Crooks who steal credit and debit card numbers are now using an off-the-shelf technology to compromise computerized store cash registers.

The skimmers are built into standard Ps2 cable connectors that plug into the back of a computer where customers can't see them. They're only about an inch long—and look so innocuous that even if employees saw them they might not suspect anything, said CNBC.

"It's a little piece of plastic, usually purple, that fits into the port where your keyboard connects to your computer," said security analyst Brian Krebs, who first reported this story on his KrebsOnSecurity blog. "It intercepts any data that is sent on that communication channel, whether it's keystrokes or somebody swiping a card through a terminal."

Ps2 keystroke loggers have been available for years. They sell for as little as $40 and are marketed as "professional surveillance products." Krebs said this is the first time he's ever heard of them being used to skim card information from a retailer.

Victor Searcy, director of the Fraud Resolution Center at IDentity Theft 911, said he wasn't surprised the bad guys have found "yet another nefarious method" for stealing consumer's personal information. said CNBC.

"This scheme, involving smaller, harder-to-detect skimming devices, puts the onus on businesses to heighten their security efforts," Searcy said.