This indicates an attack attempt to exploit a Code Injection Vulnerability in MyBB Group MyBB.
The vulnerability is due to insufficient input validation when parsing user input sent to Admin Control Panel. A remote, authenticated, attacker can exploit this vulnerability by sending a crafted request to the target server. Successful exploitation could result in arbitrary code execution with the privileges of the www-data user.
MyBB Group MyBB prior to 1.8.30
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.