GitLab has remediated an issue in GitLab EE affecting all versions from 13.1.4 before 18.10.8, 18.11 before 18.11.5, and 19.0 before 19.0.2 that under certain conditions could have allowed an authenticated user to add unauthorized email addresses to a targeted user's account due to improper sanitization of user-supplied input in certain group setting fields.
Upgrade to versions 18.10.8, 18.11.5, 19.0.2 or above.