Yahoo has confirmed 500 million users had their personal details stolen after a security breach.
The internet giant said they were currently investigating the "state-sponsored" attack, which they claimed took place in 2014.
They advised customers to "change passwords and security questions and review your accounts for suspicious activity."
Bob Lord, Yahoo's chief information security officer, said: "An increasingly connected world has come with increasingly sophisticated threats. Industry, government and users are constantly in the crosshairs of adversaries."
But how do you protect yourself online?
- Do not use the same password and security questions on different accounts
- Avoid using real words in your password
- The longer and more complex your password the better
- Use upper and lower case letters numbers, and punctuation
- Be wary of emails your receive from accounts that are not known to you and which ask for personal information, or refer you to a website which asks for personal information
- Avoid clicking on links or downloading attachments from suspicious emails
- No company will ever ask you for your password, so do not give it out
- Use antivirus software
- Make sure there is a WPA key on your wireless internet connection
- Check your account for suspicious activity