AT&T has confirmed that a security breach occurred back in April in which personal information including Social Security numbers and call records of AT&T Mobility customers being accessed. The breach has affected an unknown number of customers.

AT&T alleges that this breach was carried out by employees of one of company's service providers which has not named at this point.

According to ITWorld, the breach took place between April 9-21, but was only disclosed this week in a filing with California regulators. While AT&T wouldn't say how many customers were affected, state law requires such disclosures if an incident affects at least 500 customers in California.

"Employees of one of our service providers violated our strict privacy and security guidelines by accessing your account without authorization," the company said in a letter to affected customers. "AT&T believes the employees accessed your account as part of an effort to request codes from AT&T than are used to unlock AT&T mobile phones in the secondary mobile phone market."

An AT&T spokesman declined to address the issue.