It seems like he was looking for information on how to prevent this.
You can configure squid to only allow tunneling on certain ports like
443 and 80. You'll have to figure out what your safe ports are to
prevent legitimate traffic from being impacted.
I usually make sure the usual ports like ssh, telnet, irc are not
allowed.
Cheers
Dennis
-----Original Message-----
From: Todd Towles [mailto:toddtowles@brookshires.com]
Sent: Monday, April 18, 2005 8:20 AM
To: Joachim Schipper; pen-test@securityfocus.com
Subject: RE: Netcat through Squid HTTP Proxy
There is a POC shell program that uses XML-RPC called Monkey
shell (http://www.securiteam.com/tools/6L00F0KBFE.html). It
looks like it might require a re-code to be fully used as a
pen-test tool. But it something to look at. -
You can try HTTPTunnel as well.
httptunnel creates a bidirectional virtual data connection
tunnelled in HTTP requests. The HTTP requests can be sent via
an HTTP proxy if so desired.
This can be useful for users behind restrictive firewalls. If
WWW access is allowed through a HTTP proxy, it's possible to
use httptunnel and, say, telnet or PPP to connect to a
computer outside the firewall.
http://www.nocrew.org/software/httptunnel.html
-Todd
-----Original Message-----
From: Joachim Schipper [mailto:j.schipper@math.uu.nl]
Sent: Sunday, April 17, 2005 10:13 AM
To: pen-test@securityfocus.com
Subject: Re: Netcat through Squid HTTP Proxy
On Fri, Apr 15, 2005 at 10:40:31AM -0400, Rod S wrote:
Hello,
I have a squid proxy server running, caching and filtering
web access.
User workstations on my network are only allowed http
access through
this proxy server. The firewall (Cisco PIX) will not let
them connect
outbound to any ports.
I've done some testing and was successful in running netcat
to connect
to a remote server listening with netcat on port 80 and get
a command
prompt for an internal machine (which is allowed to
connect to any
outgoing ports) on that remote server. I'm wondering if
it's possible
for netcat to connect through our proxy server to a remote
machine and
send a cmd.exe shell in the same way? Any tips on
preventing this or
any other information you care to share is appreciated.
Thanks!
Rod
Dear Rod,
if I understand correctly, you can get a shell on a remote
machine and
want to allow a remote machine to get a shell on a local host. This
can be achieved quite easily - search for 'reverse shell'.
One example
which looks nice is rrs (*nix
only) - see freshmeat.net. This one cannot do HTTP
proxying, though,
so it should be augmented or wrapped in something that can.
The Hacker's Choice (www.thc.org) has just run an article on this,
including an example in Perl. If you desire something more
Windows-specific, you may want to ask Google, or any
shades-of-grey-hat site you can find. ;-)
However, simply, yes, this is possible. Quite a few of
these kinds of
reverse shells rely on HTTP CONNECT, so limiting that may
help - but
there are some seriously scary things out there, including reverse
shells that communicate over DNS or ICMP (pings etc).
A good I(P|D)S may help a little. Locking down the network
further may
help. However, it is almost impossible to keep a smart
attacker in -
make sure to keep him out.
Joachim
