Yahoo hack: 'One billion user accounts affected'
Yahoo has identified a breach from 2013 involving data of more than a billion users which is distinct from the breach reported in September.
The breach disclosed in September saw 500 million accounts accessed in 2014.
Yahoo said names, phone numbers, passwords and email addresses were stolen, but not bank and payment data.
"Yahoo believes an unauthorized third party, in August 2013, stole data associated with more than one billion user accounts," a statement said.
The firm said it is notifying potentially affected users.
Yahoo, which is being acquired by Verizon Communication Inc for $4.83 billion, has its European HQ in the Republic of Ireland.
Verizon said: "we will review the impact of this new development before reaching any final conclusions."