The incidents also shine a spotlight on what appears to be an increase in attempts by criminals to compromise PIN-based card transactions, which have long been considered very secure, analysts said.

The most recent furor was ignited last week when Citi-bank acknowledged that it had put transaction holds on an unspecified number of its MasterCard debit cards after detecting fraudulent cash withdrawals in Canada, Russia and the U.K.

In a statement, Citibank said that the fraud was the result of a "third-party business information breach" last year. To protect its customers, the company "blocked PIN-based transactions in those locations for the customers affected by the breach," the statement said.

A possible lead

A spokesman for the company refused to name the company that suffered the information breach, though a source involved with the investigation said evidence suggests that point-of-sale systems at a California store of retailer OfficeMax Inc. were somehow involved in the compromise.