US telecommunications giant AT&T says it has started notifying millions of customers about the theft of personal data recently discovered online.

The company said Saturday that a dataset found on the “dark web” contains information such as social security numbers for about 7.6 million current AT&T account holders and 65.4 million former account holders.

AT&T said it had already reset the passcodes of current users and will be communicating with account holders whose personal information was compromised.

It is not known if the data “originated from AT&T or one of its vendors”, the company said in a statement.

The compromised data is from 2019 or earlier and does not appear to include financial information or call history, it said.

In addition to passcodes and social security numbers, it may include email and mailing addresses, phone numbers and birth dates.

It’s understood the data surfaced on a hacking forum nearly two weeks ago.

An AT&T spokesperson didn’t immediately return a request for comment Saturday.

It is not the first crisis this year for the Dallas-based company. An outage in February temporarily knocked out mobile phone service for thousands of US users.

AT&T at the time blamed the incident on a technical coding error, not a malicious attack.