GitLab EE 8.14 through 12.5, 12.4.3, and 12.3.6 has Incorrect Access Control. After a project changed to private, previously forked repositories were still able to get information about the private project through the API.
https://gitlab.com/gitlab-org/gitlab/issues/28802
https://about.gitlab.com/blog/categories/releases/
https://about.gitlab.com/blog/2019/11/27/security-release-gitlab-12-5-1-released/