This indicates an attack attempt to exploit a Remote Code Execution Vulnerability in ThinkPHP.
The vulnerability is a result of the application's failure to properly sanitize user request. As a result, a remote attacker can send a crafted HTTP request to execute arbitrary code on a vulnerable server.
v5.x below v5.0.23,v5.1.31
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor: