A computer hacker gained access to more than 5.6 million Visa and MasterCard account numbers by breaching the security of a company that processes transactions for merchants, the card associations said on Tuesday.
Visa USA spokesman Mike Riley said that there has been no report of fraudulent activity involving the accounts and that Visa was monitoring the situation.
He said he could not identify the third-party processor or say exactly when or how the hacker got access to the account information, which involves some 3.4 million Visa accounts and 2.2 million MasterCard accounts.
A source with knowledge of the situation who spoke on condition of anonymity said that the incident occurred in early February.
Processors handle transactions for merchants, bundling and transmitting charges to the banks that issue the cards.
Visa said that after learning of the incident, the company's fraud team "immediately notified all affected card-issuing financial institutions and is working with the third-party payment card processor to protect against the threat of a future intrusion."
Visa added that under its zero-liability policy, customers would pay nothing in cases of unauthorized purchases.
MasterCard said that affected banks had been notified. A MasterCard spokesman did not immediately return calls seeking comment.
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