GHSA-rf44-j88r-hh8c Traefik: ForwardAuth identity spoofing via dot-form header alias

• Go github.com/traefik/traefik/v2: < 2.11.56 → fixed in 2.11.56 • Go github.com/traefik/traefik/v3: >= 3.0.0, < 3.7.12 → fixed in 3.7.12

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. Original Description 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 `underscoreHeadersStrategy: delete`. The tested PHP 8.2 built-in SAPI maps `X-Authenticated-User` and `X.Authenticated.User` to the same `HTTP_X_AUTHENTICATED_USER` server variable. In Traefik's tested HTTP/1 backend path, the client value is serialized last and overrides the identity asserted by ForwardAuth. A client whom ForwardAuth permits as a lower-privilege identity can therefore be treated by the backend as another user or role. Details At `v3.7.10`, `pkg/server/server_entrypoint_tcp.go:800-818` removes or rejects only names containing `_`. After successful authentication, `pkg/middlewares/auth/forward.go:314-326` deletes and replaces only the canonical `authResponseHeaders` key. The dot alias remains in `req.Header` and the standard reverse proxy forwards both legal field names. Go's HTTP/1 writer sorts header names lexically, placing `X-Authenticated-User` before `X.Authenticated.User`. PHP then collapses both into one `$_SERVER` key, so the attacker value deterministically wins. This is an incomplete-fix sibling of `GHSA-x677-9fxg-v5c5`: the published underscore input is blocked by the new entry-point strategy, while the dot input bypasses that mitigation on the current stable release. PoC traefik-dot-forwardauth-poc.zip run: ```bash docker compose up -d bash verify.sh docker compose down ``` The decisive request is: ```http GET /probe HTTP/1.1 Host: 127.0.0.1:18080 X.Authenticated.User: admin Connection: close ``` Expected backend identity: `lab-user`, as returned by ForwardAuth. Observed on `v3.7.10`: `admin`. The script also verifies that requests without the alias, with the canonical header, and with the already-fixed underscore alias all produce `lab-user`. Impact Applications that authorize requests using a ForwardAuth-provided identity header can receive an attacker-selected username or role instead. This was runtime-verified with PHP 8.2.27 and 8.2.33; impact on other normalization-prone backends is conditional. A lower-privilege permitted client may consequently impersonate another user or administrative role, affecting confidentiality and integrity. This report does not claim bypass of a ForwardAuth denial: the auth service must first permit the request. ---