Pages are not blocked as expected when using the 'Endpoint Full Web Control' feature of Sophos Anti-Virus. This issue only occurs when using a Sophos Web Appliance as proxy server.
First seen in
Sophos Web Appliance
Sophos Anti-Virus for Windows 2000+ 10.0
The IP address of the Web Appliance has been explicitly allowed in the Web Appliance policy. When used as a proxy the IP address of the Web Appliance will be the destination address for every HTTP request, therefore this should not be explicitly allowed.
What To Do
Login to the Sophos Web Appliance. Remove the web appliance IP address from:
Configuration | Group Policy | Local site list
This IP address should not be allowed by tag or have it's default risk-class/category changed.