This indicates an attack attempt to exploit a Remote File Disclosure vulnerability in Sophos Web Appliance.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when parsing HTTP requests. A remote attacker may be able to exploit this to view arbitrary local files within the context of the application, via a crafted HTTP request.
Sophos Web Appliance 18.104.22.168
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply the most recent upgrade or patch from the vendor.