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.
Attempts to reach some of the U.S. cardholders affected were unsuccessful, since many of the phone numbers were disconnected or incorrect.
But another person in the list, Sydney resident Julian Bale, said the information was very old. The home address published for him is seven years out of date, and an e-mail address published at least four years old, Bale said in a phone interview Tuesday.
The e-mail address listed came from a hosting provider that Bale had paid through PayPal, which means the company would not have had his credit card details. The data dump claims to have a MasterCard number for Bale.
The majority of the data appeared to come from U.S. cardholders, although other people listed purportedly live in countries including Egypt, Cambodia, Israel, Turkey, Pakistan and elsewhere. The data includes only five digits of the credit card numbers and no expiration dates or three-digit security codes.
Reckz0r wrote on Twitter that he deleted a message containing the Visa number for Reid Hoffman, the founder of the social-networking site LinkedIn, for fear his Twitter account would be suspended.
The mix of international addresses indicates that the target could have been an international payment processor, said Dan Clements, who runs CloudeyeZ, a security consultancy that monitors underground forums for sensitive data and tries to match the data with its owner.
Those payment processors are a rich target for hackers. In April, the payment processor Global Payments said hackers stole details for 1.5 million credit and debit cards, including account numbers, card expiration dates but not names of consumers or Social Security numbers. Global Payments processes payments for about one million storefronts worldwide.