GitLab CE/EE, versions 11.3 before 11.3.11, 11.4 before 11.4.8, and 11.5 before 11.5.1, are vulnerable to an XSS vulnerability in Markdown fields via unrecognized HTML tags.
https://gitlab.com/gitlab-org/gitlab-ce/issues/52392
https://about.gitlab.com/2018/11/28/security-release-gitlab-11-dot-5-dot-1-released/