Summary

The ForwardAuth middleware, even when configured with trustForwardHeader: false, still derives the X-Forwarded-Port header sent to the authentication service by reading the attacker-controlled X-Forwarded-Proto header from the original incoming request. This allows an unauthenticated remote attacker to cause Traefik to forward X-Forwarded-Port: 443 to the auth service on a plain HTTP connection, creating an inconsistency that can bypass port-based authorization checks.

Details

The fix introduced in commit 5e1de2258 (released as part of the April 2026 security advisory GHSA-6384-m2mw-rf54) correctly strips all X-Forwarded-* headers from the forwarded auth request when trustForwardHeader=false, and reconstructs X-Forwarded-Proto from the actual TLS state of the connection (req.TLS).

However, the reconstruction of X-Forwarded-Port is delegated to the helper forwardedPort(req) which receives the original request (req) rather than the sanitized forward request (forwardReq):

// pkg/middlewares/auth/forward.go – writeHeader()
if !trustForwardHeader {
    forwardedheaders.DeleteXForwardedHeaders(forwardReq.Header) // strips all X-Fwd-* from forwardReq
}
// ...
if _, ok := forwardReq.Header[forwardedheaders.XForwardedPort]; !ok {
    forwardReq.Header.Set(forwardedheaders.XForwardedPort, forwardedPort(req)) // ← req = ORIGINAL
}...
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The inconsistency between Proto=http and Port=443 is exploitable against any authentication service that gates access based on X-Forwarded-Port.

PoC

Traefik configuration:

  middlewares:
    my-auth:
      forwardAuth:
        address: "http://auth-service/"
        trustForwardHeader: false  # security setting, but still bypassable
  routers:
    api:
      rule: "PathPrefix(`/api`)"...
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Attack:

Plain HTTP connection, no TLS – but spoofs port 443 curl -H "X-Forwarded-Proto: https" http://traefik.example.com/api/admin Auth service receives X-Forwarded-Port: 443 → grants access

Verification: Enable Traefik debug logging and observe X-Forwarded-Port: 443 in the auth request while the connection is plain HTTP.

Impact

Any deployment using the ForwardAuth middleware with trustForwardHeader: false where the downstream authentication service uses X-Forwarded-Port to make authorization decisions is vulnerable to privilege escalation. An unauthenticated attacker can bypass port-based security checks (e.g., "only allow requests arriving on HTTPS port 443") by injecting a single X-Forwarded-Proto: https header on a plain HTTP connection.

This is a regression of the incomplete fix for GHSA-6384-m2mw-rf54: while the X-Forwarded-Prefix and X-Forwarded-Proto spoofing vectors were addressed, the X-Forwarded-Port vector was missed.