CVE-2026-88011

## Summary There is a medium severity vulnerability in Traefik's handling of request headers whose name aliases another header name. Go canonicalizes header names on dashes only, so `X-Auth-User`, `X_Auth_User` and `X.Auth.User` are three distinct headers to Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX, and others) collapse all of them into the same variable. A client can therefore smuggle an alias of a header that Traefik manages past the middleware managing it — for example a dot-form `X.Authenticated.User` alongside the canonical `X-Authenticated-User` written by the ForwardAuth middleware — and have such a backend read the client-supplied value instead of the identity Traefik asserted. Any header Traefik sets is exposed, not only ForwardAuth's. This is an incomplete-fix sibling of GHSA-x677-9fxg-v5c5, which blocked only the underscore form. The mitigation is the new `aliasHeadersStrategy` entry point option. It defaults to `keep`, which preserves the previous behavior for backwards compatibility, so it must be explicitly set to `delete` or `reject` to take effect. Traefik v1.x, the v2 releases up to v2.11.55 and the v3 releases from v3.0.0 to v3.7.11 are affected. The unmaintained lines among them will not receive a patch of their own, and the remedy for their users is to upgrade to v2.11.56 or v3.7.12 and set `aliasHeadersStrategy`. ## Patches - https://github.com/traefik/traefik/releases/tag/v2.11.56 - https://github.com/traefik/traefik/releases/tag/v3.7.12 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> ### Summary Traefik's ForwardAuth middleware removes the configured canonical identity header before copying the value returned by the auth service. However, a client-supplied dot-form alias such as `X.Authenticated.User` survives both this replacement and