One of the largest carriers in the US, AT&T has roughly 200 million wireless customers, but only a small percentage of the total has been impacted by the incident.

“Approximately 9 million wireless accounts had their Customer Proprietary Network Information accessed,” AT&T said in an emailed statement.

“We recently determined that an unauthorized person breached a vendor’s system and gained access to your ‘customer proprietary network information’ (CPNI),” the company told impacted customers in an email notification, copies of which were shared by users on AT&T’s community forums.