Mantis 0.17.4a and earlier allows remote attackers to view private bugs by modifying the f_id bug ID parameter to (1) bug_update_advanced_page.php, (2) bug_update_page.php, (3) view_bug_advanced_page.php, or (4) view_bug_page.php.
http://www.securityfocus.com/bid/5563
http://www.iss.net/security_center/static/9954.php
http://www.debian.org/security/2002/dsa-161