According to a filing with the SEC, Yahoo is investigating how much Yahoo employees knew about the data breach of 500 million user credentials in 2014.

You'll find the key part in "security incident" in the SEC filing.
[sec.gov...]

Earlier stories
E.U. Regulator Commands Yahoo Cooperate With Authorities Over Data Breach [webmasterworld.com]
Verizon Indicates Yahoo Data Breach is Significant Event and Could Halt Acquisition [webmasterworld.com]
Yahoo Confirms 500 Million Accounts Hacked [webmasterworld.com]