User names and passwords of its email customers have been stolen and are used to access multiple accounts. Yahoo did not say how many accounts have been affected, and neither they are sure about the source of the leaked users’ credentials. It appears to have come from a third party database being compromised, and not an infiltration of Yahoo’s own servers. Yahoo users can prevent account hijacks by using a strong and unique password. You can use’Random strong password generator’ feature of DuckDuckGo to get a unique & strong password.
Source: https://thehackernews.com/2014/01/yahoo-mail-hacked-change-your-account.html