Canadian citizen Karim Baratov has pleaded guilty to charges relating to the hacking of Yahoo in 2014.
The hack involved spear-phishing Yahoo employees and compromising 500 million accounts.
Baratov admitted he was tasked with hacking the email accounts of people of interest by the Federal Security Service of Russia.
Baratov said he sent gatherer email passwords to his alleged handler, Dmitry Aleksandrovich Dokuchaev.
Dokuchaev was arrested by Russian authorities in December and charged with treason.
Two other men were indicted in February and remain in Russia, along with Dokuchaev.
Baratov is being held in Northern California without bail. His sentencing hearing is scheduled for February 2018.