The impersonate feature in Gitlab 8.7.0, 8.6.0 through 8.6.7, 8.5.0 through 8.5.11, 8.4.0 through 8.4.9, 8.3.0 through 8.3.8, and 8.2.0 through 8.2.4 allows remote authenticated users to "log in" as any other user via unspecified vectors.
https://www.exploit-db.com/exploits/40236/
https://gitlab.com/gitlab-org/gitlab-ce/issues/15548
https://euvd.enisa.europa.eu/vulnerability/EUVD-2016-5340
https://about.gitlab.com/2016/05/02/cve-2016-4340-patches/
http://packetstormsecurity.com/files/138368/GitLab-Impersonate-Privilege-Escalation.html