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.
An issue was discovered in GitLab Community and Enterprise Edition before 11.4.13, 11.5.x before 11.5.6, and 11.6.x before 11.6.1. It has Incorrect Access Control.