When requests to the internal network for webhooks are enabled, a server-side request forgery vulnerability in GitLab affecting all versions starting from 10.5 was possible to exploit for an unauthenticated attacker even on a GitLab instance where registration is disabled
https://hackerone.com/reports/1059596
https://gitlab.com/gitlab-org/gitlab/-/issues/294178
https://gitlab.com/gitlab-org/cves/-/blob/master/2021/CVE-2021-22175.json