Yahoo now believes all Yahoo user accounts were affected by the August 2013 data breach, which affected 1 billion accounts. The data breach was attributed to state-sponsored hackers. Yahoo has also started notifying the affected account holders, requiring them to change their passwords immediately. This breach is separate from the 2014 breach disclosed by Yahoo in September last year, affecting as many as 500 million user accounts. You should now change your passwords immediately and enable two-factor authentication (2FA) and enable 2FA.
Source: https://thehackernews.com/2017/10/yahoo-email-hacked.html