Facebook Inc has discovered a security issue affecting about 50 million user accounts, the company said on Friday.

Facebook said attackers stole Facebook access tokens through its “view as” feature, which they could then use to take over people’s accounts. “View as” is a feature that allows users to see what their own profile looks like to someone else.

“Since we’ve only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed,” the company said in a blog post https://newsroom.fb.com/news/2018/09/security-update.

Facebook shares fell 3 percent to $163.78 in afternoon trading.