A hacker nicknamed "" posted a link to the data dump on , also writing on Twitter that he had "penetrated over 79 large banks" and holds 50GB of data on MasterCard and Visa cardholders. No card numbers were released, however.

A payment card industry source, who did not want to be identified by name, said the data may have been copied from another website that released it more than two weeks ago, and the details appeared to be "old."

"We see people try to dump stuff all the time and claim that it is real," the source said.

MasterCard and Visa, which are aware of the breach, do not actually hold information on individual cardholders. That information is held by banks, as well as the many companies involved in processing credit-card transactions.