Facebook discovered a security issue that allowed hackers to access information that could have let them take over around 50 million accounts, the company announced Friday. "This is a very serious security issue, and we're taking it very seriously," said CEO Mark Zuckerberg on a call with reporters. Facebook shares, which were already down about 1.5 percent before the announcement, extended losses after the disclosure and ended down 2.6 percent. The company said in a blog post that its engineering team found on Tuesday that attackers identified a weakness in Facebook's code regarding its "View As" feature. Facebook became aware of a potential attack after it noticed a spike in user activity on Sept. 16. "View As" lets users see what their profile looks like to other users on the platform. This vulnerability, which consisted of three separate bugs, also allowed the hackers to get access tokens — digital keys which let people stay logged into the service without having to re-enter their password — which could be used to control other people's accounts.

Almost 50 million accounts had their access tokens taken, and Facebook has reset those tokens. The company also reset tokens for an additional 40 million accounts who used the "View As" feature in the last year as a precautionary measure, for a total of 90 million accounts. Facebook had 2.23 billion monthly active users as of June 30. The reset will require these users to re-enter their password when they return to Facebook or access an app that uses Facebook Login. They will also receive a notification at the top of their News Feed explaining what happened. In addition, the company suspended the "View As" feature while it reviews its security. Facebook said it fixed the issue on Thursday night and has notified law enforcement including the FBI and the Irish Data Protection Commission in order to any address General Data Protection Regulation (GDPR) issues.