Facebook says the accounts of 50 million users have been exposed after a “security issue”.

The breach occurred on Tuesday after hackers exploited a vulnerability in Facebook’s code that impacted ‘View As’, a feature which allows people see what their profile looks like to someone else.

In a statement on Friday, Guy Rosen, Facebook vice-president, management, said the attackers stole access tokens which they could use to take over accounts of users.

Rosen said the company has reset over 50 million access tokens of those affected.

He said the company is turning off the ‘View As’ feature in order to conduct a thorough security review.

“On the afternoon of Tuesday, September 25, our engineering team discovered a security issue affecting almost 50 million accounts. We’re taking this incredibly seriously and wanted to let everyone know what’s happened and the immediate action we’ve taken to protect people’s security,” the statement read.

“We’re also taking the precautionary step of resetting access tokens foranother 40 million accounts that have been subject to a “View As” look-up in the last year. As a result, around 90 million people will now have to log back in to Facebook, or any of their apps that use Facebook login.

“After they have logged back in, people will get a notification at the top of their News Feed explaining what happened.

“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.”

Rosen said the company is yet to know whether the affected accounts were misused or if any information was accessed.

“We also don’t know who’s behind these attacks or where they’re based. We’re working hard to better understand these details — and we will update this post when we have more information, or if the facts change. In addition, if we find more affected accounts, we will immediately reset their access tokens,” the statement read.

“People’s privacy and security is incredibly important, and we’re sorry this happened. It’s why we’ve taken immediate action tosecure these accounts and let users know what happened. There’s no need for anyone to change their passwords.

“But people who are having trouble logging back into Facebook — for example because they’ve forgotten their password — should visit our Help Center. And if anyone wants to take the precautionary action of logging out of Facebook, they should visit the “Security and Login” section in settings. It lists the places people are logged into Facebook with a one-click option to log out of them all.”

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