CVE-2026-88877

Traefik Proxy v3.7.0 through v3.7.11, when running the Kubernetes ingress-nginx provider, mishandles Ingresses that combine an authentication annotation (e.g. BasicAuth) with the nginx.ingress.kubernetes.io/from-to-www-redirect annotation, generating an extra 'sibling' router that matches on the host alone, carries only a RedirectRegex middleware, and points at the parent router's protected backend service. An attacker triggers the flaw by sending a request whose Host header carries a non-numeric or empty port (for example 'Host: www.example.com:x'), which the host matcher's canonicalization via net.SplitHostPort selects for the sibling router; the redirect pattern does not match, and because RedirectRegex is not a terminal handler the request is proxied straight to the backend with none of the Ingress's annotation-derived middlewares applied. The result is a complete bypass of authentication and of every other annotation-derived control on that route, including source-IP allowlisting, granting unauthenticated access to otherwise protected services. Only Traefik v3.7.0-v3.7.11 deployments using the Kubernetes ingress-nginx provider with Ingresses carrying both annotations are affected; Traefik v2 and v3 releases before v3.7.0 are not. The flaw is rated 9.3 (critical) under CVSS 4.0, is not in CISA's KEV, and there is no known public proof-of-concept or reported exploitation.

What to do: Upgrade to Traefik v3.7.12 or later. Where an immediate upgrade is not possible, remove the nginx.ingress.kubernetes.io/from-to-www-redirect annotation from any Ingress that also carries an authentication annotation, or enforce access control at another layer (e.g. network policies or an upstream gateway) until patched. Audit clusters for Ingresses combining these two annotations to identify which protected routes may be exposed to unauthenticated access.