This indicates an attack attempt against a Remote Code Execution vulnerability in Barracuda Spam Firewall.
This is due to filters for user inputs failing to properly sanitize the f parameter value that is passed to "img.pl". An attacker may include shell commands by supplying an injection string through the URL.
Barracuda Networks Barracuda Spam Firewall 3.1.17 firmware.
System Compromise: Remote attackers can gain control of vulnerable systems
Upgrade to Barracuda Spam Firewall 3.1.18.