An issue was discovered in MantisBT before 2.24.3. When editing an Issue in a Project where a Custom Field with a crafted Regular Expression property is used, improper escaping of the corresponding form input's pattern attribute allows HTML injection and, if CSP settings permit, execution of arbitrary JavaScript.
https://mantisbt.org/bugs/view.php?id=27275
http://github.com/mantisbt/mantisbt/commit/221cf323f16a9738a5b27aaba94758f11281d85c