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CVE-2026-88009 – Path-Scoped Authorization & Routing Bypass – Traefik < 2.11.57 / 3.0.0–3.7.1

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Summary

CVE-2026-88009 is a vulnerability in Traefik, the popular reverse proxy and load balancer, caused by inconsistent interpretation of HTTP/1.x request targets between Traefik’s routing engine and backend servers. Traefik affected by this issue evaluates routing, authorization, and logging decisions against a normalized path of "/" while forwarding the attacker’s original, unmodified request target to the backend, allowing path-scoped routing, middleware, and access controls to be bypassed. The issue carries a CVSS v4.0 base score of 8.8 (High) and requires no authentication or user interaction to exploit.

Technical details

  • Root cause: Traefik accepts HTTP/1.x requests whose request-target is in "rootless"/opaque form (e.g. GET http:http://internal-vhost/admin HTTP/1.1). Go’s URL parser stores this as URL.Opaque with an empty URL.Path, which Traefik normalizes to / for all routing, path-sanitization, middleware, and access-log decisions — while the proxy forwards the original opaque target byte-for-byte to the backend.
  • Trigger conditions: A remote, unauthenticated attacker sends a crafted HTTP/1.x request with a rootless/opaque request-target to a Traefik entrypoint using default settings. Exploitation impact depends on the backend server behavior — the backend must accept and interpret the opaque target as a path (confirmed with fasthttp/Fiber and Python’s http.server; most other tested servers such as nginx, Apache, Tomcat 9, and Node.js reject it with a 400 error).
  • Attack vector: Network, no privileges or user interaction required.
  • Impact: Cross-vhost routing bypass (host-only routers matching unintended backends), authorization bypass (path-scoped forwardAuth policies fail open against opaque targets), and access-log evasion (all such requests are recorded identically as GET / HTTP/1.1). The encodedCharacters.allowEncodedSlash=false hardening does not detect the equivalent payload delivered via an opaque target.

Affected software

  • Traefik v2.x through v2.11.56
  • Traefik v3.0.0 through v3.7.12

Severity

  • CVSS v4.0 Base Score: 8.8 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to Traefik v2.11.57 or v3.7.13, which deny requests carrying an opaque request-target.
  • If immediate patching is not possible: Review backend servers for lenient handling of opaque/rootless HTTP request-targets, and do not rely on Traefik path-scoped routing, forwardAuth policies, or access logs as the sole authorization or audit control until patched. Note that Traefik v3.0–v3.6 branches are end-of-life and should be upgraded to v3.7.13 or later.

References

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