is in the process of notifying millions of customers that their personal data has been leaked in a "dark web" breach that contained information including Social Security numbers. The carrier confirmed on Saturday that a dataset found on the " " contains information including Social Security numbers for around 7.6 million current account holders as well as 65.4 million former account holders. The company has confirmed it has already reset the passcodes of current users. It added it would be speaking to account holders whose sensitive personal information was compromised in the massive leak. In a statement released on Saturday, AT&T said: "As of today, this incident has not had a material impact on AT&T's operations." An investigation is now underway to determine how the leak, which appears to have happened two weeks ago, came about. A preliminary review found the leaked data was from around 2019 or earlier and included personal information such as names, home addresses, phone numbers, and dates of birth, as well as Social Security numbers. It's believed the data does not contain personal financial information or call history relating to users. AT&T has said users who are impacted will receive an email, and has encouraged all users to set up fraud alerts and monitor their account activity and credit reports. It is not known if the data "originated from AT&T or one of its vendors", the company added in its statement. The source of the leak remains unclear, however, the company said there is no evidence so far that the leak was caused by unauthorized access to the company's systems. AT&T began the process of resetting millions of passcodes after the outlet informed the carrier that leaked data contained encrypted passcodes that could be used to access customers' accounts. According to a security researcher who looked at the leaked data, the encrypted passcodes are easy to decipher, reported the outlet. AT&T's full statement read: "AT&T has launched a robust investigation supported by internal and external cybersecurity experts. Based on our preliminary analysis, the data set appears to be from 2019 or earlier, impacting approximately 7.6 million current AT&T account holders and 65.4 million former account holders." "AT&T does not have evidence of unauthorized access to its systems resulting in exfiltration of the data set," the statement added. AT&T account passcodes are generally made up of four numbers that are used as an extra security step when accessing a customer's account, whether that's on the phone with AT&T customer service, in retail stores, or online. Sharing tips on how customers could better protect their accounts, AT&T said: "We encourage customers to remain vigilant by monitoring account activity and credit reports. You can set up free fraud alerts from nationwide credit bureaus -
Equifax, Experian, and TransUnion."