An issue was discovered in GitLab Community and Enterprise Edition 11.9.x and 11.10.x before 11.10.1. Merge requests created by email could be used to bypass push rules in certain situations.
https://gitlab.com/gitlab-org/gitlab-ee/issues/11302
https://about.gitlab.com/2019/08/29/security-release-gitlab-12-dot-2-dot-3-released/