GitLab has remediated an issue in GitLab CE/EE affecting all versions from 18.2 before 18.8.9, 18.9 before 18.9.5, and 18.10 before 18.10.3 that could have allowed an authenticated user with custom role permissions to demote or remove higher-privileged group members due to improper authorization checks on member management operations.
Upgrade to versions 18.8.9, 18.9.5, 18.10.3 or above.