Yahoo said on Wednesday it had discovered another major cyberattack, saying data from more than 1bn user accounts was compromised in August 2013, making it the largest such breach in history.
Yahoo said the stolen user account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords and, in some cases, encrypted or unencrypted security questions and answers.
The number of affected accounts was double the number implicated in a 2014 breach that the internet company disclosed in September 2016 and blamed on hackers working on behalf of a government.
“An unauthorised party” broke into the accounts, Yahoo said in a statement posted on its website. The company believes the hacks are connected and that the breaches are “state-sponsored”.
Yahoo said this case “is likely distinct from the incident the company disclosed on September 22, 2016”.
Verizon, which is in the process of buying Yahoo for $4.8bn (£3.8bn), said of the 2013 hack: “We will review the impact of this new development before reaching any final conclusions.”
The breach disclosed in September had already threatened to derail the deal or result in a reduction in the sale price.
Yahoo is notifying all the users affected and asking them to change their passwords and security questions.