source: http://www.securityfocus.com/bid/3067/info

A vulnerability has been discovered in Horde Imp which may allow an attacker to access arbitrary system files. The issue occurs due to insufficient sanity checks on user-supplied URI parameters.

By specifying a malicious INBOX file in a request, the contents of the file may be disclosed to a remote attacker. All files would be accessed with the privileges of the user invoking Imp. 

http://vulnerableserver/horde/imp/mailbox.php?mailbox=/etc/passwd