This humble-looking piece of circuitry, little larger than a coin, can be used to wirelessly fool magnetic stripe readers into thinking you've swiped your card, by generating hefty magnetic fields that mimic the data held on it.
Developed by Samy Kamkar, MagSpoof was developed as a way "to store all of your credit cards and magstripes in one device". He explains how it works:
MagSpoof emulates a magnetic stripe by quickly changing the polarization of an electromagnet, producing a magnetic field similar to that of a normal magnetic stripe as if it's being swiped. What's incredible is that the magstripe reader requires no form of wireless receiver, NFC, or RFID -- MagSpoof works wirelessly, even with standard magstripe readers. The stronger the electromagnet, the further away you can use it (a few inches in its current iteration).
It's actually an incredibly simple device, which uses some simple off-the-shelf electronic components: an Atmel ATtiny85 microcontroller, an L293D H-bridge to drive the electromagnet, a coil of simple magnetic wire, a small 100mAh 3.7V lipo battery, and then a few simples LEDs, resistors and switches.
Then it's just a matter of building it -- but there's a wiring diagram to help with that -- and sprinkling Kamkar's code over the device. You can find out how to make your own over at GitHub, and there's a full explainer about how it works in the video below.
It's worth noting that Kamkar hasn't designed the device for nefarious purposes. "MagSpoof does not enable you to use credit cards that you are not legally authorised to use," he writes on GitHub, adding that "MagSpoof requires you to have/own the magstripes that you wish to emulate."