https://www.cnbc.com/2018/09/28/facebook-says-it-has-discovered-security-issue-affecting-nearly-50-million-accounts-investigation-in-early-stages.html

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 on Friday.

The company said in a blog post its engineering team found attackers found a weakness in Facebook's code regarding its "View As" feature on Tuesday. "View As" lets users see what their profile looks like to other users on the platform. This vulnerability 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.