MantisBT before 1.2.18 allows remote authenticated users to bypass the $g_download_attachments_threshold and $g_view_attachments_threshold restrictions and read attachments for private projects by leveraging access to a project that does not restrict access to attachments and a request to the download URL.
https://github.com/mantisbt/mantisbt/commit/5f0b150b
https://exchange.xforce.ibmcloud.com/vulnerabilities/98731
http://www.securityfocus.com/bid/71104
http://www.openwall.com/lists/oss-security/2014/11/15/6
http://www.mantisbt.org/bugs/view.php?id=17742
http://www.debian.org/security/2015/dsa-3120