Facebook said that it is working with the FBI and other law enforcement agencies to help investigate the security issue.
Guy Rosen, Facebook’s VP of Product Management, wrote in a Security Update post that the investigation is still in early stages. But, he says, “it’s clear that attackers exploited a vulnerability in Facebook’s code that impacted “View As”, a feature that lets people see what their own profile looks like to someone else. This allowed them to steal Facebook access tokens which they could then use to take over people’s accounts. 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.”
Rosen goes on to say that people’s security and privacy is incredibly important which is why they’ve taken immediate action. Facebook will update the post once they have more information.
Full Security Update post here: Security Update