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.
https://www.talosintelligence.com/vulnerability_reports/TALOS-2019-0837
https://www.talosintelligence.com/vulnerability_reports/TALOS-2019-0836