Traefik Kubernetes CRD allows unauthorized cross-namespace middleware binding
Date: 2026-04
Affected: go/github.com/traefik/traefik, go/github.com/traefik/traefik/v2, go/github.com/traefik/traefik/v3
Attack vector: rce
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-39858
CVE-2026-41174
CWE: CWE-290
CWE-306
CWE-653
CWE-863
CVSS: 6.4 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N)

## 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 `


Mitigations:

Architectural layers (MAESTRO):