CVE-2026-88008

Traefik Proxy versions from 2.11.26 until 2.11.57, and 3.x versions before 3.7.13, incorrectly forward the client-supplied hop-by-hop headers 'Connection: Upgrade', 'Upgrade: h2c', and 'HTTP2-Settings' to shared backend services. If a backend accepts the h2c (cleartext HTTP/2) upgrade and returns 101 Switching Protocols, Traefik enters a raw tunnel and stops applying routers, BasicAuth, ForwardAuth, IPAllowList, RateLimit middleware, and access logging, metrics, and tracing to subsequent HTTP/2 requests on that connection. An unauthenticated attacker can therefore send a request through an unprotected route to establish the tunnel and then reach protected paths on the same backend without credentials - an authentication/authorization bypass (CWE-444, CWE-863) rated 7.0 High in CVSS 4.0. Only deployments where a backend accepts h2c upgrades and where both protected and unprotected routes point to that same backend are exploitable, an attack requirement (AT:P) that excludes many installs. There is no public proof-of-concept, the flaw is not in the CISA KEV catalog, and no in-the-wild exploitation is known; it is fixed in 2.11.57 and 3.7.13.

What to do: Upgrade to Traefik 2.11.57 (2.x) or 3.7.13 (3.x) or later. Until patched, determine whether any backends accept cleartext HTTP/2 upgrade requests and whether protected and unprotected routes share the same backend; disable h2c on backends or ensure no unauthenticated route shares a backend with protected paths to mitigate. No public exploit exists yet, so patching before one appears should be the priority.