Four men in total were charged, two of whom belong to Russia's FSB.
We Live Security explains:
The United State Department of Justice (DOJ) has charged four men, including two officials of Russia’s FSB intelligence agency, in connection with a hacking attack against Yahoo that saw the details of 500 million users stolen and the use of forged cookies to break into accounts.
In September last year, Yahoo revealed that in late 2014 an unnamed “state-sponsored actor” had accessed the account information of some approximately 500 million users including names, email addresses, telephone numbers, dates of birth, hashed passwords and, in some cases, encrypted or unencrypted security questions and answers.
Yahoo believes that hackers managed to break into its internal systems, accessing proprietary code that allowed the attackers to forge cookies granting access to accounts without needing a password.