Personal data belonging to 73 million current or former AT&T customers has been leaked online.
Information including addresses, social security numbers and passcodes was published on the dark web, the US telecoms giant said.
AT&T said it had not identified evidence indicating the data had been stolen but had brought in cybersecurity experts to investigate.
The company said it had reset customers' passcodes.
They were urged by the company to "remain vigilant by monitoring account activity and credit reports".
The data involved in the breach appears to be from 2019 or earlier and is linked to 7.6 million customers and 65.4 million former account holders.