Twenty-nine million Facebook users have been affected by a hack which allowed accounts to be taken over, the company has disclosed. Attackers exploited a vulnerability in code connected with ‘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 post, which originally said 90 million accounts were affected, was updated 13.10.18. to reflect Facebook’s reassessment of the impact; read Reuter’s report here.]

After the hack was disclosed, the Information Commissioner’s Office said it would be asking how many UK account holders had been affected.

“It’s always the company’s responsibility to identify when UK citizens have been affected as part of a data breach and take steps to reduce any harm to consumers,” said James Dipple-Johnstone, deputy commissioner of operations. “We will be making enquiries with Facebook and our overseas counterparts to establish the scale of the breach and if any UK citizens have been affected.”

Facebook’s engineering team discovered a security issue affecting almost 50 million accounts on Tuesday. “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,” said Guy Rosen, its vice-president of product management.

Rosen said the vulnerability had been fixed and law enforcement had been notified. Facebook has reset the access tokens of the almost 50 million accounts it knows were affected. As a “precautionary step”, it has also of reset access tokens for another 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,” said Rosen. He added: “We’re temporarily turning off the ‘View As’ feature while we conduct a thorough security review.”

The attack exploited the complex interaction of multiple issues in Facebook code, Rosen explained. It stemmed from a change it made to its 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.

“Since we’ve only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed,” said Rosen.

“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.”

Rosen said there was 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 Facebook’s Help Center.

If anyone wanted 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.