source: http://www.securityfocus.com/bid/41521/info

Yappa (Yet Another PHP Photo Album) is prone to multiple remote command-execution vulnerabilities because it fails to properly validate user-supplied input.

An attacker can exploit these issues to execute arbitrary commands within the context of the vulnerable system.

Yappa 3.1.2 is vulnerable; other versions may also be affected. 

http://www.example.com/yappa/yappa.php?thedir=[ command you ]
http://www.example.com/yappa/yappa.php?image=[ command you] 