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CVE-2026-88007 – Authentication Bypass / Backend Session Hijacking – Traefik 2.11.0–2.11.56 and 3.0.

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Summary

CVE-2026-88007 is an improper authentication / incorrect authorization vulnerability in Traefik, the open source HTTP reverse proxy and load balancer. When Traefik’s HTTP/3 entrypoint is enabled, it fails to isolate backend connection state the way the HTTP/1.1 and HTTP/2 paths do, allowing an unrelated client to inherit a backend connection that was authenticated for another user via NTLM or Negotiate (Kerberos). The issue carries a CVSS v4.0 base score of 9.1 (Critical) and lets an unauthenticated attacker read victim-only data and perform actions as the victim without ever presenting valid credentials.

Technical details

  • Root cause: The HTTP/3 request path does not call the connection-scoped transport initialization (AddTransportOnContext) that the HTTP/1.1 and HTTP/2 paths use to give each frontend connection its own backend transport. As a result, the kerberosRoundTripper falls back to a shared backend transport instead of a connection-dedicated one.
  • Trigger conditions: HTTP/3 must be enabled on the entrypoint, the backend must use connection-bound authentication (NTLM or Negotiate/Kerberos), and backend keep-alive must be in use. Backends using ordinary per-request authentication are not affected.
  • Attack vector: Network — no authentication or user interaction is required. A second, unrelated HTTP/3 client can be handed a backend connection that was already authenticated as a victim.
  • Impact: The attacker inherits the victim’s authenticated backend identity, enabling disclosure of victim-only data and execution of state-changing requests as the victim, without ever supplying the victim’s credentials.

Affected software

  • Traefik >= 2.11.0, < 2.11.57
  • Traefik >= 3.0.0, < 3.7.13

Severity

  • CVSS v4.0 Base Score: 9.1 (Critical)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to Traefik 2.11.57 or 3.7.13, which correctly initialize the connection-scoped backend transport for the HTTP/3 path.
  • If patching is not immediately possible: Disable HTTP/3 on entrypoints that front backends using connection-bound NTLM or Negotiate/Kerberos authentication, or disable backend keep-alive for those routes, until the upgrade can be applied.

References

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