include/auth/auth.php in Simple Invoices before 2007 03 05 does not use the login system to protect print preview pages for invoices, which might allow attackers to obtain sensitive information.
https://sourceforge.net/project/shownotes.php?group_id=164303&release_id=491300
http://www.securityfocus.com/bid/22818
http://secunia.com/advisories/24402