AT&T confirmed on Saturday that a total of 73 million current and former customer accounts were leaked to the dark web around two weeks ago.
The company said it is assessing the breach and whether the data, which includes Social Security numbers, came from AT&T itself or one of its vendors.
The data appears to be from 2019 or earlier, cybersecurity experts hired by AT&T to do a "robust investigation" found, affecting 7.6 million current account holders, and approximately 65.4 million former account holders.
"Currently, AT&T does not have evidence of unauthorized access to its systems," the company said in a release. "The company is communicating proactively with those impacted and will be offering credit monitoring at our expense where applicable."
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AT&T said it encouraged current and former customers to visit its account safety page for more information.
The company said it has already reached out to the 7.6 million current customers and reset their passcodes and are in the process of reaching out to former customers.
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"We encourage customers to remain vigilant by monitoring account activity and credit reports," AT&T said. "You can set up free fraud alerts from nationwide credit bureaus — Equifax, Experian, and TransUnion."
A year ago, the company revealed around 9 million customers had been hacked by one of its vendors, exposing Customer Proprietary Network Information (CPNI), which includes information like first names, wireless account numbers, wireless phone numbers and email addresses.
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An AT&T spokesperson said at the time, "A small percentage of impacted customers also had exposure of rate plan name, past due amount, monthly payment amount, various monthly charges and/or minutes used. The information was several years old."