Kibana contains an embedded version of the Chromium browser that the Reporting feature uses to generate the downloadable reports. If a user with permissions to generate reports is able to render arbitrary HTML with this browser, they may be able to leverage known Chromium vulnerabilities to conduct further attacks. Kibana contains a number of protections to prevent this browser from rendering arbitrary content.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-7-13-0-and-6-8-16-security-update/273964/1