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 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.

Although Facebook didn’t mention this in their post, one other major unanswered question about this incident is whether the access tokens could have let attackers interactively log in to third-party sites as the user. Tens of thousands of Web sites let users log in using nothing more than their Facebook profile credentials.

I have asked for clarification from Facebook on this point and will update this post when and if I receive a response. I would have expected Facebook to mention this as a mitigating factor if authorized logins at third-party sites were not impacted.

Facebook says there is no need for users to reset their passwords as a result of this breach, although that is certainly an option.

More importantly, it’s a good idea for all Facebook users to review their login activity. This page should let you view which devices are logged in to your account and approximately where in the world those devices are at the moment. That page also has an option to force a simultaneous logout of all devices connected to your account.

Check back for updates as more information becomes available.