This indicates an attack attempt to exploit a Remote Command Execution vulnerability in pfSense Apcupsd package.
The vulnerability is due to an error when the vulnerable system handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Apcupsd prior to 0.3.91_5
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.