Yahoo has tripled down on what was already the largest data breach in history, saying it affected all 3 billion of its users, not the 1 billion it revealed late last year.
The company announced it was providing notice to additional user accounts affected by the August 2013 data theft.
The breach was previously disclosed by the company in December.
The stolen information included names, email addresses, phone numbers, birthdates and security questions and answers.
Following its acquisition by Verizon in June, Yahoo said it obtained new intelligence while investigating the breach with help from outside forensic experts.
It said the stolen customer information did not include passwords in clear text, payment card data or bank account information.
Yahoo had already required users to change their passwords and invalidate security questions so they could not be used to hack into accounts.
"Whether it's 1 billion or 3 billion is largely immaterial. Assume it affects you," Sam Curry, chief security officer for Boston-based firm Cybereason, said.
"Privacy is really the victim here."
The disclosure is also a huge embarrassment for Verizon, which has just started running TV ads for its new subsidiary, Oath, which will consist of Yahoo and AOL services.
In January, the ABC revealed thousands of Australian government officials were among the victims of the breach.