This indicates an attack attempt against a remote Code Execution vulnerability in vBSEO.
This issue is caused by an error when vBSEO handles malicious php request. It may allow remote attackers inject arbitrary code by sending a crafted php request.
vBSEO 3.5.0, 3.5.1, 3.5.2
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's web site for suggested workaround.