RE: Netcat through Squid HTTP Proxy
From: JB (pentest_at_jitonline.net)
Date: 04/20/05
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Date: Tue, 19 Apr 2005 18:43:48 -0400 (EDT) To: pen-test@securityfocus.com
I have one word... OpenVPN (http://openvpn.net/)- It can tunnel through
proxies, use almost any open port, and is pretty difficult to detect...
JB
> Well you have to understand how they do it to prevent it. Proxy will
> never be totally secure, it has to be a multiple level protection idea. If
> you allow tunneling on 80 and 443, then Stunnel and other things will
> still work. But it is better than it was before, and to pass the proxy
> would recommend know-how and some pre-work.
>
> Packet Inspection could be used to filter known bad traffic, but that
> type of protection isn't cheap. 443 tunnel hide all the traffic from the
> proxy and most IDS/IPS system so client security becomes important. AV on
> the client or possible Host IPS, if money isn't a big deal.
>
>> -----Original Message-----
>> From: Henderson, Dennis K. [mailto:Dennis.Henderson@umb.com]
>> Sent: Monday, April 18, 2005 11:28 AM
>> To: Todd Towles; Joachim Schipper; pen-test@securityfocus.com
>> Subject: RE: Netcat through Squid HTTP Proxy
>>
>>
>> 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
>>>>
>>>>
>>>
>>
>
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- In reply to: Todd Towles: "RE: Netcat through Squid HTTP Proxy"
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