Karim Baratov, the 22-year-old charged in connection with a massive hack of Yahoo, pleaded not guilty to all charges against him in a San Francisco courtroom on Wednesday morning, according to one of his lawyers.
U.S. authorities allege Baratov, a Kazakhstan-born Canadian citizen, was a "hacker for hire" with Russian ties.
His U.S.-based lawyer, Andrew Mancilla, confirmed his client was extradited from Canada on Tuesday and is in federal custody south of the border.
Baratov, who owned a home in Ancaster, Ont., is expected back in court next Tuesday morning.
He waived his right to an extradition hearing last Friday in a Hamilton court.
Baratov, apparently eager to get his case dealt with, elected to move forward with fighting the charges immediately in the U.S.
Baratov was arrested March 14 in Hamilton under the Extradition Act after U.S. authorities indicted him and three others on computer hacking, economic espionage and other crimes.
He has been held without bail since his arrest after an Ontario Superior Court judge ruled in April that he was too much of a flight risk to be released prior to an extradition hearing.
Yahoo said in September 2016 that information from at least 500 million user accounts had been stolen in a cyberattack two years earlier. Baratov is accused of hacking 80 Yahoo accounts, and if convicted, faces at least 20 years in a U.S. prison.
The U.S. also charged two Russian intelligence officers and a fourth man with hacking Yahoo accounts.
Baratov's lawyers have said their client had no idea who he was dealing with, or exactly what he was doing.