Facebook says it recently discovered a security breach affecting nearly 50 million user accounts.
The hack is the latest setback for Facebook during a year of tumult for the global social media service.
In a blog post, the company says hackers exploited its "View As" feature, which lets people see what their profiles look like to someone else.
Facebook says it has taken steps to fix the security problem and alerted law enforcement.
To deal with the issue, Facebook reset some logins, so 90 million people have been logged out and will have to log in again.
That includes anyone who has been subject to a "View As" lookup in the past year.
Facebook says it doesn't know who is behind the attacks or where they're based.
News broke early this year that a Trump-linked data analytics firm, Cambridge Analytica, had gained access to personal data from millions of user profiles.
Then, a congressional investigation found that agents from Russia and other countries have been posting fake political ads since at least 2016.
Facebook chief executive Mark Zuckerberg appeared at a Congressional hearing over Facebook's privacy policies in April.
In a call with reporters, Zuckerberg said that the company doesn't know yet if any of the accounts that were hacked were misused.
Facebook has more than 2 billion users worldwide.
