This indicates an attack attempt to exploit a Command Injection Vulnerability in Netgate pfSense pfBlockerNG.
This vulnerability is due to improper input validation for the Host HTTP header. A remote, unauthenticated attacker could exploit this vulnerability by sending a crafted request to the target server. Successfully exploiting this vulnerability could result in OS command injection in the context of root.
Netgate pfSense pfBlockerNG 2.1.4_27 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.