This was a bad week for America’s mobile network operators, starting with a massive data breach at T-Mobile US and ending with another reported cyberattack at AT&T.

A cybercriminal claims to have personal data on 70 million AT&T customers, according to a Restore Privacy report. The data includes names, phone numbers, physical addresses, email addresses, social security numbers, and dates of birth.

AT&T, following a quick investigation, denies it was attacked. “Based on our investigation yesterday, the information that appeared in an internet chat room does not appear to have come from our systems,” an AT&T spokesperson said in a statement to SDxCentral. 

The hacker, which posted details about the intrusion on an underground forum that included a sample of the data, is reportedly trying to sell the information for $1 million. Restore Privacy described the individual as a “well-known threat actor” that has previously exploited other organizations, including MeetMindful, Pixlr, Tokopedia, and the financial service Dave.

The reported cyberattack at AT&T comes just days after T-Mobile suffered perhaps the largest carrier data breach on record. T-Mobile claims it was a “highly sophisticated cyberattack” and thus far admits personal data on at least 54 million people was exposed and stolen. 

Mobile network operators’ massive customer bases and the trove of personal data they store on customers make them an “ideal target for cybercriminals,” Forrester Research analyst Allie Mellen told SDxCentral, responding to the T-Mobile attack.

Cybercriminals “can sell off the personal data of mobile network operators’ customers” and nation-state attackers “can use data these providers have to gather call and location information on specific individuals,” she added. 

This marks the fifth publicly acknowledged data breach at T-Mobile in three years, and the Federal Communications Commission earlier this week opened an investigation into the latest incident. Unfortunately, for millions of T-Mobile customers and potentially millions more at AT&T, the damage is already done.