CVE-2026-18571: Keycloak-services: keycloak-services: fgap v2 group assignment bypass during user creation

A Broken Access Control vulnerability was identified in Keycloaks Fine-Grained Admin Permissions V2 (FGAP V2) implementation. The flaw exists in the POST /admin/realms/{realm}/users endpoint, where the server fails to validate group membership permissions within the user creation payload. While Keycloak correctly enforces manage-membership scopes for existing users, it misses this check during the initial user creation process. A sub-administrator with user create scope but restricted group manage-membership scope can successfully assign a new user to unauthorized groups by including them in the creation request. Concrete impact: An attacker with sub-admin privileges can elevate the permissions of new users, grant them access to restricted data by placing them in sensitive groups, and bypass the security boundaries established by FGAP V2.