Introduction: what is this about?
You are a young infosec student. You are passionate about Physical Penetration Testing and Intrusions. You really really like stuff like the Under the Door tool but you hate the fact that it costs 30$ and the shipping is even double of that because you live in EU. Nothing can stop you tho so you make yourself one since DIY is cool and you can build one for less than 8$. It doesnt end there and you want to find ways to hack doors even cheaper and more fancy and you figure out how to do it! Let's see what we need and in what circumstances we can do such attack:
That's really it. You can even substitute the ruler with anything thin and long you can get able to be slided under the door. A ruler is just the first cheap and common thing I've found. They also often have a small hole on one of the ends and we will make use of that a lot! What is "really" needed (and to be honest, even in this case it's not 100% necessary and any string will do) is a fishing line. Why that? 1) it's cheap 2) really cheap, you can get meters and meters of it for a handful of cents, but most important it's 3) very thin and 4) strong and hard to snap.
The bottom of a door will likely always have about a centimeter of space open. A UTD tool abuses that and we will do the same with other sides of the door and we will be able to do it even when that space is really really small, since a fishing line is just a fration of a millimeter wide!
What we are trying to do is create a system we can manipulate in order to wrap the fishing line around the door handle on the other side and pull the string to turn it down and open the door.
We can do so in two ways, depending on which sides we can use to slide the fishing line to the other side. Then we will try to make this position, and by pulling the string we will be able to open the door by turning the handle.
Better Pictures and Videos to come!
Knots to know
Blood Knot: There are just two knots needed. One is the Dropper Knot and the other one helps in the Left and Right method when you need to connect both the fishing lines and it's the Blood Knot, but any should work.