## Summary There is a medium severity vulnerability in Traefik's Kubernetes CRD provider. When `providers.kubernetesCRD.allowCrossNamespace` is disabled — the default — cross-namespace `@kubernetescrd` references are rejected for middlewares, TLS options and HTTP/TCP ServersTransports, but the same restriction was not applied to `TraefikService` backend references resolved by the service resolver. A tenant confined by RBAC to a single namespace can therefore bind its own router to a `TraefikService` owned by another namespace and expose or reroute that namespace's backend, defeating the namespace isolation `allowCrossNamespace=false` is meant to enforce. Traefik v2 releases and the unmaintained v3 minor lines below v3.6 are affected and will not receive a patch on their own line; the remedy for those users is upgrading to a maintained, patched release. ## Patches - https://github.com/traefik/traefik/releases/tag/v2.11.54 - https://github.com/traefik/traefik/releases/tag/v3.6.25 - https://github.com/traefik/traefik/releases/tag/v3.7.10 ## 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 When `providers.kubernetesCRD.allowCrossNamespace=false` (the default), Traefik correctly rejects cross-namespace `@kubernetescrd` references for middlewares, TLS options, and HTTP/TCP `ServersTransport`, but it does not apply the same restriction to service (`TraefikService`) backendRefs. As a result, a Kubernetes tenant who is confined by RBAC to their own namespace can bind their own router to a `TraefikService` owned by another namespace simply by referencing it as `<victim-namespace>-<name>@kubernetescrd`, defeating the namespace-isolation boundary that `allowCrossNamespace=false` is meant to enforce. This is the service-resolver sibling of the cross-namespace isolation family that Traefik has been fixing one resolver at a time (`df00d82f` / CVE-2026-41174 for Chain middlewares, and `67501cbe` for TCP `ServersTransport`, which shipped in v3.7.7 only four days before the analyzed commit). The `TraefikService` resolver in `configBuilder.nameAndService` was never given the guard its sibling resolvers received. ### Details ### Root cause `nameAndService` only performs the same-namespace check (`isNamespaceAllowed`) inside the branch that handles names without an `@` separator. For names that contain an `@` separator (that is, `@kubernetescrd` cross-namespace references) it applies only the `crossProviderNamespaces` allowlist check, and that check returns `true` by default because a `nil` allowlist means "unrestricted". It never applies the `!allowCrossNamespace && strings.HasSuffix(name, "@kubernetescrd")` rejection that the sibling resolvers all apply, so `allowCrossNamespace=false` is effectively never consulted for `@kubernetescrd` service references. ### Vulnerable code ```go // pkg/provider/kubernetes/crd/kubernetes_http.go:662-695 — nameAndService (VULNERABLE) func (c configBuilder) nameAndService(ctx context.Context, parentNamespace string, service traefikv1alpha1.LoadBalancerSpec) (string, *dynamic.Service, error) { svcCtx := log.Ctx(ctx).With().Str(logs.ServiceName, service.Name).Logger().WithContext(ctx) if !strings.Contains(service.Name, providerNamespaceSeparator) { // 665: only names WITHOUT "@" service = *service.DeepCopy() service.Namespace = namespaceOrParentNamespace(service.Namespace, parentNamespace) if !isNamespaceAllowed(c.allowCrossNamespace, parentNamespace, service.Namespace) { // 669 return "", nil, fmt.Errorf("service %s/%s not in the parent resource namespace %s", ...) } } // 674: for "@"-names, the ONLY gate is crossProviderNamespaces, which defaults to allow-all (nil). if !isCrossProviderNamespaceAllowed(c.crossProviderNamespaces, parentNamespace) && strings.Contains(service.Name, providerNamespaceSeparator) { return "", nil, fmt.Errorf("service %q reference is not allowed: ...", service.Name) } // ^-- MISSING: no `!c.allowCrossNamespace && strings.HasSuffix(service.Name, "@"+ProviderName)` rejection. switch service.Kind { case "TraefikService": return fullServiceName(svcCtx, service, intstr.FromInt(0)), nil, nil // 690: returns the cross-namespace reference ... } } ``` For comparison, the sibling resolver used for middleware and TLS references does carry the guard: ```go // pkg/provider/kubernetes/crd/kubernetes.go:1653-1668 — resolveReference (CORRECT) func resolveReference(ctx context.Context, parentNs, ns, name string, crossProviderNamespaces []string, allowCrossNamespace bool) (string, error) { if strings.Contains(name, providerNamespaceSeparator) { if !allowCrossNamespace && strings.HasSuffix(name, providerNamespaceSeparator+ProviderName) { return "", errors.New("when allowCrossNamespace is disabled, @kubernetescrd references are disallowed") // 1656 — THE GUARD } ... } ... } ```

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