Personal data belonging to 73 million current or former AT&T customers was leaked online.
The US telecommunications giant said information such as addresses, social security numbers and passcodes was exposed on the dark web.
AT&T said it has not identified any evidence that the data was stolen, but has brought in cybersecurity experts to investigate.
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They were urged by the company to “monitor your account activity and credit report and remain vigilant.”
The data involved in the breach appears to date back to 2019 or earlier and is associated with 7.6 million customers and 65.4 former account holders.
Information such as names, email addresses and dates of birth was also included, but AT&T said financial information was not included in the breach.
The company said in a statement that it was unclear whether the data originated from its own systems or via a third-party supplier.
AT&T's wireless 5G network covers approximately 290 million people across the United States, making it one of the nation's largest mobile and internet service providers.
In February, a major outage affected tens of thousands of phone users, and the company apologized and offered affected users a $5 credit.
New York prosecutors opened an investigation into the incident, and people were unable to use their cell phones for about 12 hours.