GitLab has remediated an issue in GitLab CE/EE affecting all versions from 9.1 before 18.11.7, 19.0 before 19.0.4, and 19.1 before 19.1.2 that under certain conditions could have allowed an unauthenticated user to determine the existence of a private project due to improper authorization controls on cross-project reference pages.
https://hackerone.com/reports/3704739
https://gitlab.com/gitlab-org/gitlab/-/work_items/597947
https://docs.gitlab.com/releases/patches/patch-release-gitlab-19-1-2-released/