Gardener implements the automated management and operation of Kubernetes clusters as a service. Prior to 1.142.6, 1.143.3, 1.144.2, and 1.145.0, the customverbauthorizer admission plugin's mustCheckProjectMembers manage-members check compares changes to User subjects but does not account for Group or ServiceAccount subjects in Project.spec.members. A project administrator who lacks manage-members permission can add arbitrary Group or ServiceAccount subjects, including the system:authenticated Group, and thereby grant broad project access. The resulting access can include Shoots, Secrets, and cloud provider credentials. This issue is fixed in versions 1.142.6, 1.143.3, 1.144.2, and 1.145.0.

Affected: Go: github.com/gardener/gardener, Go: gardener/gardener

## 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 mat…