Facebook says it discovered a security breach earlier this week that affected almost 50 million accounts. The company says it has fixed the breach, which allowed hackers to take over people's accounts.

The social-media network said in a statement Friday that it has told law enforcement authorities about the breach. Shares declined about 3 percent on the news.

There was a loophole in Facebook's code for a feature called "View As" that let people see what their account looks like to someone else. The vulnerability allowed people to steal access tokens -- digital keys that keep people logged into Facebook so they don't need to re-enter passwords. Once logged in, the attackers could take control.

"This attack exploited the complex interaction of multiple issues in our code. It stemmed from a change we made to our video uploading feature in July 2017, which impacted 'View As,'" Facebook said in the statement. "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."

User data leaks, security breaches and the spread of misinformation have forced Facebook to confront hostile congressional hearings and uproar from users. This week's breach adds to concern that Facebook is collecting too much personal information and not looking after it properly. Data is the lifeblood of its advertising business, so any limits on its activities that stem from these missteps could crimp the company's earning power.

While access codes were taken from 50 million accounts in the recent breach, Facebook said it doesn't know whether any personal information was gathered or misused from those accounts.

Everyone whose profile used the "View As" tool in the last year will have to log in to Facebook again, and any apps that used Facebook to log in. From there, they'll be able to see a statement from Facebook explaining what happened. The company estimated that about 90 million people will have to log in again.

-- Facebook News