The breach, Facebook’s worst ever, has exacerbated concerns among users, lawmakers and investors that the company is not doing enough to safeguard data, particularly in the wake of the Cambridge Analytica data scandal.
Still, hackers neither accessed personal messages nor financial data and did not use Facebook logins to access other websites, all of which would have been a cause for greater concern.
Facebook originally had said in late September hackers stole digital login codes to take over nearly 50 million user accounts.
On Friday, the company revealed that stolen data on 14 million users included birth dates, employers, education and lists of friends. For 15 million users, it was restricted to just name and contact details.
All of those could help a fraudsters 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.
Facebook said it will send customised messages in the coming days to affected users to explain what information the attackers accessed and how they can protect themselves, including from suspicious emails, text messages or calls.
A company executive said on a conference call that Facebook will not provide country-by-country breakdowns of the affected users.
The hackers used an automated programme to move from account to account and harvest the data quickly.
"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 here
The social network in late September did not confirm if information had actually been stolen.“There’s not much more that Facebook can do,” said Michael Pachter, an analyst with Wedbush Securities. “The stolen data is likely to be used by the hackers, so this problem is likely to persist for quite some time.”
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” feature. It determined that it was an attack on Sept. 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 did not rule out the possibility of smaller-scale attacks and said it would continue to investigate.