From: Niels Bakker (niels=bugtraqbakker.net)
Date: Wed Mar 05 2003 - 14:44:11 CST

• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

* bit_logics-mail.com [Wed 05 Mar 2003, 21:35 CET]:
[..]
> C:\>telnet www.blockedsite.com 80
>
> GET / HTTP/1.1
> Host: www.blockedsite.com
>
> Given the nature of Telnet, the request is sent to the server one
> character at a time; obviously, the filter cannot examine packets with a
> single character of valid data, so each packet makes it through with no

Actually, in these situations, telnet works line-based. That's also why
backspace works (modulo matching terminal emulator and stty settings).

> problem. The blocked server waits until it receives all packets, then
> pieces them together and responds to the request. Incoming traffic isn't
> monitored, so the user is easily able to receive the source code of the
> page he requested via telnet.

Does a filtering product exist that has not had this flaw in the past?

> Unfortunately, I do not have the necessary equipment at my disposal to
> further test the exploit, although I know for a fact that it works, at
> least on firewalls with basic filter configurations. I also have yet to
> come up with a successful work-around for this bypass, as it occurs at a
> very low level. If anyone has any ideas, I'm all ears. Thanks.

Force all HTTP traffic via a proxy that sends out its own HTTP requests
in one packet; don't try to solve social problems with technical
solutions; and above all, realise that filtering in this way is utterly
useless censorship.

        -- Niels.

--
subvertise me

• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]