ISA SecureNAT and Firewall Clients Can Bypass Websense Content Filtering
Websense content filtering software is an excellent add-on product for the Microsoft ISA firewall. If you have deployed the ISA firewall to secure outbound Internet access for your organization, I would strongly encourage you to consider this powerful software suite. An ISA/Websense solution will provide unparalleled protection against spyware, malicious mobile code, and many other threats. Websense features granular, user and group based filtering policies and with its modular design is highly scalable. With Websense Web Security you also get features such as enhanced security categories as well as real-time security updates, ensuring that your users are protected at all times, even from today’s advanced blended threats.
Although under most circumstances the Websense content filtering does a fantastic job, it is possible to bypass Websense filtering policies if the software is not configured correctly. For instance, if a workstation is configured as a web proxy client to the ISA firewall, communication is filtered properly. If the workstation also has the Firewall Client installed, the user can simply disable any proxy settings in their web browser, resulting in their HTTP communication being sent to the ISA firewall by the Firewall Client. Due to some vagaries with the inner workings of the Websense filtering mechanism, this communication may not be filtered.
The good news is that this issue is simple to resolve. A little known, and perhaps often overlooked, configuration change needs to be made if you expect content filtering to work correctly for ISA SecureNAT and Firewall Clients. That change is documented in the Websense Installation Guide Supplement for Microsoft ISA Server. On page 13 under the ‘Configuring the ISAPI Filter’ section you will see that in order to correctly filter SecureNAT and Firewall Clients you must create a file called ‘ignore.txt’ in the Windows\System32 folder. This file should contain the hostname of the ISA firewall that the filtering plug-in is installed on (note also that this entry should be in all caps). Recycle the Websense services in order for this change to take effect.