Quick Proxy Detection: “

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Just a quicky post on how in Firefox you can detect proxies using image tags. Firefox (and possibly other browsers but I first saw it in Firefox) use [ ] to denote IPv6 (I believe that’s it’s original intention anyway) but it also works in IPv4.

Something as simple as http://[123.123.123.123]/img.jpg?unique_id embedded into a page could be used to see if the user is using a proxy, which, as far as I’ve seen – at least using Apache’s proxy, doesn’t understand that syntax and therefore won’t fetch the image. This does give false positives when using something that blocks cross domain requests, and robots that try to stay on the same domain. Anyway, this might be helpful to someone.

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(Via ha.ckers.org web application security lab.)