Yahoo is investigating whether the details of 200 million of its users’ accounts have been posted on a dark web marketplace.
A hacker known as ‘Peace’ claimed to be selling the details – usernames, passwords and dates of birth from 2012 – for three bitcoins (£1,360).
Yahoo said in a statement that it was treating the situation “very seriously” and “working to determine the facts”.
“Yahoo works hard to keep our users safe, and we always encourage our users to create strong passwords, or give up passwords altogether by using Yahoo Account Key, and use different passwords for different platforms,” it said.
The hacker has previously been linked to breaches at MySpace and LinkedIn.
Motherboard, which first reported the alleged breach, tested whether a sample of the offered accounts were genuine. It found that many were, but others corresponded to old accounts.
Last month in was announced that Verizon will complete the acquisition of Yahoo’s core business for around $4.83 billion (£3.69bn).
The Daily Mail was at one point rumoured to be interested in purchasing the internet giant.