Investing.com - Facebook revealed on Friday a security issue that enabled attackers to take over user accounts.
The security issue affected almost 50 million accounts on Tuesday, Facebook said in a statement, adding that it is taking the issue "incredibly seriously."
Shares of Facebook (NASDAQ:FB) fell more than 3% in midday trading.
"Our investigation is still in its early stages," the company said. "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," Facebook added.
Facebook says is has fixed the vulnerability and told law enforcement.
"We also don’t know who’s behind these attacks or where they’re based," the company added. "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."