GitLab EE, versions 11.x before 11.3.11, 11.4 before 11.4.8, and 11.5 before 11.5.1, is vulnerable to an insecure direct object reference vulnerability that allows authenticated, but unauthorized, users to view members and milestone details of private groups.
https://gitlab.com/gitlab-org/gitlab-ce/issues/52522
https://about.gitlab.com/2018/11/28/security-release-gitlab-11-dot-5-dot-1-released/