Yahoo CEO Marissa Mayer talks about strategic alternatives
Yahoo CEO Marissa Mayer Reuters

Yahoo has revealed the largest online security breach in history on Wednesday, admitting to a 2013 cyber attack that compromised more than one billion user accounts. The latest disclosure comes on top of the massive data breach in 2014 that the company reveled in September this year.

"Yahoo believes an unauthorized third party, in August 2013, stole data associated with more than 1 billion user accounts ... The company has not been able to identify the intrusion associated with this theft. Yahoo believes this incident is likely distinct from the incident the company disclosed on Sept. 22, 2016, the company said in a blog post.

Woman dies after hospital carries out wrong blood type transfusion

The latest data breach revelation gives Verizon more room to push for a better deal with Yahoo after it agreed to buy the struggling internet company for early $4.8 billion. There have been reports that Verizon-Yahoo deal would fall through after Yahoo announced in September that 500 million user accounts had been compromised in a cyber attack.

Following this Verizon had expressed reservations about the deal. Verizon's president of innovation and new business, Marni Walden, said the company would "protect our shareholders and our investors, so we're not going to jump off a cliff blindly."

Yahoo said on Wednesday the August 2013 hack gave away hundreds of millions of user names, email addresses, telephone numbers, dates of birth and encrypted passwords. The lost data also included security questions and answers, some of which were not encrypted.

"Yahoo is notifying potentially affected users and has taken steps to secure their accounts, including requiring users to change their passwords ... Yahoo has also invalidated unencrypted security questions and answers so that they cannot be used to access an account," the company said in a statement.

The latest hack muddled the deal with Verizon, which is scheduled to be completed in the first quarter of next year. However, a Verizon statement issued on Wednesday said the company continues to work towards integration with Yahoo.

"As we've said all along, we will evaluate the situation as Yahoo continues its investigation .. We will review the impact of this new development before reaching any final conclusions," a Verizon statement said.

Yahoo shares were down 2.5 percent in extended trading while Verizon shares largely stayed the course.