What if Yahoo hackers have my details?
23 September 2016 Last updated at 19:00 BST
Yahoo says "state-sponsored" hackers stole the data of about 500 million users in what could be the largest publicly disclosed cyber-breach in history.
In the UK it is believed data on about eight million user accounts was taken in the hack.
The BBC's technology reporter Chris Foxx explains how you can stay safe following the data breach.