The Auth0 integration in GitLab before 10.3.9, 10.4.x before 10.4.6, and 10.5.x before 10.5.6 has an incorrect omniauth-auth0 configuration, leading to signing in unintended users.
https://www.debian.org/security/2018/dsa-4206
https://about.gitlab.com/2018/03/20/critical-security-release-gitlab-10-dot-5-dot-6-released/