https://finance.yahoo.com/news/former-yahoo-ceo-apologizes-data-152829874.html

"Unfortunately, while all our measures helped Yahoo successfully defend against the barrage of attacks by both private and state-sponsored hackers, Russian agents intruded on our systems and stole our users’ data."

Verizon, the largest U.S. wireless operator, acquired most of Yahoo Inc's assets in June, the same month Mayer stepped down. Verizon disclosed last month that a 2013 Yahoo data breach affected all 3 billion of its accounts, compared with an estimate of more than 1 billion disclosed in December.