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CVE-2026-53434 – Authentication Bypass via CRL Revocation Bypass – Apache Tomcat 9.0.83–9.0.118, …

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Summary

CVE-2026-53434 is a "Detection of Error Condition Without Action" vulnerability in Apache Tomcat affecting all three actively maintained release branches. When the FFM (Foreign Function & Memory) based connector is configured with invalid or malformed Certificate Revocation Lists (CRLs), Tomcat silently ignores those CRLs rather than failing closed — allowing revoked client certificates to be accepted. With a CVSS v3.1 base score of 9.1 (Critical), this flaw constitutes an authentication bypass for any deployment relying on mutual TLS with CRL-based client certificate revocation via the FFM connector.

Technical details

  • Root cause: Tomcat’s FFM connector fails to detect and act on an error condition when configured CRLs are invalid or malformed. Instead of halting or rejecting the misconfigured revocation check, Tomcat silently discards the CRL and proceeds with TLS handshake completion.
  • Trigger conditions: The vulnerability is only triggerable when: (1) the FFM-based connector is explicitly configured (non-default), and (2) CRL-based certificate revocation is enabled on that connector. Standard connectors (NIO, APR) and deployments without CRL validation are not affected.
  • Attack vector: Network-accessible; the affected component is a TLS/HTTPS listener reachable over the internet. No user interaction or prior privileges are required per the CVSS vector. In practice, an attacker must possess a certificate that was issued by the trusted Certificate Authority and has been subsequently revoked.
  • Impact: Revoked client certificates pass validation unchecked, effectively bypassing the server’s authentication boundary. This enables unauthorized access to resources protected by mutual TLS client certificate authentication, with high impact to both Confidentiality and Integrity (CVSS C:H, I:H).

Note on severity ratings: Apache’s own advisory rates this vulnerability Low, reflecting the realistic exploitation barrier — the attacker must already hold a certificate previously issued by the trusted CA. NVD’s CVSS score of 9.1 represents the theoretical maximum impact under the CVSS methodology. Organizations using Tomcat with mutual TLS and CRL-based revocation on the FFM connector should treat this as a priority patch.

Affected software

  • Apache Tomcat 11.x: 11.0.0-M1 through 11.0.22
  • Apache Tomcat 10.1.x: 10.1.0-M7 through 10.1.55
  • Apache Tomcat 9.0.x: 9.0.83 through 9.0.118

Severity

CVSS v3.1 Base Score: 9.1 (Critical)
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 patched release:
    • Apache Tomcat 11.0.23 or later
    • Apache Tomcat 10.1.56 or later
    • Apache Tomcat 9.0.119 or later
  • If patching is not immediately possible:
    • Disable the FFM-based connector and revert to the standard NIO connector, which is not affected by this flaw.
    • Alternatively, remove CRL configuration from the FFM connector to eliminate the vulnerable code path until an upgrade can be applied.
    • Restrict network access to Tomcat HTTPS endpoints to trusted client networks where certificate issuance is tightly controlled.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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