A massive data breach has put hundreds of millions of email addresses at risk.
Millions of unique passwords have also been jeopardized and posted to a hacking forum.
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The mishap was first reported by security researcher Troy Hunt.
He runs a website where users can search to see if their email address or password has been compromised.
To check if your account has been compromised - click here.
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Hunt says it's difficult to see where the info originated from --- but it could have come from more than 2,000 leaked databases.
This type of breach has happened before -- but never on this scale.