Facebook said today some 90 million of its users may get forcibly logged out of their accounts after the company fixed a rather glaring security vulnerability in its Web site that may have let attackers hijack user profiles.

In a short blog post published this afternoon, Facebook said hackers have been exploiting a vulnerability in Facebook’s site code that impacted a feature called “View As,” which lets users see how their profile appears to other people.

“This allowed them to steal Facebook access tokens which they could then use to take over people’s accounts,” Facebook wrote. “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.”

Facebook said it was removing the insecure “View As” feature, and resetting the access tokens of 50 million accounts that the company said it knows were affected, as well as the tokens for another 40 million users that may have been impacted over the past year.

The company said it was just beginning its investigation, and that it doesn’t yet know some basic facts about the incident, such as whether these accounts were misused, if any private information was accessed, or who might be responsible for these attacks.

“We have invalidated data access for third-party apps for the affected individuals,” the spokesperson said, referring to the 90 million account that were forcibly logged out today and presented with a notification about the incident at the top of their feed.