It wasn't the best pieces of planning by a group of criminals looking to skim the bank cards of guests at a Las Vegas hotel. The hapless crooks picked the wrong target when they installed a card-skimming device in an ATM about to be used by 100s of guests at a computer security conference.

Attendees at the Defcon conference noticed that at ATM placed in the Riviera Hotel, which plays host to the annual event, didn't quite look right, according to a senior conference organiser who identified himself only as Priest. "They looked at the screen where there would normally be a camera," he said. "It was a little bit too dark, so someone shined a flashlight in there and there was a PC."

The ATM looked like a working system, but when people would put their cards in the machine, it would scan their card information and record the PIN numbers they entered. He didn't know how long the ATM had been at the Riviera.

Conference organisers notified local law enforcement who hauled away the machine on said Priest.

Credit card skimmers - small devices installed on top of card readers to steal information - and fake ATM machines are a common problem. Once the criminal records the card information and PIN number, he can use that to create a fake ATM card and then empty the victim's account.

Previously unsophisticated criminal gangs are increasingly using these devices, Priest said. "They're realising that this is a great way to make money without getting caught." The criminals probably didn't realise that they were installing their ATM in a hotel that was soon going to be flooded with more than 8,000 security professionals, he added.

They were smart enough to place the machine in one of the few spots in the hotel where there was no security camera to catch them, Priest said. "It was literally right next to the hotel security entrance."