GitLab CE/EE, versions 8.6 up to 11.x before 11.3.11, 11.4 before 11.4.8, and 11.5 before 11.5.1, are vulnerable to an access control issue that allows a Guest user to make changes to or delete their own comments on an issue, after the issue was made Confidential.
https://gitlab.com/gitlab-org/gitlab-ce/issues/51238
https://about.gitlab.com/2018/11/28/security-release-gitlab-11-dot-5-dot-1-released/