Facebook Inc (FB.O) on Friday said attackers in the mass security breach it announced late September accessed the accounts of about 30 million people in total and stole name and contact details for 29 million.
Facebook found no theft of highly personal messages or financial data, and saw no use of Facebook logins to access other websites, all of which would have been cause for greater concern.
Instead, stolen data on 14 million users included birth-dates, employers, education and lists of friends.
All of those could help a fraudster pose as Facebook, the employer or a friend.
They could then craft a more sophisticated email aimed at tricking users into providing login information on a fake page or into clicking on an attachment that would infect their computers.
“We’re cooperating with the FBI, which is actively investigating and asked us not to discuss who may be behind this attack,” Facebook said on a blog post.
The social network said in late September that hackers stole digital login codes allowing them to take over almost 50 million user accounts in its worst security breach ever.
However, Facebook did not confirm if information had actually been stolen.
Facebook’s latest vulnerability has existed since July 2017, but the company first identified it in mid-September after spotting a fairly large increase in use of its “view as” privacy feature.
It determined that it was an attack on September 25.
“Within two days, we closed the vulnerability, stopped the attack, and secured people’s accounts by restoring the access tokens for people who were potentially exposed,” Facebook said.
The “view as” feature allows users to check their privacy settings by giving them a glimpse of what their profile looks like to others.
But a trio of errors in Facebook’s software enabled someone accessing the feature to post and browse from Facebook accounts of other users.
Facebook shares fell 2.6 per cent after the breach was announced in September and they were down more than 1 per cent following the updated disclosures on Friday.