An issue was discovered in GitLab Community and Enterprise Edition before 11.1.7, 11.2.x before 11.2.4, and 11.3.x before 11.3.1. There is Cross Site Request Forgery (CSRF) in the Slack integration for issuing slash commands.
https://about.gitlab.com/releases/2018/10/01/security-release-gitlab-11-dot-3-dot-1-released/