Hacker sold Yahoo!’s over 1 Billion-user database on the Dark Web last August for $300,000. Three different buyers, including two “prominent spammers” and the third is believed to be involved in espionage tactics. The database includes backup email addresses and unencrypted security questions and answers that could provide quick access to users accounts via password reset option. The hacker group that breached Yahoo! sold the database is based in Eastern Europe, but the company still does not know if this information is accurate or not.
Source: https://thehackernews.com/2016/12/yahoo-hacking.html