Hackers are forging cookies or files used in the authentication process to access their accounts, instead of stealing passwords. The IT giant disclosed the alarming data in response to the massive data breach disclosed last year that created serious problems for the company. In 2013, hackers broke into the systems of Yahoo and accessed one billion user accounts containing names, addresses, phone numbers, and hashed passwords easy to crack. Media speculated the involvement of state-sponsored hackers. The breach had an impact on the sale of the company to telecom giant Verizon for $4.8 billion.”]
Source: https://securityaffairs.co/wordpress/56321/breaking-news/yahoo-breach-notifications.html