Multiple versions of GitLab expose sensitive user credentials when assigning a user to an issue or merge request. A fix was included in versions 8.15.8, 8.16.7, and 8.17.4, which were released on March 20th 2017 at 23:59 UTC.
https://gitlab.com/gitlab-org/gitlab-ce/issues/29661
https://gitlab.com/gitlab-org/gitlab-ce/commit/cdf396f456472ef8decd9598daa8dc0097cd30c5
https://gitlab.com/gitlab-org/gitlab-ce/commit/a70346fc6530aa28a98e4aa4cf0f40e2c3bcef6b
https://gitlab.com/gitlab-org/gitlab-ce/commit/43f5a2739dbf8f5c4c16a79f98e2630888f6b5d1
https://about.gitlab.com/2017/03/20/gitlab-8-dot-17-dot-4-security-release/