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CVE-2026-85597 – mTLS/Client-Certificate Authentication Bypass – Traefik (< v2.11.55, v3.0.0–v3.7.10

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Summary

CVE-2026-85597 is a high-severity incorrect authorization vulnerability (CWE-863) in Traefik, an HTTP reverse proxy and load balancer. It allows an unauthenticated remote attacker to bypass client-certificate (mTLS) authentication on protected backends by exploiting how Traefik resolves conflicting TLS options across multi-host routers. The flaw carries a CVSS v3.1-equivalent severity of 8.2 (HIGH) and affects both the Traefik v2 and v3 release lines.

Technical details

  • Root cause: Traefik resolves and stores a single TLS options value per router/host-name rule rather than independently per host. When two routers on the same entry point serve the same host name with different TLS options, Traefik arbitrates the conflict by falling back to the default TLS options.
  • Trigger condition: For a multi-host router rule, this fallback is incorrectly applied to every host name in the rule, not only the one in conflict. A router enforcing client-certificate authentication (clientAuth) for one of its hosts stops enforcing it as soon as any other router creates a TLS option conflict on another host in the same rule.
  • Attack vector: Network-based (AV:N), no authentication or user interaction required; the attacker only needs to reach the exposed entry point and cause/exploit a TLS option conflict, which does not require valid client credentials.
  • Impact: Confidentiality impact only — unauthenticated clients without a valid client certificate can reach backends that were intended to be protected by mutual TLS. Single-host router configurations are not affected.

Affected software

  • Traefik v2 line: all versions prior to 2.11.55
  • Traefik v3 line: 3.0.0 through 3.7.12 (vendor advisory: 3.0.0–3.7.10, patched in 3.7.11; a follow-up release 3.7.12 addressed a related TLS option name collision)
  • Versions below v2.11 and below v3.7 are unmaintained and do not receive patches

Severity

  • CVSS Score: 8.2 (HIGH)
  • Vector (CVSS v4.0 as published): CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to Traefik v2.11.55 (v2 line) or v3.7.11 or later (v3 line, v3.7.12 recommended).
  • After upgrading, explicitly enable the new core.strictTLSOptions static configuration option — it defaults to false for backward compatibility. Enabling it disables the insecure fallback and instead marks conflicting routers as errors (failing closed) rather than silently weakening TLS enforcement.
  • If immediate patching is not possible: restructure multi-host router rules into separate single-host routers to eliminate ambiguous TLS option resolution across hosts sharing a rule.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Page title: Traefik, Traefik Proxy
  • content-security-policy response header: frame-src 'self' https://traefik.io https://*.traefik.io;

References

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