Yahoo Finance 500 million Yahoo customers have data stolen
Data from at least 500 million Yahoo (Hanover: YHO.HA - news) users was "stolen" during an attack in 2014, the internet company has said.
The attack may have been "state-sponsored" but "the investigation has found no evidence that the state-sponsored actor is currently in Yahoo's network", Yahoo said.
"Yahoo is working closely with law enforcement on this matter," it added.
The data stolen may have included names, dates of birth, and hashed passwords but may not have included unprotected passwords, payment card data or bank account information, the company said.
Alex Holden, founder of Hold Security, which has been tracking the flow of stolen Yahoo credentials on the underground web, told the New York Times that the attack is "one of the biggest breaches of people's privacy and very far reaching".
He added: "The stolen Yahoo data is critical because it not only leads to a single system but to users' connections to their banks, social media profiles, other financial services and users' friends and family."
Prior to this, the biggest breach was thought to have been the MySpace hack, which was revealed earlier this year and affected 360 million users.
Potentially affected users will be notified, asked to change their passwords and use other ways of verifying their account.
The company says that, while the breach happened late in 2014, it was only recently found as part of an internal investigation.
It is not clear how the news will affect Yahoo's plans to sell its email service and other core internet properties to Verizon Communications (LSE: 0Q1S.L - news) .
The $4.8bn (£3.7bn) deal was announced in July but Verizon (NYSE: VZ - news) has said it was only told of the data breach in the last two days.
In a statement, Verizon said: "We will evaluate as the investigation continues through the lens of overall Verizon interests...Until then, we are not in position to further comment."
The deal is expected to close in the first quarter of next year, which may give them some room to renegotiate the purchase price or even to walk away.
