Traefik HTTP/3 Backend NTLM Connection Reuse

Details

The issue is caused by a protocol-parity gap between the normal TCP HTTP entrypoint path and the HTTP/3 entrypoint path.

For HTTP/1.1 and HTTP/2, Traefik explicitly creates a connection-scoped holder that can later store a dedicated RoundTripper for NTLM or Negotiate:

// pkg/server/server_entrypoint_tcp.go:691-703
var connContext multipleConnContext
connContext.AddConnContextFunc(func(ctx context.Context, c net.Conn) context.Context {
	// This adds an empty struct in order to store a RoundTripper in the ConnContext in case of Kerberos or NTLM.
	ctx = service.AddTransportOnContext(ctx)

	if tlsConn, ok := c.(*tls.Conn); ok {
		if tlsConnWithOptionsName, ok := tlsConn.NetConn().(tcp.TLSConn); ok {
			return tcp.AddTLSOptionsNameInContext(ctx, tlsConnWithOptionsName.TLSOptionsName)
		}
	}

	return ctx
})

That helper installs the per-connection holder, and kerberosRoundTripper depends on it. If the holder is absent, it falls back to the shared original backend transport. If NTLM or Negotiate is detected, it stores a dedicated cloned RoundTripper into that holder so future requests stay on the authenticated backend connection:

// pkg/server/service/transport.go:374-402
func AddTransportOnContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, transportKey, &stickyRoundTripper{})
}

type kerberosRoundTripper struct {
	new                  func() http.RoundTripper
	OriginalRoundTripper http.RoundTripper
}

func (k *kerberosRoundTripper) RoundTrip(request *http.Request) (*http.Response, error) {
	value, ok := request.Context().Value(transportKey).(*stickyRoundTripper)
	if !ok {
		return k.OriginalRoundTripper.RoundTrip(request)
	}

	if value.RoundTripper != nil {
		return value.RoundTripper.RoundTrip(request)
	}

	resp, err := k.OriginalRoundTripper.RoundTrip(request)

	// If we found that we are authenticating with Kerberos (Negotiate) or NTLM.
	// We put a dedicated roundTripper in the ConnContext.
	// This will stick the next calls to the same connection with the backend.
	if err == nil && containsNTLMorNegotiate(resp.Header.Values("WWW-Authenticate")) {
		value.RoundTripper = k.new()
	}
	return resp, err
}

For HTTP/3, the server reuses the normal HTTPS handler chain, but its ConnContext only propagates the TLS options name and does not call service.AddTransportOnContext:

// pkg/server/server_entrypoint_tcp_http3.go:65-80
h3.Server = &http3.Server{
	Addr:      config.GetAddress(),
	Port:      config.HTTP3.AdvertisedPort,
	Handler:   httpsServer.Server.(*http.Server).Handler,
	TLSConfig: &tls.Config{GetConfigForClient: h3.getTLSConfigForClient},
	QUICConfig: &quic.Config{
		Allow0RTT: false,
	},
	ConnContext: func(ctx context.Context, c *quic.Conn) context.Context {
		tlsOptionsName, err := h3.getTLSOptionsName(c)
		if err != nil {
			log.Error().Msgf("Error getting TLS options name for client: %v", err)
			return ctx
		}
		return tcp.AddTLSOptionsNameInContext(ctx, tlsOptionsName)
	},
}

This means HTTP/3 requests reach the same reverse-proxy and backend transport logic as HTTPS, but without the connection-scoped transport holder that NTLM and Negotiate isolation relies on.

In practice, the flow is:

1. A victim authenticates through Traefik to a backend that binds identity to the backend TCP connection using NTLM or Negotiate.
2. Because the HTTP/3 request context does not contain transportKey, kerberosRoundTripper uses the shared OriginalRoundTripper.
3. No frontend-connection-specific dedicated backend pool is installed for that HTTP/3 client.
4. A second unrelated HTTP/3 client can be assigned the same backend TCP connection after the victim has authenticated it.
5. That second client inherits the victim's backend identity without sending the victim's credentials.

The attached verifier demonstrates both the negative control and the exploit path:

• HTTPS/HTTP/1.1 control case: the attacker uses a separate frontend connection and correctly receives 401
• HTTP/3 exploit case: the attacker uses a separate HTTP/3 client with no Authorization header, reads resource=secret actor=victim, executes action=transfer actor=victim to=attacker amount=5000, and hits the same backend TCP connection identifier as the victim

PoC

See the reproduction materials at: https://gist.github.com/OneZ3r0/41da8e8b79ebbe444a94f8a2a3a30895

The gist can also be downloaded as a ZIP archive.

Files included in this gist:

• run.sh
• Dockerfile
• .dockerignore
• go.mod
• go.sum
• verify.go

The package is intentionally kept as a single-container reproduction:

1. run.sh builds a local image for the pinned target commit
2. the Dockerfile builds both Traefik and the verifier during image build
3. the container runs the verifier directly as its entrypoint
4. the verifier starts a synthetic backend, launches Traefik, runs the HTTPS/HTTP/1.1 control case, then runs the HTTP/3 exploit case

Run:

./run.sh

run.sh defaults to the validated commit above. To override it explicitly:

PRODUCT_COMMIT=f2d0794417e4d06343e6e7c4722143f5b34bee45 ./run.sh

Expected terminal result:

REPRODUCED: HTTP/1.1 isolates the authenticated backend connection, but HTTP/3 reuses the victim-authenticated backend connection for a different client and executes an unauthorized state-changing request as the victim.

Important observed behavior from the PoC:

• the HTTP/1.1 control case succeeds only if a fresh attacker connection receives 401
• the HTTP/3 exploit case succeeds only if the attacker reads victim-only data without sending Authorization
• the HTTP/3 exploit case succeeds only if the attacker performs /transfer?to=attacker&amount=5000 as actor=victim
• the HTTP/3 exploit case succeeds only if the attacker uses the same backend TCP connection identifier as the victim

Environment notes:

• Docker is required
• the build fetches the target Traefik source from GitHub
• no production credentials or external NTLM service are required; the verifier includes a synthetic NTLM-like backend specifically to demonstrate connection-bound identity reuse

Impact

This is a cross-client authorization bypass affecting deployments that expose HTTP/3 routes to backends using connection-bound NTLM or Negotiate authentication with persistent backend connection reuse.

In the verified reproduction, an unauthenticated second client can:

• read victim-only data
• perform a state-changing action as the victim
• reuse a backend TCP connection that has already been authenticated as the victim

Attack prerequisites:

• HTTP/3 enabled on the Traefik entrypoint
• a routed backend using connection-bound NTLM or Negotiate authentication
• backend keep-alive and backend connection reuse enabled
• the attacker can reach the same route as the victim

Deployments using ordinary per-request authentication are not affected by this specific issue.