This code allows a malicious website to detect whether the user is browsing through a proxy or not by using image tags. Proxies are often used by corporations, political dissidents, and privacy conscience Internet users because they can provide additional security or anonymous Internet browsing.
Firefox uses square brackets [ ] to denote IPv6 addresses, but this notation also works to describe IPv4 addresses (I'm not sure exactly why).
So, if we embed an image with src="http://[74.207.246.197]/pic.jpg" into a page, Firefox automatically resolves [74.207.246.197] into the IP address 74.207.246.197.
However, if the user is browsing through a proxy, this automatic resolution doesn't happen. Instead, Firefox asks the proxy to do a DNS lookup for the "domain" [74.207.246.197], which obviously fails since it's not a valid domain name.
Most proxies don't know how to handle the bracketed domain, so the DNS lookup fails. I've tested this on Tor (popular proxy for anonymous Internet browsing), PHP Proxy and CGI Proxy (the top two web-based proxies), and Proxify (popular commercial web proxy).
So, if the image fails to load, we know that the user is browsing through a proxy. Add some Javascript to detect when the image fails to load and you've got a working proxy detector.
View the demo. (Works in: Firefox 3, Safari 5)
This, of course, assumes that the user is not blocking cross-domain requests. Also, my implementation requires Javascript to be enabled, but that's not a necessity.
On my blog.
Copyright (c) 2012 Feross Aboukhadijeh
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