Facebook on Friday disclosed a breach of its network that affected almost 50 million user accounts. The social networking giant said that attackers exploited a vulnerability in Facebook’s code that let them steal access tokens — digital keys that are used to keep users logged in when they enter their username and password — when users switched over to a public profile view via the “View As” feature.

The access tokens allowed the attackers to take over user accounts, however it’s still unclear whether user data was accessed and misused.

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Facebook said it has secured its network and affected user accounts since engineering discovered the attack on September 25. The bug was fixed and Facebook said it has notified law enforcement.

Meantime, the company has reset the access tokens on all of the affected user accounts, as well as on another 40 million accounts that were subject to a “View As” look-up in the last year.

Anyone impacted by the reset will need to log back in to Facebook and on any apps that use Facebook Login. Once logged back in, affected users will see a notification at the top of News Feed alerting them to the incident.

Facebook has also disabled the “View As” feature while it conducts a security review.

“Since we’ve only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed,” Facebook said in a blog post. “We also don’t know who’s behind these attacks or…