In all versions of GitLab CE/EE since version 11.3, the endpoint for auto-completing Assignee discloses the members of private groups.
{ "cpes": [ "cpe:2.3:a:gitlab:gitlab:*:*:*:*:community:*:*:*", "cpe:2.3:a:gitlab:gitlab:*:*:*:*:enterprise:*:*:*", "cpe:2.3:a:gitlab:gitlab:*:*:*:*:*:*:*:*" ], "severity": "Medium" }