This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in Sophos Web Appliance.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when parsing HTTP request.
A remote attacker may be able to exploit this to execute arbitrary script code within the context of the application, via a crafted HTTP request.
Sophos Web Appliance 184.108.40.206
System Compromise: Remote attackers can execute arbitrary script code in the context of the application.
Apply the most recent upgrade or patch from the vendor.