cheap homemade device that you can learn about on the internet. >> reporter: just blocks from times square, tourists trust hotels like the hilton garden inn for a safe night's sleep. but when abc news checked in, we discovered a major security hole. electronic locks on guest rooms can open without a key. >> i'm going to insert this marker into the bottom of the door and the door is going to open up. >> reporter: you are in my room with your hand on my ipad. >> reporter: this security consultant nick percoco showed us how easy it is to break in with a device that costs just $50 to build. >> i can go down the entire hallway and unlock every door. >> reporter: it also worked at the nearby holiday inn express, where once again, no key, no problem. hackers first exposed the problem earlier this summer, posting videos like this one on youtube, using more primitive gadgets. now the tools can be hidden in magic markers, even iphone cases. and the how-to guide is just a click away online. big named hotels reportedly bought millions of the faulty