About 15 million T-Mobile US (TMUS) customers' personal information could be at risk. Experian (EXPGY), the vendor that processes T-Mobile's credit applications, recently notified the wireless company of the data breach. Dublin, Ireland-based Experian sounded the alarm after finding out that hackers in September broke into its computer system, and stole two years’ worth of records containing names, social security numbers, addresses, birthdays, and identifications numbers, such as military ID, driver's license or passport number. But, no card or banking information was stolen. The breach was discovered on September 15 and T-Mobile was notified about a week afterwards, the company said. Overall, this data breach affects customers who applied for T-Mobile financing or postpaid services from September 1, 2013 to September 16, 2015. Responding to the hack, T-Mobile CEO John Legere said, 'Obviously I am incredibly angry about this data breach and we will institute a thorough review of our relationship with Experian, but right now my top concern and first focus is assisting any and all consumers affected. I take our customer and prospective customer privacy VERY seriously.' After discovering the breach, Experian said it immediately secured its server and contacted both the U.S. and international law enforcement. TheStreet’s U-Jin Lee reports from New York.