What happened? Hackers infiltrated the computers of CardSystems Solutions – a credit card processing center for MasterCard International. As many as 40 million credit card numbers may have been stolen, including American Express, MasterCard, Visa, Discover and other cards. NOTE: Social security numbers or other items of personal information are not at risk. Be aware that your bank or credit union may choose to place a temporary freeze on your credit card account to limit chances of unauthorized use.