It’s a sad commentary on the state of the world when it becomes a good practice to closely inspect the card reader on every ATM and gas pump for the presence of a skimmer. The trouble is, even physically yanking on the reader may not be enough, as more sophisticated skimmers now reside safely inside the device, sipping on the serial comms output of the reader and caching it for later pickup via Bluetooth. Devilishly clever stuff.

Luckily, there’s an app to detect these devices, and the prudent consumer might take solace when a quick scan of the area reveals no skimmers in operation. But is that enough? After all, how do you know the smartphone app is working? This skimmer scammer scanner — or is that a skimmer scanner scammer? — should help you prove you’re being as safe as possible.

The basic problem that [Ben Kolin] is trying to solve here is: how do you prove a negative? In other words, one could easily write an app with a hard-coded “This Area Certified Zebra-Free” message and market it as a “Zebra Detector,” and 99.999% of the time, it’ll give you the right results. [Ben]’s build provides the zebra, as it were, by posing as an active skimmer to convince the scanner app that a malicious Bluetooth site is nearby. It’s a quick and dirty build with a Nano and a Bluetooth module and a half-dozen lines of code. But it does the trick.

Need a primer on the nefarious world of skimming? Here’s an overview of how easy skimming has become, and a teardown of a skimmer captured in the wild.