This is just a simple proof of concept where I use Windows Filter Platform as a covert channel for incoming communications. The main idea is explained in my article Knock! Knock! The postman is here! (abusing Mailslots and PortKnocking for connectionless shells), and it can be summarized as using source ports to carry the encoded messages. We use WFP to get the information from the events, so we can hit a well-known port (UDP 123, for example) and vary the source port to encode 2 bytes of information per request.

On the other hand, if you are interested on playing with Port Knocking in Windows the code can be helpful to you.

To use the WFP APIs you need to run the executable as a privileged user (administrator). Run the wfp-reader.exe in your VM and then in other VM execute this small snippet:

import sys
from scapy.all import *


def textToPorts(text):
 chunks = [text[i:i+2] for i in range(0, len(text), 2)]
 for chunk in chunks:
     send(IP(dst=sys.argv[1])/UDP(dport=123,sport=int("0x" + chunk[::-1].encode("hex"), 16))/Raw(load="Use stealthier packet in a real operation, pls"))

if __name__ == "__main__":
 while 1:
     command = raw_input("Insert text> ")
     textToPorts(command)

This python will encode your text inside the source port of UDP packets sent to the remote server at port 123.

➜  working sudo python messenger.py 10.0.2.7
Insert text> Hello World!
.
Sent 1 packets.
.
Sent 1 packets.
.
Sent 1 packets.
.
Sent 1 packets.
.
Sent 1 packets.
.
Sent 1 packets.

And at wfp-reader.exe you are going to see the messages:

C:\Users\avispa.marina\Source\Repos\wfp-reader\wfp-reader\bin\Debug\netcoreapp3.1>wfp-reader.exe
        -=[Proof of concept - Covert Channel using Windows Filtering Platform]=-


Hello World!

Juan Manuel Fernandez (@TheXC3LL)