What happened

Facebook said hackers exploited a bug that affected its ‘View As’ feature, which lets people see what their profiles look like to someone else. That would let attackers steal the ‘access tokens’ Facebook uses to keep people logged in. Possession of those tokens would allow attackers to “seize control” of user accounts.

To deal with the issue, Facebook reset some logins, so 90 million people have been logged out and will have to log in again. That includes anyone who has been subject to a ‘View As’ lookup in the past year.