The /rest/api/1.0/render resource in Jira before version 8.4.0 allows remote anonymous attackers to determine if an attachment with a specific name exists and if an issue key is valid via a missing permissions check.
The AccessLogFilter class in Jira before version 8.4.0 allows remote anonymous attackers to learn details about other users, including their username, via an information expose through caching vulnerability when Jira is configured with a reverse Proxy and or a load balancer with caching or a CDN.
The Webwork action Cross-Site Request Forgery (CSRF) protection implementation in Jira before version 8.4.0 allows remote attackers to bypass its protection via "cookie tossing" a CSRF cookie from a subdomain of a Jira instance.