An input validation problem was discovered in the GitHub service integration which could result in an attacker being able to make arbitrary POST requests in a GitLab instance's internal network. This vulnerability was addressed in 12.1.2, 12.0.4, and 11.11.6.
https://hackerone.com/reports/446593
https://gitlab.com//gitlab-org/gitlab-ce/issues/54649
https://about.gitlab.com/2019/07/29/security-release-gitlab-12-dot-1-dot-2-released/