Facebook has said 50 million user accounts have been affected by a security breach.
The social media giant has not yet determined whether the accounts were misused or information was accessed.
Nor does it know who is behind the breach or where they are based. Police have been informed.
The breach stemmed, Facebook said, from a change it made to its video uploading feature in July 2017.
About 90 million people will now have to log back in to their accounts, after an additional 40 million accounts were reset as a precautionary measure.
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