Executive brief
Traefik is a widely-used HTTP reverse proxy and load balancer for Kubernetes. In affected versions, a specific annotation combination causes the system to create an extra route that bypasses all security middlewares, including authentication (BasicAuth, IP allowlisting) and forwards requests directly to protected backends. An attacker can exploit this by sending requests with malformed port numbers in the Host header, completely circumventing login requirements and access controls.
Technical details
The vulnerability is an authorization bypass in Traefik's Kubernetes ingress-nginx provider. When an Ingress resource carries both an authentication annotation (e.g., basic-auth) and the nginx.ingress.kubernetes.io/from-to-www-redirect annotation, the provider creates a "sibling" router that matches only on the host with a RedirectRegex middleware but points to the same protected backend service. RedirectRegex is not a terminal handler, so unmatched requests pass through to the backend. The redirect pattern only accepts numeric ports (regex: (https?)://[^/:]+(:[0-9]+)?/(.*)), but Traefik's host matcher canonicalizes authorities via net.SplitHostPort, which accepts non-numeric ports. A request with a malformed port (e.g., Host: www.example.com:x) bypasses the redirect pattern, selects the sibling router lacking all middlewares, and reaches the protected backend unauthenticated. This affects all annotation-derived middlewares: authentication, source-IP allowlisting, and others. Versions >= v3.7.0 and <= v3.7.11 are vulnerable; the fix is available in v3.7.12.
Affected products
- Traefik Traefik >= v3.7.0, <= v3.7.11
Timeline
- 2026-09-10: disclosed: CVE-2026-88877 published
- 2026: patched: Fixed in v3.7.12