In GitLab EE 10.5 through 12.5.3, 12.4.5, and 12.3.8, when transferring a public project to a private group, private code would be disclosed via the Group Search API provided by the Elasticsearch integration.
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.