GitHub Security Advisory: GHSA-xhjw-95fp-8vgq
Package: github.com/traefik/traefik/v3 (GO)
Severity: MODERATE (CVSS 6.4)
CWE: CWE-653: Improper Isolation or Compartmentalization, CWE-863: Incorrect Authorization
Vulnerable versions: >= 3.0.0-beta1, < 3.6.14
Patched in: 3.6.14

Summary: Traefik Kubernetes CRD allows unauthorized cross-namespace middleware binding

Details:
## Summary

There is a vulnerability in Traefik's Kubernetes CRD provider cross-namespace isolation enforcement.

When `providers.kubernetesCRD.allowCrossNamespace=false`, Traefik correctly rejects direct cross-namespace middleware references from `IngressRoute` objects, but fails to apply the same restriction to middleware references nested inside a `Chain` middleware's `spec.chain.middlewares[]`. An actor with permission to create or update Traefik CRDs in their own namespace can exploit this to cause Traefik to resolve and apply middleware objects from another namespace, bypassing the documented isolation boundary.

## Patches

- https://github.com/traefik/traefik/releases/tag/v2.11.43
- https://github.com/traefik/traefik/releases/tag/v3.6.14
- https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2

## For more information

If there are any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).

<details>
<summary>Original Description</summary>

### Summary
When `providers.kubernetesCRD.allowCrossNamespace=false`, Traefik still allows a namespace-local `Middleware` of type `Chain` to reference middleware objects from another namespace via `spec.chain.middlewares[].namespace`.

This bypasses the documented cross-namespace restriction and allows an actor with permission to create or update Traefik CRDs in namespace A to bind middleware defined in namespace B to routes in namespace A.

### Details
Traefik documents `allowCrossNamespace` as the control that governs whether `IngressRoute` objects may reference resources in other namespaces.

Direct middleware references from `IngressRoute.routes[].middlewares[]` are validated in `pkg/provider/kubernetes/crd/kubernetes_http.go` by `makeMiddlewareKeys(...)`, which rejects cross-namespace references when `allowCrossNamespace` is disabled.

However, nested middleware references inside `Middleware.spec.chain.middlewares[]` follow a different code path. `createChainMiddleware(...)` in `pkg/provider/kubernetes/crd/kubernetes.go` does not receive or enforce `allowCrossNamespace`; it resolves `mi.Namespace` (or defaults to the current namespace) and appends `makeID(ns, mi.Name)` unconditionally.

At runtime, `pkg/server/middleware/middlewares.go` qualifies and builds `config.Chain.Middlewares`, so the cross-namespace middleware is actually loaded and used.

This was verified on the current `master` at commit `786f7192e11878dfaa634f8263bf79bb730a71cb`.

This appears related to earlier cross-namespace hardening work, but the surviving issue is a distinct nested `Chain` middleware code path rather than the already-guarded direct reference path.

### Expected behavior
When `providers.kubernetesCRD.allowCrossNamespace=false`, any middleware reference that resolves to an object in another namespace should be rejected, whether referenced directly from an `IngressRoute` or indirectly through a local `Chain` middleware.

### Actual behavior
A namespace-loca