Automating Man-in-the-Middle SSHv2 attacks
Recently during an internal penetration test, I was performing ARP spoofing and i discovered a SSH connection from the administrator computer to another box.
That sounds like the correct way to access remote hosts securely. However, the problem was that the company was using a network switch that was vulnerable to ARP spoofing.
I came across the below article about performing ARP spoofing and MITM SSH connections to steal credentials.
When performing arp spoofing and performing a mitm attack on SSH, the victim does get an alert message saying that there is a key mismatch but most people just ignore them anyway.
Below is the link to the original article.
http://woff.hu/tools/ssh2-mitm-like-attack-with-jmitm2/
In the article, the author demonstrates the use of a software called JMITM2 (http://www.david-guembel.de/index.php?id=6) which is sort of like a honey pot that proxies SSH connections between the victim and the target SSH server.
However, there are a number of steps to be done manually to execute this attack during an internal penetration test.
1. Check if network is vulnerable to ARP spoofing
2. Check if there are any active SSH connections in the network
2. Identify the victim computer and SSH server
3. Modify the configuration files of JMITM2
4. Modifying iptables
5. ARP spoofing
6. Checking JMITM2 console for credentials
7. Re-arp the router and victim host with the correct MAC addresses of each.
It would save a great amount of time to automate these steps. I wrote a script that does just that.
Running the command below checks the network for active SSH connections (via ARP spoofing) and then automates the whole attack to outputs any credentials captured to the console.
python2.7 mitmSSH.py -analyze
If you know the victim host IP and SSH server, you can use the below command
python2.7 mitmSSH.py -host victims -ssh sshServerIP
This script has only been tested on Kali Linux.
There are a couple of things that are still in the works to improve the script.
1. Switching from intercepter-ng for ARP spoofing to scapy.
The script can be grabbed from the below link
https://github.com/milo2012/pentest_automation/blob/master/mitmSSH.py
Milo2012's Security Blog 
SSH MITM doesn’t work if the admin is using key based authentication, even if they accept the new host key.
Yes thats correct. That is one way to prevent the attack
Thanks for sharing the information
And if they have connected even once before, known_hosts will prevent them from entering their credentials until they delete the key from the file, with a big fat warning that someone may be intercepting the connection.