Facebook says it recently discovered a security breach affecting nearly 50 million user accounts.

The hack is the latest setback for Facebook during a year of tumult for the global social media service.

In a blog post, the company says hackers exploited a bug that affected its “View As” feature, which lets people see what their profiles look like to someone else. That would let attackers steal the “access tokens” Facebook uses to keep people logged in. Possession of those tokens would allow attackers to “seize control” of user accounts, Facebook said.

Facebook says it has taken steps to fix the security problem and alerted law enforcement.

To deal with the issue, Facebook reset some logins, so 90 million people have been logged out and will have to log in again. That includes anyone who has been subject to a “View As” lookup in the past year.

Facebook says it doesn't know who is behind the attacks or where they're based. In a call with reporters on Friday, CEO Mark Zuckerberg said that the company doesn't know yet if any of the accounts that were hacked were misused.

Jake Williams, a security expert at Rendition Infosec, said the stolen access tokens would have likely allowed attackers to view private posts and probably post status updates or shared posts as the compromised user, but wouldn't affect passwords.

“The bigger concern (and something we don't know yet) is whether third-party applications were impacted,” Williams said in a text exchange.

Facebook offers a login service for third parties to allow users to log into their apps using Facebook. In other words, Facebook is providing the identity management for countless other sites and services. These access tokens that were stolen show when a user is logged into Facebook and that may be enough to access a user's account on a third party site.

Explore: A timeline of Facebook's most notable missteps and promises around privacy

News broke early this year that data analytics firm that once worked for the Trump campaign, Cambridge Analytica, had gained access to personal data from millions of user profiles.

Then a congressional investigation found that agents from Russia and other countries have been posting fake political ads since at least 2016.

Facebook CEO Mark Zuckerberg appeared at a Congressional hearing over Facebook's privacy policies in April.

Facebook has more than 2 billion users worldwide.

The company said people do not need to change their Facebook passwords, but anyone having trouble logging on should visit the site's help centre.

Those who want to log out can visit the “Security and Login” section of their settings, which lists the places that people are logged into Facebook. It has a one-click option of logging out of all locations.

Ed Mierzwinski, the senior director of consumer advocacy group US PIRG, said the breach was “very troubling.”

“It's yet another warning that Congress must not enact any national data security or data breach legislation that weakens current state privacy laws, preempts the rights of states to pass new laws that protect their consumers better, or denies their attorneys general rights to investigate violations of or enforce those laws,” he said in a statement.

Wedbush analyst Michael Pachter said “the most important point is that we found out from them,” meaning Facebook, as opposed to a third party.

“As a user, I want Facebook to proactively protect my data and let me know when it's compromised,” he said. “Shareholders should ultimately approve of Facebook's handling of the issue.”