Hackers stole personal data from all three billion Yahoo user accounts as part of a cyber attack that occurred in 2013, the tech giant said.
The company, now part of Oath, initially said that personal information including names, email addresses and security questions relating to one billion accounts were all accessed by a "third-party".
Outside forensic experts were brought in following Yahoo being acquired by Verizon, and the company has now tripled the number of accounts it believes were compromised.
A statement said the user account information that was stolen did not include passwords in clear text, payment card data, or bank account information and that all affected customers have been contacted.
The company's chief information security officer Chandra McMahon said: "Verizon is committed to the highest standards of accountability and transparency, and we proactively work to ensure the safety and security of our users and networks in an evolving landscape of online threats."