This indicates an attack attempt to exploit a Remote Command Injection vulnerability in pfSense.
The vulnerability is due to an error when the vulnerable system handles a maliciously crafted HTTP request. An attacker can exploit this by injecting commands onto a request to execute arbitrary commands within the context of the application.
pfSense 2.4.1 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.