This indicates an attack attempt against a remote Code Execution vulnerability in pfSense XMLRPC.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application.
pfSense 2.3.4 through 2.4.4-p3
System compromise: Remote attackers can gain control of vulnerable systems.
Apply the appropriate patch from the vendor or upgrade to a non-vulnerable version.