This indicates an attack attempt to exploit a Remote Command Execution Vulnerability in vBulletin.
The vulnerability is due to an input validation error while parsing a HTTP request in the vulnerable module. A remote attacker could exploit this to execute arbitrary code execution within the context of the application, via a crafted HTTP request.
vBulletin Version 5
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are not aware of any vendor supplied patch for this issue