In all versions of GitLab EE starting from 13.10 before 14.1.7, all versions starting from 14.2 before 14.2.5, and all versions starting from 14.3 before 14.3.1 a specific API endpoint may reveal details about a private group and other sensitive info inside issue and merge request templates.
https://hackerone.com/reports/1255128
https://gitlab.com/gitlab-org/gitlab/-/issues/336446
https://gitlab.com/gitlab-org/cves/-/blob/master/2021/CVE-2021-39888.json