Unrestricted upload of a file with dangerous type in Kibana can lead to arbitrary JavaScript execution in a victim’s browser (XSS) via crafted HTML and JavaScript files. The attacker must have access to the Synthetics app AND/OR have access to write to the synthetics indices.
https://discuss.elastic.co/t/kibana-7-17-24-and-8-12-0-security-update-esa-2024-20/377712