The breach is even bigger than Yahoo's previous hack, when the details of 500m accounts were stolen - at the time, that was the biggest hack in history.
This chart, put together for City A.M. by Statista, shows the largest hacks ever, and when they were revealed.
What actually happened?
Details including names, email addresses, dates of birth, passwords and phone numbers were stolen, although card details and bank account information were safe.
The company added it was worried about so-called forged cookies, which hackers used to access user accounts by the back door, bypassing password verification.
"In October, Verizon’s general counsel told reporters that it was reasonable that the first breach may be a material change to the plans for the $4.8bn acquisition," said Ashley Winton, chairman of the UK data protection forum and of law firm Paul Hastings.
"It has become clear that good cyber security due diligence is now an essential step for M&A or private equity activity, especially if you are the acquirer or investor seeking a discount.”