpretation of HTTP Requests) affecting Apache Tomcat. The flaw is a regression introduced by the fix for a prior vulnerability, CVE-2026-41293, and causes Tomcat to mix up HTTP/2 request headers between separate requests. The issue carries a CVSS v3.1 base score of 9.1 (Critical) and is remotely exploitable over the network without authentication.

Technical details

  • Root cause: a regression in the fix applied for CVE-2026-41293 reintroduces inconsistent interpretation of HTTP/2 requests within Tomcat’s request-processing code.
  • The regression can cause request headers from one HTTP/2 request to be mixed up with (attributed to) another request handled by the server.
  • Attack vector: network-based, no authentication or user interaction required, exploitable by sending crafted HTTP/2 requests to an affected Tomcat instance, particularly where requests may traverse a proxy or intermediary.
  • Impact: header mix-up between requests can allow an attacker to manipulate how request data is interpreted, potentially exposing or altering data intended for other requests/users (high confidentiality and integrity impact).

Affected software

  • Apache Tomcat 11.0.22 through 11.0.25
  • Apache Tomcat 10.1.55 through 10.1.59
  • Apache Tomcat 9.0.118 through 9.0.121

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: Upgrade to a fixed release — Apache Tomcat 11.0.26, 10.1.60, or 9.0.122, depending on the deployed branch.
  • If patching cannot be applied immediately: remove or disable the bundled examples web application on affected installations, and restrict/limit exposure of HTTP/2 endpoints where feasible until the upgrade can be completed.

References

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