Facebook discovered a security issue that allowed hackers to access information that could have let them take over around 50 million accounts, the company announced on Friday. “This is a very serious security issue, and we’re taking it very seriously,” said CEO Mark Zuckerberg said on a call with reporters. Facebook, which was already trading down about 1.5 percent before the announcement, extended losses to as much as 3.5 percent after the disclosure.

The company said in a blog post its engineering team found that attackers identified a weakness in Facebook’s code regarding its “View As” feature on Tuesday. Facebook became aware of a potential attack after it noticed a spike in user activity on Sept. 16. “View As” lets users see what their profile looks like to other users on the platform. This vulnerability, which consisted of three separate bugs, also allowed the hackers to get access tokens — digital keys which let people stay logged into the service without having to re-enter their password — READ MORE