🛡️ CVE-2026-54761 — traefik

Traefik: Kubernetes Gateway crossProviderNamespaces bypass allows HTTPRoute outside the allowlist to expose internal Traefik services

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 Kuber