Yahoo has confirmed personal data from 500 million accounts has been stolen in a massive security breach.
The hack dates back to late 2014.
The stolen data includes users' names, email addresses, telephone numbers, dates of birth, hashed passwords and security questions for verifying an account holder's identity.
Yahoo is blaming the hack on a "state-sponsored actor".
The web company is recommending that users change their passwords if they have not done so since 2014.