Facebook says it is taking steps to fix a security problem that has affected nearly 50 million user accounts.

The company says hackers exploited its “View As” feature and the social media company says it has alerted law enforcement about the security breach.

“This allowed them to steal Facebook access tokens which they could then use to take over people’s accounts,” a news release from the company states.

“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.”

To deal with the issue, Facebook reset some logins, so 90 million people have been logged out and will have to log in again.

The “View As” feature will also be temporarily turned off while the company conducts its investigation.

“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 — and we will update this post when we have more information, or if the facts change.”