In all versions of GitLab EE since version 8.13, an endpoint discloses names of private groups that have access to a project to low privileged users that are part of that project.
https://hackerone.com/reports/447817
https://gitlab.com/gitlab-org/gitlab/-/issues/25414
https://gitlab.com/gitlab-org/cves/-/blob/master/2021/CVE-2021-39884.json