It's not just MasterCard holders who are affected. MasterCard says its security division detected multiple instances of fraud that tracked back to CardSystems Solutions Incorporated, which processes credit card and other payments for banks and merchants.

The compromised data included names, banks and account numbers, but not addresses or Social Security numbers. Such data could be used to steal funds, but not identities.

CardSystems says it first learned of a potential breach on May 22nd. CardSystems says it was told by the FBI not to release any information to the public. The company says it's surprised by MasterCard's decision to go public.