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Any circuit level tunneling through a proxy server such as SOCKS or SSL, will allow most protocols to be passed through a standard proxy gateway. Whenever you see a statement like this, you should remember that it implies that the protocol is not actually understood but merely transparently transmits it. For instance, the popular tunneling protocol SSL is able to tunnel virtually any TCP based protocol without problem, it’s often used to add some protection for weak protocols like FTP and Telnet.
But it can create a little bit of a headache for a proxy administrator. Not only can all sorts of protocols be allowed access to a network but often the administrator has no knowledge of the contents due to encryption. There are some short terms solutions which will provide a limited amount of protection – for example blocking access based on port numbers. That is only allow specific ports to be tunneled such as 443 for HTTPS, 636 for secure LDAP. This can work well but remember some advanced security programs like Identity Cloaker allow the configuration of the port, allowing protocols and applications to be tunneled on non standard ports -a bit like a proxy unblocker . It is therefore not an ideal solution and one that cannot be relied upon in the longer terms to keep a network and proxy secure.
The obvious solution of course is to utilise a proxy server that can verify the protocol that is being transmitted. This requires an awful lot more intelligence built into the proxies but it is possible. It does require a bigger overhead, it does make the proxy server more expensive and perhaps more complicated and trickier to manage. However without this sort of intelligence or something similar you will get the possibility of an FTP session being set up through an SSL tunnel for example.
In some ways proxies already do some of this, and protocols that are proxied rather than tunneled at the application level cannot be exploited like this. Examples include HTTP, FTP and even Gophur cannot be used to trick entry, simply because there is no ‘dumb’, direct tunnel the proxy understands and will only relay legitimate responses.