Internet giant Yahoo has revealed that one billion user accounts were hacked in 2013. The breach is separate and distinct from an announcement in September that 500 million accounts had been penetrated by a "state-sponsored actor".
In a statement released by Yahoo, a spokesman said the company was working with law enforcement officials to track down the origin of the hack and did not know yet who is responsible.
The statement said: "Yahoo believes an unauthorised third party, in August 2013, stole data associated with more than one billion user accounts.
"The company has not been able to identify the intrusion associated with this theft. Yahoo believes this incident is likely distinct from the incident the company disclosed on September 22, 2016.
"For potentially affected accounts, the stolen user account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords (using MD5) and, in some cases, encrypted or unencrypted security questions and answers.
"An investigation indicates that the stolen information did not include passwords in clear text, payment card data or bank account information. Payment card data and bank account information are not stored in the system the company believes was affected."
Yahoo said it was notifying affected users and will advise them on what steps to take to secure their accounts.
Earlier in December, the company confirmed it had fixed a highly critical cross-site scripting (XSS) security flaw in its email system that would have allowed attackers to access any email.
It was after Finland-based security researcher Jouko Pynnonen discovered the flaw in a Yahoo bug bounty programme, earning him $10,000 (£7,970).
Yahoo also confirmed, with the help of outside forensic experts, it was investigating the creation of forged cookies that could allow an intruder to access users' accounts without a password.
The statement added: "Based on the ongoing investigation, the company believes an unauthorised third party accessed the company's proprietary code to learn how to forge cookies.
"The outside forensic experts have identified user accounts for which they believe forged cookies were taken or used. Yahoo is notifying the affected account holders, and has invalidated the forged cookies."