The credit monitoring firm Experian suffered a major breach in fall 2015 when hackers were able to steal the personal information of 15 million consumers who had applied for T-Mobile wireless service.

Announced on: September 15, 2015

Description of breach: An unauthorized party accessed T-Mobile (the wireless service provider) customer data that is housed in an Experian (the credit monitoring firm) server.

Information exposed: Experian’s consumer credit database was not assessed in this incident, and no payment card or banking information was obtained. However, records containing a name, address, Social Security number, date of birth, identification number (typically a driver’s license, military ID, or passport number) and additional information used in T-Mobile’s own credit assessment were accessed. No payment card or banking information was obtained.

Affected customers: Estimated 15 million, including some current T-Mobile customers as well as consumers who applied for T-Mobile USA postpaid services or device financing, which required a credit check, from Sept. 1, 2013 through Sept. 16, 2015

Official information from T-Mobile: https://www.t-mobile.com/landing/experian-data-breach-faq.html

Help for victims: Experian offered complimentary identity resolution services to affected customers.

More coverage of the breach: Reuters