Some proxy sites and applications use HTTPS/SSL Certificates to evade detection.
Some proxy sites and applications (such as Ultrasurf) also use HTTPS/SSL Certificates to evade detection. Proxy tools and applications that use HTTPS can be particularly difficult to detect and block because they allow web sites and media files to be viewed secretly within a secure tunnel where content is encrypted.
They also present a much more serious security problem since the secure tunnels they use allow malware and other web-related threats to sneak into networks undetected.
Examples of such 'secure' proxies include Ultrasurf, TOR and VTunnel. These proxy types cannot be blocked using domain or URL blocklists and so the fix that most security vendors recommend is a (far from practical) blanket block on all HTTPS traffic.
Guardian Web Filters can perform real-time analysis of all content travelling within secure tunnels (HTTPS Filtering) to accurately detect and block secure proxies like Ultrasurf.