I made this for Vulnhub, which if you haven't heard is a fantastic site to download vulnerable VM's that you can hack into. Super fun, and it is a great way to learn.
Pretty much thought of a pretty neat idea I hadn't seen done before with a VM, and I wanted to turn it into reality!
Your job is to escalate to root, and find the flag.
Gotta give a huge shoutout to c0ne for helping to creating the binary challenge, and rasta_mouse and recrudesce for testing :)
Grab it here: https://www.vulnhub.com/entry/knock-knock-11,105/
Feel free to contact me on Freenode in #vulnhub as zer0w1re with any questions, or comments :D