The Skull CTF

by Wokwi

A mind-bending hardware puzzle in three acts

View all updates Dec 10, 2020

Spiralling Deeper

by Uri Shaked

Ahoy Mateys!


Safety First

Thank you for joining our AMA session yesterday! Based on the questions that were asked, we’ve decided that we should probably specify a few safety instructions to ensure that The Skull is used responsibly. For your convenience, we’ve summarized the most important points here:

  • The Skull is NOT edible. Please do not consume it as food.
  • Do not reheat it in a microwave.
  • Do not smoke it.
  • Do not surgically implant it in your head. (Or anywhere else, for that matter.)
  • Do not try to feed The Skull.
  • Keep away from children and furious clowns.
  • The Skull is not suitable for use as an emergency light. (We tried it. Never again.)
  • Do not, under any circumstances, use The Skull as a weapon of war.

The Virus

We believe in you. Eventually, you’ll figure out how to hack into The Skull’s brain and control her mind. But her story doesn’t have to end there! For those of you with a great deal of courage—and not much going on in the self-preservation department—we’ve created a bonus challenge called "The Virus."

To overcome this challenge, you will need to craft a spell with the ability to copy itself into the brain of an uninfected Skull and spread, from there, to other uninfected Skulls that do not take precautions. The infection rate does not have to be 100%, but you do need to demonstrate an infection chain of at least two Skulls.

Suppose, for example, you have two Skulls: Napoleon Bone Apart and Dee Ceased. Your job is to infect Napoleon manually, then watch in horror as he infects Dee. You could then factory reset Napoleon, and Dee would infect him right back.

For your own protection, please wear sterilized gloves while working on this challenge. And clean your keyboard with isopropyl alcohol when you are done.

Medulla Oblongata

Solving The Virus is hard. We know because we tried it. And, well, we wish you luck. But making the Virus was a ton of fun! So we decided to make something even more challenging. We call it medulla oblongata. We’ve hidden another flag in the depths of The Skull’s brain—a place not easily accessed, even by the most powerful spells—and your mission is to figure out how to sneak into that Inner Sanctum and extract the second flag. Because, as we all know, one flag is never enough!

Be brave.

Reprogramming & Troubleshooting

Some of you will receive a programming header with your Skull. While we don’t know what evil plans you are hatching in that black heart of yours, we are here to help. We have created the Reprogramming Guide, which walks you through the process of connecting the programmer to The Skull and uploading new code. (Or re-uploading the firmware. Just for the sake of it. Because why not?)

Things go wrong. They always do. So we also came up with a Troubleshooting Guide for your Skull.

The Source Code

We’ve set up an FTP server to share The Skull’s firmware with the world. As we did it, Chrome decided to drop FTP support. And so did Firefox. But we trust you to find your way in…


Happy diggin’!

About the Author

Uri Shaked


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Product Choices


The Skull

A single The Skull CTF board (☠)


SparkFun Programmer

From the Crowd Supply Basics project.

Design your own puzzles and load them into The Skull's brain using this programmer! Mini USB cable not included.

AVR Pocket Programmer by SparkFun


EconoStik Header: DR VT SMT 2x3

From the Crowd Supply Basics project.

Use with the SparkFun Programmer.

One Amphenol FCI EconoStik™ 2.54mm Header. SMD/SMT Mounting style, Straight angled male pins, gold-plating. Voltage rating: 110 V



Wokwi creates tools for the Arduino and maker community, such as AVR8js, the Open source Arduino Simulator, and GoodArduinoCode.

Uri Shaked

JA Electronics


PCB Fabrication & PCB Assembly

See Also

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