RE: Netcat through Squid HTTP Proxy

["Henderson, Dennis K." <Dennis.Henderson () umb com>]

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