. >> reporter: security expert chris hadnagy found this for sale on the dark web it's as thin as a credit card. >> and it slides into the slot and down it's not even detectable. >> reporter: this one uses blue tooth so criminals just -- >> connect to the blue tooth wireless, download all the cards, act like you're pumping gas, and leave. >> reporter: another has a cell phone chip so thieves don't even have to go back to the scene to collect your data. >> it calls to a server and downloads all the numbers to a file for them to obtain. >> reporter: almost impossible to detect. but sometimes the bad guys leave clues. >> if the security tape is at all altered, that's a huge thing to walk away turn on your blue tooth when you're next to an atm or gas pump, see if you see any weird networks popping up. >> reporter: if you wind up getting skimmed, is it up to you as the consumer to make sure everything is right? >> it's absolutely up to you. >> reporter: attorney diane finguera. >> when you notice an unauthorized transaction, if you report it within two days of that unauthorized activity, then t