Direct static code injection vulnerability in phpBook 1.3.2 and earlier allows remote attackers to execute arbitrary PHP code via the e-mail field (mail variable) in a new message, which is written to a PHP file.
http://www.vupen.com/english/advisories/2006/0002
http://www.securityfocus.com/bid/16106
http://www.securityfocus.com/archive/1/420698/100/0/threaded