The /rest/api/latest/groupuserpicker resource in Jira before version 8.4.0 allows remote attackers to enumerate usernames via an information disclosure vulnerability.
https://github.com/pwnosec/jira-vuln-scanner
https://github.com/imhunterand/JiraCVE
https://github.com/und3sc0n0c1d0/UserEnumJira
https://github.com/mufeedvh/CVE-2019-8449
https://jira.atlassian.com/browse/JRASERVER-69796
http://packetstormsecurity.com/files/156172/Jira-8.3.4-Information-Disclosure.html