Cyber attackers stole data from 29 million Facebook accounts using an automated program that moved from one friend to the next, Facebook Inc announced on Friday, as the social media company said its largest-ever data theft hit fewer than the 50 million profiles it initially reported.
The company said it would message affected users over the coming days to tell them what type of information had been accessed in the attack.
The breach has left users more vulnerable to targeted phishing attacks and could deepen unease about posting to a service whose privacy, moderation and security practices have been called into question by a series of scandals, cybersecurity experts and financial analysts said.
The attackers took profile details such as birth dates, employers, education history, religious preference, types of devices used, pages followed and recent searches and location check-ins from 14 million users.
For the other 15 million users, the breach was restricted to name and contact details. In addition, attackers could see the posts and lists of friends and groups of about 400,000 users.
Lawmakers and investors have grown more concerned that Facebook is not doing enough to safeguard data.
The company’s shares rose 0.25 per cent on Friday as Wall Street rebounded after a six-day losing streak. The Nasdaq composite index gained 2.29 per cent.
Facebook cut the number of affected users from its original estimate after investigators reviewed activity on accounts that may have been affected. Still, cyber security experts warned that attackers could use stolen information in targeted phishing scams.
“The bottom line is that all this data is still out there,” said Corey Milligan, a senior researcher with cyber-security firm Armor Inc.
Facebook Vice President Guy Rosen told reporters that the US Federal Bureau of Investigation has asked the company to limit descriptions of the attackers due to an ongoing inquiry.
Rosen revealed that while the attackers’ intent has not been determined, they did not appear to be motivated by the upcoming US mid-term Congressional election on Nov. 6.
He said the attack affected a “broad” spectrum of users, but declined to break down the number affected by country, reports Reuters.
Facebook said it was continuing to investigate whether the attackers took actions beyond stealing data, such as posting from accounts, but had not found additional misuse.
Hackers did not steal personal messages or financial data and did not use their access to accounts to access users’ accounts on other websites, Facebook said.
