GHSA-92mv-8f8w-wq52

Traefik's XForwarded middleware (removeConnectionHeaders) tries to prevent clients from using the Connection header to strip trusted X-Forwarded-* headers, but the protection compares the Connection tokens case-sensitively while the deletion is case-insensitive.

As a result, a remote unauthenticated client can send a lowercase token like Connection: x-real-ip and still trigger deletion of traefik-managed X-Real-Ip (and similarly named headers in the managed list).

This can cause downstream routing, scheme, and header-based authn/authz decisions to be evaluated with missing trusted forwarding identity headers.

Severity

CRITICAL

Rationale: the PoC demonstrates an end-to-end access control bypass pattern when a downstream service uses proxy-provided identity headers (for example, X-Real-Ip) for IP allowlists or trust decisions. A remote unauthenticated client can strip the traefik-managed identity header via a lowercase Connection token, causing the downstream service to evaluate the request without the expected header signal.

Relevant Links

• Repository: https://github.com/traefik/traefik
• Pinned commit: a4a91344edcdd6276c1b766ca19ee3f0e346480f
• Callsite (pinned): https://github.com/traefik/traefik/blob/a4a91344edcdd6276c1b766ca19ee3f0e346480f/pkg/middlewares/forwardedheaders/forwarded_header.go#L225

Vulnerability Details

Root Cause

removeConnectionHeaders uses a case-sensitive membership check for protected header names when inspecting Connection tokens, but it deletes headers via net/http which treats header names case-insensitively. A lowercase token bypasses the protection check and still triggers deletion.

Attacker Control / Attack Path

Remote unauthenticated HTTP client (untrusted IP) sends Connection: x-real-ip, and Traefik deletes the generated X-Real-Ip header.

Proof of Concept

The attached poc.zip contains a deterministic, make-based integration PoC with a canonical run and a negative control.

Canonical (vulnerable):

Output contains:

Control (same env, no lowercase token):

Output contains:

Expected: Connection tokens are handled case-insensitively and protected identity headers (for example, X-Real-Ip and X-Forwarded-*) are not deleted due to client-supplied Connection options (regardless of token casing).

Actual: Lowercase Connection tokens bypass the protection check and still trigger deletion of traefik-managed identity headers (for example, X-Real-Ip).

Recommended Fix

• Case-fold (or otherwise canonicalize) Connection header tokens before comparing them against protected header names.
• Add a regression test covering lowercase tokens (for example, Connection: x-real-ip).

Fix accepted when: a request with Connection: x-real-ip does not cause deletion of traefik-managed X-Real-Ip, and a regression test covers this behavior.