This indicates an attack attempt to exploit a remote Command Execution vulnerability in FreePBX.
The vulnerability is due to insufficient input validation in the application when handling a crafted HTTP request. A remote attacker may be able to execute arbitrary commands in the security context of the target user, via a crafted request.
FreePBX Project FreePBX prior to 10.13.66
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the latest patch from the vendor.