"It’s clear that attackers exploited a vulnerability in Facebook’s code that impacted 'View As', a feature that lets people see what their own profile looks like to someone else. This allowed them to steal Facebook access tokens which they could then use to take over people’s accounts," Guy Rosen, Facebook's vice-president of product management, announced in a company blog post.
Access tokens are the equivalent of digital keys that keep people logged in to Facebook so they don’t need to re-enter their password every time they use the app.
Facebook's engineering team discovered the attack on Tuesday, September 25, and the company said they have informed law enforcement.
"We have reset the access tokens of the almost 50 million accounts we know were affected to protect their security. We’re also taking the precautionary step of resetting access tokens for another 40 million accounts that have been subject to a 'View As' look-up in the last year," Rosen said.
"As a result, around 90 million people will now have to log back in to Facebook, or any of their apps that use Facebook Login. After they have logged back in, people will get a notification at the top of their News Feed explaining what happened," he added.
The “View As” feature hit by the attack has also been turned off.
"Since we’ve only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed. We also don’t know who’s behind these attacks or where they’re based. We’re working hard to better understand these details," Rosen explained.
Hundreds of millions of people have accounts in the social media giant. Facebook is based in Menlo Park, California.
Shares of Facebook were down 3.6% to $162.74 by midsession on Friday.