This indicates an attack attempt against a Command Injection vulnerability in pfSense system.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when parsing HTTP requests. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
pfSense version 2.3.1_1 or prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.