This is why over 90 million users, including yours truly, had to log back in to the Facebook app.

Facebook chief executive Mark Zuckerberg said engineers discovered the breach on Tuesday, and patched it on Thursday night.

Facebook says that they've already fixed the vulnerability and informed law enforcement.

"Our investigation is still in its early stages", stated Guy Rosen, VP of Product Management, for Facebook.

The tokens are used to keep people logged into their Facebook account; a third-party can use them to pretend to be that person and log into their account.

The latest security incident reportedly stemmed from a change made to Facebook's video uploading feature in July 2017. The company said that it was also notifying almost 90 million users who would be required to log into their Facebook account and reset the password. These users will have to log into their Facebook account and change their passwords immediately.

Facebook have assured users that the 50 million people affected have had their tokens reset.

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The social media site do not know who is behind the attack or where they are based, according to a statement.

The same was taken with a further 40 million accounts as a precautionary measure and the "view as" feature has been disabled.

The attackers exploited a feature called "View as" that lets users see their Facebook page the way someone else would. It is not clear whether the affected accounts were misused or had information accessed. After all, the Cambridge Analytica fiasco was worse than Facebook initially believed or admitted. Once these users do sign-in, they will receive a notification in their Facebook News Feed with an explanation about what occurred. We'll update this post as more information becomes available.

So if you had to re-enter your Facebook password today, this is why. It also has no idea who was behind the attacks. "We need to prevent this from happening in the first place".

He added: "People's privacy and security is incredibly important, and we're sorry this happened".

Facebook has more than 2 billion users worldwide.