This indicates an attack attempt to exploit a Arbitrary Command Execution Vulnerability in vBulletin 5.
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 from 5.1.4 to 5.1.9
System Compromise: Remote attackers can gain control of vulnerable systems.
Update the patch