The data set appears to be from 2019 or earlier, impacting approximately 73 million current AT&T account holders and approximately 65.4 million former account holders, the company said.
"With respect to the balance of the data set, which includes personal information such as social security numbers, the source of the data is still being assessed," AT&T said.
"Currently, AT&T does not have evidence of unauthorized access to its systems resulting in exfiltration of the data set. The company is communicating proactively with those impacted and will be offering credit monitoring at our expense where applicable. We encourage current and former customers with questions to visit www.att.com/accountsafety for more information."
AT&T has not yet determined if the data leak originated from AT&T or a vendor.
Back in February, AT&T was hit with a widespread outage impacting upwards of 70,000 customers.
Editor's note: A previous version of this story erroneously stated that 7.6 million customers were impacted. The story has been updated for accuracy to reflect that 73 million customers were impacted.
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