SAN FRANCISCO – A 20-year-old hacker on Monday admitted to surreptitiously seizing control of hundreds of thousands of Internet-connected computers, using the zombie network to serve pop-up ads and renting it to people who mounted attacks on Web sites and sent out spam.

Jeanson James Ancheta of Downey, Calif., pleaded guilty in Los Angeles federal court to four felony charges for crimes, including infecting machines at two U.S. military sites, that earned him more than $61,000, said federal prosecutor James Aquilina.

Under a plea agreement, which still must be approved by a judge, Ancheta faces up to six years in prison and must pay the federal government restitution. He also will forfeit his profits and a 1993 BMW. Sentencing is scheduled for May 1.

Prosecutors called the case the first to target profits derived from use of “botnets,” large numbers of computers that hackers commandeer for various nefarious deeds. The “zombie” machines’ owners are unaware parasitic programs have been installed on them and are being controlled remotely.

According to a November indictment, Ancheta advertised his network of “bots,” short for robots, on Internet chat channels. A Web site Ancheta maintained included a schedule of prices he charged people who wanted to rent out the machines, along with guidelines on how many bots were required to bring down a particular type of Web site.

Botnets are increasingly being used to overwhelm Web sites with streams of data, often by extortionists. They feed off vulnerabilities in computers that run Microsoft Corp.’s Windows operating system, typically machines whose owners haven’t bothered to install security patches.

The indictment charged Ancheta with 17 counts of conspiracy, fraud and other crimes connected to a 14-month hacking spree that started in June 2004 and that authorities say continued even after FBI agents raided his house the following December.

“Part of what’s most troubling about those who commit these kinds of offenses is they think they’ll never be caught,” said Aquilina, who spent more than a year investigating Ancheta and several of Ancheta’s online associates who remain uncharged co-conspirators.

Ancheta’s attorney, federal public defender Greg Wesley, did not immediately return phone calls seeking comment.

The guilty plea comes less than a week after the FBI released a report that estimates viruses, worms and Trojan horse programs like the ones Ancheta employed cost U.S. organizations $11.9 billion each year.