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In a blog post, Facebook says 50 million of its 2.23 billion active users have been affected by a reported security breach.

Facebook’s engineering team detected a security issue on Sept. 25, with hackers exploiting the vulnerability of its “View As” feature that allows people to view their profile as somebody else.

This allowed them to steal Facebook access tokens which they could then use to take over people’s accounts. Access tokens are digital keys that keep people logged in to Facebook instead of logging in every time they want to connect on the social media network`.

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.

“This attack exploited the complex interaction of multiple issues in our code,” the company says. “It stemmed from a change we made to our video uploading feature in July 2017, which impacted ‘View As.’ The attackers not only needed to find this vulnerability and use it to get an access token, they then had to pivot from that account to others to steal more tokens.”

While the incident is under investigation, the company is temporarily turning the “View As” feature.

The company has reset access tokens of the users the security researchers believe were affected by the breach. Extending the precautionary measures, Facebook is also resetting access tokens of another 40 million accounts that have been subject to a “View As” look-up in the last year.

People who suddenly needed to log in might be one of those 50 million Facebook is certain to have been affected and the 40 million the researchers believed to might have been subjected to the breach.

As a result, around 90 million people will now have to log in back 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.

The company did not reveal if “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 — and we will update this post when we have more information, or if the facts change. In addition, if we find more affected accounts, we will immediately reset their access tokens.”