An issue was discovered in file_download.php in MantisBT before 2.24.3. Users without access to view private issue notes are able to download the (supposedly private) attachments linked to these notes by accessing the corresponding file download URL directly.
https://mantisbt.org/bugs/view.php?id=27039
http://github.com/mantisbt/mantisbt/commit/9de20c09e5a557e57159a61657ce62f1a4f578fe
http://github.com/mantisbt/mantisbt/commit/5595c90f11c48164331a20bb9c66098980516e93