Both MasterCard and Visa says they are investigating potential security breaches of their cardholders' account information.
MasterCard said Friday it has notified law enforcement agencies and issuers of the MasterCard accounts that are potentially at risk. The credit card company only processes transactions -- the credit cards are issued to cardholders by banks such as Citibank and also a number of retailers.
MasterCard wouldn't say how many cardholders, banks or issuing companies are affected.
The company advises cardholders who have concerns that they should contact the financial institution that issued their cards.
MasterCard says its own systems have not been compromised.
The breach is also said to have involved Visa cards as well.