SAN FRANCISCO -- A group of Israeli researchers said it has found a weakness in the way banks keep ATM PIN codes secret.

The problem is in the way PIN codes are transported through bank networks.

"If they're right, this would be a serious attack on the entire ATM PIN system," msnbc.com technology writer Bob Sullivan said.

It all starts when a customer enters a PIN code into ATM.

"You might think the PIN goes directly from the ATM machine to your bank, but often, that's not the case," Sullivan said. Instead, it passes through a series of switches, where it's encrypted and decrypted, which is the point where the PIN codes are vulnerable, Sullivan said.

The U.S. Secret Service is investigating the report's findings.