In the biggest-ever security breach after Cambridge Analytica scandal, Facebook on Friday admitted hackers broke into nearly 50 million users' accounts by stealing their "access tokens" or digital keys. This allowed them to steal Facebook access tokens which they could then use to take over people's accounts, Facebook said in a statement.

Access tokens are the equivalent of digital keys that keep people logged in to Facebook so they do not need to re-enter their password every time they use the app. "Our investigation is still in its early stages. But 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," said Guy Rosen, VP of Product Management.