Yahoo said that a massive attack on its network in 2014 allowed hackers to steal data from half a billion users and may have been "state-sponsored".

Yahoo, which confirmed details of the breach months after reports of a major hack, said on Thursday that its investigation concluded that "certain user account information was stolen" and that the attack came from "what it believes is a state-sponsored actor".

"Based on the ongoing investigation, Yahoo believes that information associated with at least 500 million user accounts was stolen," said a statement by the US internet giant in what is likely the largest-ever breach for a single organization.