If you use Facebook, it's probably a good idea to change your password.
Officials with the social media giant announced in a blog post that 50 million accounts were affected by a security issue exploiting Facebook's 'View As' feature that allows users to see how their profile looks like to other people.
The engineering team discovered the vulnerability on Tuesday, September 25th.
Officials say "We're taking this incredibly seriously and wanted to let everyone know what's happened and the immediate action we've taken to protect people's security."
The blog post goes on to say, "Since we've only just started our investigation, we have yet to determine whether these accounts were misused or any information accessed. We also don't know who’s behind these attacks or where they're based."
Facebook has reset some logins as part of the steps they are taking to protect users - around 90 million people have been logged out and will have to log in again.
Because the details of the attack are sparse, many experts recommend users consider changing their passwords.
It's been a tumultuous year for Facebook; earlier this year Cambridge Analytica, a data analytics firm, gained access to personal data from millions of users, while later on a US congressional investigation revealed agents from other countries (including Russia) were posting fake political ads.