The socketless backdoor with a bandwidth of roughly one byte per second!
SnailDoor provides a (very slow) backdoor when you are unable to create sockets on the remote machine (for sending or receiving), but a web server is running and you can host static files.
SnailDoor creates a file for every possible byte, then polls those files regularly, looking for a change in access time. When a client requests one of the byte-files this updates the access time, and SnailDoor reads in the appropriate byte.
When SnailDoor reads a newline it executes the command now in its buffer, and saves the results to another hosted file (output.txt).
Tada! You have a painfully slow remote shell!
I made this tool for fun and to demonstrate an unusual technique. It is not intended for criminal purposes, and you should check the terms of service for any system you mean to deploy SnailDoor on.
Further, SnailDoor has no means of authentication. If you leave SnailDoor running, realize you are giving the whole world a (very slow) shell on your machine.