Yahoo has said more than one billion user accounts may have been affected in a hacking attack dating back to 2013.
The internet giant said the hack appears separate from a breach disclosed in September, when some 500 million accounts were accessed in 2014.
Yahoo said names, phone numbers, passwords and email addresses were stolen, but not bank and payment data.
The company, which is being taken over by Verizon, said it is working closely with the police and authorities.