The world’s two largest credit card processors have notified U.S. banks of a potential security breach that may affect more than 10 million cardholders, Reuters reported on Friday. MasterCard and Visa have said that the issue was the result of a third-party vendor and not their own internal systems. MasterCard said it has taken the proper steps by alerting law enforcement officials and hiring an independent data-security organization to review the possible breach. “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,” the company said in a statement. “If cardholders have any concerns about their individual accounts, they should contact their issuing financial institution.” Visa made sure to emphasize that its customers are not responsible for any potential fraudulent charges.