GitHub Security Advisory: GHSA-5m6w-wvh7-57vm
Package: github.com/traefik/traefik/v2 (GO)
Severity: HIGH (CVSS 10.0)
CWE: CWE-290: Authentication Bypass by Spoofing, CWE-306: Missing Authentication for Critical Function
Vulnerable versions: < 2.11.43
Patched in: 2.11.43

Summary: Traefik: Pre-authentication decision bypass due to forwarded alias spoofing

Details:
## Summary

There is a high severity authentication bypass vulnerability in Traefik's `ForwardAuth` and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only canonical header names (e.g., `X-Forwarded-Proto`) and does not strip or normalize alias variants that use underscores instead of dashes (e.g., `X_Forwarded_Proto`). These unsanitized alias headers are forwarded intact to the authentication backend. When the backend normalizes underscore and dash header forms equivalently, an attacker can inject spoofed trust context — such as a trusted scheme or host — through the alias headers and bypass authentication on protected routes without valid credentials.

## Patches

- https://github.com/traefik/traefik/releases/tag/v2.11.43
- https://github.com/traefik/traefik/releases/tag/v3.6.14
- https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2

## For more information

If there are any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).

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

### Summary
An authentication bypass arises from chaining two bugs: incomplete forwarded-header sanitization at ingress and overly permissive header forwarding in pre-auth subrequests. While canonical `X-Forwarded-*` headers are handled, alias variants (e.g., underscore forms) are neither normalized nor stripped consistently. When downstream auth services normalize these headers, attackers can inject trusted context and bypass authentication on protected routes without credentials.

### Details
This issue results from the interaction between forwarded-header handling and auth subrequest construction, creating a trust boundary mismatch.

At ingress, Traefik defines a fixed set of canonical forwarded headers (`X-Forwarded-Proto`, `X-Forwarded-For`, etc.):

Reference : [`pkg/middlewares/forwardedheaders/forwarded_header.go#L29-L36`](https://github.com/traefik/traefik/blob/174e5d81111d8e9fb3d3c81cf6d22f3e33eb4f78/pkg/middlewares/forwardedheaders/forwarded_header.go#L29-L36)

```go
var xHeaders = []string{
	xForwardedProto,
	xForwardedFor,
	xForwardedHost,
	xForwardedPort,
```

This logic focuses exclusively on canonical header names and does not account for alias forms such as `X_Forwarded_Proto`. As a result, while standard headers may be sanitized or rewritten, semantically equivalent variants can pass through unchanged.

During ForwardAuth processing, request headers are copied wholesale into the auth subrequest:

Reference : [`pkg/middlewares/auth/forward.go#L401-L408`](https://github.com/traefik/traefik/blob/174e5d81111d8e9fb3d3c81cf6d22f3e33eb4f78/pkg/middlewares/auth/forward.go#L401-L408)

```go
utils.CopyHeaders(forwardReq.Header, req.Header)
RemoveConnectionHeaders(forwardReq)
utils.RemoveHeaders(forwardReq.Header, hopHeaders...)
```

This implementation forwards nearly all client-supplied headers to the auth backend, with filtering limited to hop-by-hop h