At least 15 million T-Mobile customers' personal data was stolen by hackers. The data was stolen when hackers accessed customers' personal information through the company that T-Mobile uses to process credit checks when signing up new customers for their service.

T-Mobile uses the credit report company Experian to see if a potential subscriber is eligible for wireless service. Experian runs a credit check on these customers, and based on the results, T-Mobile decides what rates and promotions they qualify for.

Experian announced Thursday that it had experienced a data breach that affects customer data from Sept. 1, 2013 to Sept. 15 2015, CNET reports. Data stolen includes names, birth dates, addresses, Social Security numbers and drivers' license numbers. Credit card numbers and payment information were not stolen, Experian said.

Customers who went through the credit check process with T-Mobile during that time period. These customers could be current or former subscribers or even people who just tried to become a subscriber that went through the credit check, but did not sign up.

T-Mobile is just one of the high-profile companies to be affected by a data breach. Recently, the U.S. government was breached and 4 million federal workers data was stolen. Additionally, health insurer Excellus BlueCross BlueShield was hacked into and 10 million health records were stolen. Last year, Target and Home Depot were two major companies to suffer a data breach.

"This data breach is certainly a big deal," said Jonathan Bowers, a fraud and data specialist at fraud prevention provider Trustev. "Give a fraudster your comprehensive personal information, they can steal your identity and take out lines of credit that destroy your finances for years to come." 

T-Mobile's CEO John Legere informed customers of the data breach through a tweet, blog post and FAQ page.

"Obviously I'm 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," he said. 

T-Mobile has almost 60 million subscribers, which means that more than a quarter of its customers are affected by this data breach.

Experian is taking responsibility for the breach and said they will inform those who are affected by letters next week.

A free two years of credit monitoring and identity protection services will be offered by Experian to any T-Mobile customer, even if they have not been affected. The offer is from Experian's Protect My ID.

"We take privacy very seriously and we understand that this news is both stressful and frustrating," said Craig Boundy, chief executive of Experian North America.