Affected versions of Atlassian Jira Server and Data Center allow an unauthenticated user to enumerate users via an Information Disclosure vulnerability in the /ViewUserHover.jspa endpoint. The affected versions are before version 7.13.6, from version 8.0.0 before 8.5.7, and from version 8.6.0 before 8.12.0.
https://jira.atlassian.com/browse/JRASERVER-71560
http://packetstormsecurity.com/files/161730/Atlassian-JIRA-8.11.1-User-Enumeration.html