An issue was discovered in MantisBT before 2.24.3. Improper escaping of a custom field's name allows an attacker to inject HTML and, if CSP settings permit, achieve execution of arbitrary JavaScript when attempting to update said custom field via bug_actiongroup_page.php.
https://mantisbt.org/bugs/view.php?id=27304
http://github.com/mantisbt/mantisbt/commit/8c6f4d8859785b67fb80ac65100ac5259ed9237d