NEW YORK CITY - Facebook said 50 million user accounts are impacted by a security flaw that allowed attackers to steal access tokens, allowing them to take over user accounts. 

The company said it reset the access tokens of the 50 million accounts that were affected by the attack, but that it is taking a "precautionary step" of resetting tokens for another 40 million accounts. 

"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,"  Guy Rosen, vice president of product management, wrote in a blog post on Facebook. 

The "View As" feature allows users to see what their account looks like to other users. 

The security flaw allowed hackers "to steal Facebook access tokens which they could then use to take over people's accounts," Rosen wrote. "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."

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