GHSA-3g6v-2r68-prfc Traefik: Kubernetes Gateway crossProviderNamespaces bypass allows HTTPRoute outside the allowlist to expose internal Traefik services

• Go github.com/traefik/traefik/v3: < 3.6.21 → fixed in 3.6.21 • Go github.com/traefik/traefik/v2: <= 2.11.50 (no fix yet) • Go github.com/traefik/traefik: <= 1.7.34 (no fix yet) • Go github.com/traefik/traefik/v3: >= 3.7.0-ea.1, < 3.7.5 → fixed in 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. Original Description 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: …