The script attempted to exploit the Horde/IMP Plesk Webmail Exploit in vulnerable versions of the Plesk control panel. By injecting malicious PHP code in the username field, successful attackers are able to bypass authentication and upload files to the targeted server. These types of attacks could be one avenue used in the DarkLeech compromises. Although not as common as the Plesk remote access vulnerability (CVE-2012-1557) described in the report, it does appear that this vulnerability is being actively exploited. 

via Possible Exploit Vector for DarkLeech Compromises.