Facebook today revealed that more nearly 50 million accounts have been affected by a "security issue." Discovered by Facebook's engineering team on September 25, the issue allowed attackers to take over people's accounts by stealing Facebook access tokens.
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. 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.
This attack exploited the complex interaction of multiple issues in our code. It stemmed from a change we made to our video uploading feature in July 2017, which impacted "View As." The attackers not only needed to find this vulnerability and use it to get an access token, they then had to pivot from that account to others to steal more tokens.
In response, Facebook has reset the access tokens of the nearly 50 million accounts that it is aware were affected by the breach. Further, the company says it is resetting tokens for an additional 50 million accounts as a precautionary measure.
As a result, people who have had their security tokens reset will have to log back into the Facebook and any of the Facebook apps they were previously logged into.
Given the avenue of attack, Facebook has also opted to turn off the "View As" feature as it conducts a security review. Currently, there's no indication as to who was behind the attack, but Facebook says it has reached out to law enforcement and has fixed the vulnerability.