A recent online security breach involving the left of 360,000 credit card numbers will cost Citigroup $2.7 million, the company confirmed to U.S. government officials on Monday. Hackers infiltrated Citigroup servers last month and stole account numbers and personal information associated with over 360,000 Citi-branded credit cards. According to Citigroup, personal information and card numbers from approximately 3,400 cardholders was subsequently used to make about $2.7 million in unauthorized purchases. Citigroup stated that affected customers would be reimbursed for the fraudulent charges. No arrests have been made in association with the breach.