Yahoos latest filing to the US Securities and Exchange Commission reveals that at least some of its employees were aware in 2014 that its systems had been breached. Yahoo is currently facing 23 lawsuits over the security breach, which was only made public in September 2016. The company disclosed the Security Incident on September 22, 2016, and began notifying potentially affected users, regulators, and other stakeholders. An Independent Committee of the Board, advised by independent counsel and a forensic expert, is investigating the scope of knowledge within the Company in 2014 and thereafter regarding this access.”]
Source: https://grahamcluley.com/yahoo-staff-knew-2014-hacked/