AT&T alerts 9 million customers of data breach after vendor hack. AT&T is notifying roughly 9 million customers that some of their information has been exposed after one of its marketing vendors was hacked in January. AT&T is notifying roughly 9 million customers that some of their information was exposed after a marketing vendor was hacked in January. “Customer Proprietary Network Information from some wireless accounts was exposed, such as the number of lines on an account or wireless rate plan,” AT&T told BleepingComputer.