The name of the payment processor is not disclosed, but Krebs writes that Visa and MasterCard began alerting banks across the US and warning that specific payment cards may have suffered fraud because of the alleged breach.
MasterCard today did confirm that it is in the midst of an investigation regarding a potential breach. Visa indicated it would shortly release its own statement on the matter.
MasterCard issued a comment, saying it is "currently investigating a potential account data compromise event of a US-based entity and, as a result, we have alerted payment-card issuers regarding certain MasterCard accounts that are potentially at risk.
"MasterCard is concerned whenever there is any possibility that cardholders could be inconvenienced and we continue to both monitor this event and take steps to safeguard account information. If cardholders have any concerns about their individual accounts, they should contact their issuing financial institutions.
"Law enforcement has been notified of this matter and the incident is currently the subject of an ongoing forensic review by an independent data security organisation. It is important to note that MasterCard's own systems have not been compromised in any manner."