Hackers stole credit card numbers belonging to as many as 1.5 million MasterCard and Visa customers, Global Payments, Inc. confirmed on Sunday. The international credit card processor was blocked by Visa after it reported the possibility of a major security breach on Friday. The company did not indicate how the hackers gained access to its system or who might be responsible for the attack. “Based on the forensic analysis to date, network monitoring and additional security measures, the company believes that this incident is contained,” the firm told The Wall Street Journal while noting that cardholder names, addresses and Social Security numbers were not compromised. The company did say that the credit card numbers were downloaded during the attack rather than just being accessed, however, indicating that the perpetrators may intend to use the information to create counterfeit credit cards. Affected Visa and MasterCard customers have not yet been notified that their account information was stolen.