LOS ANGELES (CBS) — An investigation was underway Friday into what officials said could be a “massive” security breach by a MasterCard contractor.

As many as 10 million Visa and MasterCard numbers may have been compromised at a third-party vendor that processes transactions for the credit card issuer.

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Former Washington Post staffer and “Krebs on Security” blogger Brian Krebs told KNX 1070 the fraud may not affect all types of customers.

“In cases where they have seen fraud related to these compromised cards, it’s been on business credit cards as opposed to consumer cards,” said Krebs.

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Visa has issued a statement acknowledging a potential data compromise at a third party, but the company denied any breach of its own systems. MasterCard also pledged to take steps to safeguard account information.

“Issuing banks are trying to figure out what — if anything — they need to do to reissue cards, they’re trying to figure out the common point of purchase on these cards to see if they can figure out where the breach actually occurred,” Krebs said.

So far, investigators say most of the cards were used at parking garages around New York City between January 21st and February 25th.

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Cardholders with concerns about their individual accounts should contact the financial institution that issued the card.