According to ZDNet the largest ever data security breach has hit MasterCard and Visa:

MasterCard International on Friday said information on more than 40 million credit cards may have been stolen.

Of those exposed accounts, about 13.9 million are for MasterCard-branded cards, the company said in a statement. Some 20 million Visa-branded cards may have been affected and the remaining accounts were other brands, including American Express and Discover.

MasterCard and Visa both say they have notified their member banks of the specific accounts involved so the banks can take action to protect cardholders…

The breach occurred at CardSystems Solutions in Tucson, Arizona, a third-party processor of payment data, according to a MasterCard statement. An intruder was able to use security vulnerabilities to infiltrate the CardSystems network and access the cardholder data, MasterCard said.

More from the Washington Post.

UPDATE 21 June: ABC News says 120,000 Australians affected

UPDATE 26 July: The Age reports that Amex and Visa have cut ties with card processor