It also appears that those behind the breach "made off with the gold" by intercepting and stealing the so-called Track 2 data from the magnetic stripe on the back of cards, which is all that's needed to create counterfeit cards, Litan said.

Dan Clements, president of , an identity protection service of Affinion Group Inc., said that he has that suggested a major compromise at a processor such as Heartland.

Typically when a card is stolen, crooks first check to see if the cards are still active by using it for some transaction -- often a very small donation to a charitable organization -- to see if it works. This sort of validity check has increased by nearly 20% over the past few months, suggesting a major compromise. But it's not clear yet if it is related to the Heartland breach, Clements said.

The Heartland compromise is the second involving a large payment processor over the past few weeks. One Dec. 23, RBS WorldPay, the payment processing division of The Royal Bank of Scotland Group, by unknown intruders, resulting in the compromise of personal information belonging to about 1.5 million card holders. The compromised information included the Social Security numbers of 1.1 million individuals using payroll cards, the company said.

The incidents suggest that cybercrooks are increasingly beginning to target payment processors, Litan said. "Attacking a processor is much more serious than attacking a retailer. A processor sits at the nerve center of the payment process,"and processes far more payment card data than any retailer, she said.