Executive brief
Gardener is an open-source Kubernetes cluster management platform. An authorization flaw in its API server allows a project administrator without explicit member-management permissions to grant full project access to arbitrary user groups—including all authenticated users—by injecting group subjects into the project member list. This bypasses the intended access control, potentially exposing cluster credentials and workload data to unauthorized users.
Technical details
The vulnerability is an authorization bypass in the `customverbauthorizer` admission plugin's `manage-members` check. The `mustCheckProjectMembers()` function compares old and new project member lists but only examines `User`-kind subjects (via the `isHumanUser()` function), ignoring `Group`-kind subjects. An authenticated project administrator can leverage this blind spot to add arbitrary groups—such as `system:authenticated` (all authenticated users) or `system:unauthenticated`—to the project member list without triggering the authorization gate. The attack requires project admin role but no `manage-members` permission; it is network-accessible and requires only valid Kubernetes authentication. The impact is privilege escalation and unauthorized access to all project resources, including Shoots and cloud provider secrets.
Affected products
- Gardener Gardener all versions before fix
Timeline
- 2026-09-22: disclosed
- other: CVE-2026-79767 assigned