Facebook disclosed a widespread security flaw that could have allowed hackers or other malicious third parties to access an affected user’s account by gleaning their security token. It also says it’s fixed the issue and alerted law enforcement, indicating that this is not an accidental engineering mistake, but a purposeful exploit discovered and used by some third-party organization or hacker. The company says its engineering team was made aware of the issue on September 25th, but Guy Rosen, Facebook’s vice president of product management, says it’s not clear whether accounts were compromised, when the issue was exploited, or who might have been behind the attack.
“On Tuesday, we discovered that an attacker exploited a technical vulnerability to steal access tokens that would allow them to log into about 50 million people’s accounts on Facebook,” wrote CEO Mark Zuckerberg in a post to his personal Facebook page. “We do not yet know whether these accounts were misused but we are continuing to look into this and will update when we learn more.”
The flaw could have let someone exploit the “View As” feature, which lets you view your own profile as it appears to another user or to the public, as a way of evaluating your specific sharing settings. However, it appears that the feature inadvertently exposed Facebook security tokens when someone selected a profile as the desired View As target. That would let someone gain access to the person’s account. Facebook access tokens are the digital keys that allow mobile users to log in to their accounts without having to retype their passwords.
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.
On a call with reporters following the announcement, Facebook said that the video uploading feature in July of last year related to a tool that allowed users to upload birthday videos in a way that would allow the View As feature to expose secure information, but only when interacting with two other bugs. The company also confirmed that no credit card info was exposed.