CVE-2026-29054: Traefik: lowercase `Connection` tokens can delete traefik-managed forwarded identity headers (for example, `X-Real-Ip`)

Impact

There is a potential vulnerability in Traefik managing the Connection header with X-Forwarded headers.

When Traefik processes HTTP/1.1 requests, the protection put in place to prevent the removal of Traefik-managed X-Forwarded headers (such as X-Real-Ip, X-Forwarded-Host, X-Forwarded-Port, etc.) via the Connection header does not handle case sensitivity correctly. The Connection tokens are compared case-sensitively against the protected header names, but the actual header deletion operates case-insensitively. As a result, a remote unauthenticated client can use lowercase Connection tokens (e.g. Connection: x-real-ip) to bypass the protection and trigger the removal of Traefik-managed forwarded identity headers.

This is a bypass of the fix for CVE-2024-45410.

Depending on the deployment, the impact may be higher if downstream services rely on these headers (such as X-Real-Ip or X-Forwarded-) for authentication, authorization, routing, or scheme decisions.

Patches

- https://github.com/traefik/traefik/releases/tag/v2.11.38 - https://github.com/traefik/traefik/releases/tag/v3.6.9

Workarounds

No workaround available.

For more information

If there are any questions or comments about this advisory, please open an issue.

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<details> <summary>Original Description</summary>

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/forwardedheader.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):

unzip poc.zip -d poc cd poc make test

Output contains:

[CALLSITEHIT]: pkg/middlewares/forwardedheaders/forwardedheader.go:225 [PROOFMARKER]: downstreamadminbypass=1 xrealippresent=0

Control (same env, no lowercase token):

unzip poc.zip -d poc cd poc make test

Output contains:

[CALLSITEHIT]: pkg/middlewares/forwardedheaders/forwardedheader.go:225 [NCMARKER]: downstreamadminbypass=0 xrealippresent=1

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.

</details>