NEW YORK (CBS/AP) — 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 its “View As” feature, which lets people see what their profiles look like to someone else. 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.

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.

In a call with reporters on Friday, Zuckerberg said that the company doesn’t know yet if any of the accounts that were hacked were misused.

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 center . 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 U.S. 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.

Here’s the entire statement from Facebook:

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.

Here is the action we have already taken. First, we’ve fixed the vulnerability and informed law enforcement.

Second, 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.

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.

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

People’s privacy and security is incredibly important, and we’re sorry this happened. It’s why we’ve taken immediate action to secure 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.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)