The Timeline feature in my_view_page.php in MantisBT through 2.21.1 has a stored cross-site scripting (XSS) vulnerability, allowing execution of arbitrary code (if CSP settings permit it) after uploading an attachment with a crafted filename. The code is executed for any user having visibility to the issue, whenever My View Page is displayed.
https://github.com/mantisbt/mantisbt/commit/9cee1971c498bbe0a72bca1c773fae50171d8c27