Facebook has suffered a security breach affecting nearly 50 million users.
In a post on the social network’s news site, Facebook vice president of product management Guy Rosen said: “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.
“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.”
It was unclear whether any UK users had been hit, but some reported getting a message when they tried to log in that said: “Recent activity may have affected your account’s security, so we’ve locked it.
“We’ll walk you through a few steps to confirm your identity and help you access your account.”
Mr Rosen outlined the action Facebook had taken since the discovery, including fixing the issue and reporting it to law enforcement.
He said: “We have reset the access tokens of the almost 50 million accounts we know were affected to protect their security.
“We’re also taking the precautionary step of resetting 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.
“Third, we’re temporarily turning off the ‘View As’ feature while we conduct a thorough security review.”
A spokesman for the National Cyber Security Centre said: “We are investigating how this breach has affected people in the UK and advise on appropriate mitigation measures. Users should read the latest advice Facebook has published.
“Based on current information, we understand that Facebook have fixed the flaw by temporarily suspending the ‘view as’ feature.
“There is no evidence that people have to take action such as changing their passwords or deleting their profiles.
“However, users should be particularly vigilant to possible phishing attacks, as if data has been accessed it could be used to make scam messages more credible.”
Tom Watson, Labour’s Digital, Culture, Media and Sport shadow secretary, said: “Facebook reaps enormous financial value from the data of its users. One of their single biggest responsibilities is to protect that data and detect breaches. They have failed to do that.
“Facebook should have discovered this industrial scale data breach months ago. It is very disappointing that it has only come to light now.
“We need to know where affected users are and exactly how the breach happened. Facebook should not be refusing to disclose this.
“While the Tories are refusing to do anything to beef up tech regulation, Labour is clear that we need a powerful bill of digital rights and a single regulator strong enough to bring today’s tech titans to heel.”
Facebook has more than two billion users worldwide, and has been hit by a series of problems this year, including the news that data analytics firm, Cambridge Analytica, had gained access to personal data from millions of user profiles.
Adam French, Which? consumer rights expert, said: “Facebook’s data breach will deeply concern users. It is critical that the company does all that it can to ensure affected people get clear information about what has happened and assists anyone negatively impacted.
“Anyone concerned they could be at risk as a result of the hack should consider changing their password, monitor bank and other online accounts and be wary of emails regarding the breach as scammers may try to take advantage of it.”
Damian Collins, chairman of the Commons Digital, Culture, Media and Sport select committee, tweeted: “More serious questions for Mark Zuckerberg and Facebook – this is why @CommonsCMS will continue to press for him to give evidence to our parliament – Facebook Network is Breached, Putting 50 Million Users’ Data at Risk”