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EUVD-2026-323911

fkie_cve-2026-30963

Capsule is a multi-tenancy and policy-based framework for Kubernetes. To defend against namespace hijacking achieved through update/patch operations on namespaces, Capsule uses a webhook to validate update requests targeting namespaces. However, in Kubernetes, the namespace/finalize and namespace/status subresource APIs can also modify various fields of a namespace, including the metadata field. Prior to version 0.13.0, the webhook does not define interception rules for these subresources. As a result, if a tenant administrator has permission to modify namespace/status or namespace/finalize, they can successfully perform namespace hijacking. Version 0.13.0 fixes the issue. Another mitigation is to add two subresources (namespaces and snamespaces/status with namespace/finalize within it) to the resources list in the ValidatingWebhookConfiguration rules.

GHSA-2ww6-hf35-mfjm — Capsule Namespace Hijacking via subresource

Affected: Go: github.com/projectcapsule/capsule

### Summary To defend against namespace hijacking achieved through update/patch operations on namespaces, Capsule uses a webhook to validate update requests targeting namespaces. However, in Kubernetes, the namespace/finalize and namespace/status subresource APIs can also modify various fields of a namespace, including the metadata field. The webhook does not define interception rules for these subresources. As a result, if a tenant administrator has permission to modify namespace/status or namespace/finalize, they can successfully perform namespace hijacking.

### Details When Capsule uses a ValidatingWebhookConfiguration to intercept changes to namespace resources, it does not intercept modification requests initiated through namespace subresource APIs (see: https://github.com/projectcapsule/capsule/blob/main/charts/capsule/templates/validatingwebhookconfiguration.yaml#L193). Through subresource APIs, it is still possible to modify the metadata field of a namespace resource, enabling hijacking.

### PoC Open two terminals and create two tenants: ``` kubectl create -f - << EOF apiVersion: capsule.clastix.io/v1beta2 kind: Tenant metadata: name: oil spec: owners: - name: alice kind: User EOF

./hack/create-user.sh alice solar export KUBECONFIG=alice-solar.kubeconfig kubectl create namespace solar-production # alice creates the namespace ```

``` kubectl create -f - << EOF apiVersion: capsule.clastix.io/v1beta2 kind: Tenant metadata: name: attacker spec: owners:…