A 20-year-old hacker has pleaded guilty to seizing control of hundreds of thousands of internet-connected computers without their owners’ consent or knowledge.

The hijacked PCs, known as botnets, where marshalled into a "zombie" network that was rented to gangs, who used them to attack websites and to send out vast quantities of spam. The network he controlled included computers at two US military bases.

In the first case of its kind, Jeanson James Ancheta, of Downey, California, pleaded guilty in a Los Angeles court to four charges, including infecting machines at the China Lake Naval Air Facility in California and the Defence Information System Agency, in Falls Church, Virginia.

Ancheta’s 14-month hacking spree started in June 2004 and earned him $61,000 (£34,000), according to prosecutors. Court documents suggested that he had a taste for expensive goods, spending $600 a week on new clothes and car parts.

Online crime, which was once regarded as little more than a nuisance, is increasingly motivated by profit, according to many analysts. "There are a number of ways in which zombie botnets can generate healthy profits for hackers," Graham Cluley, senior technology consultant for the antivirus company Sophos said.

"They can install advertising pop-ups which generate income through affiliate schemes, rent out the network for hackers who wish to blackmail websites with denial of service attacks [which aim to overload websites by simultaneously logging on with thousands of computers], or use them to steal information or pump out spam campaigns."

The owners of the zombie machines are usually unaware that parasitic programs have been installed and that their computers are being used to break the law. Hackers can gain control of computers with malicious programs known as Trojans, which users are tricked into installing on their computers. Using an online security package that includes antivirus, anti-spyware and firewall functions minimises the risk of infection.

Prosecutors said this was the first case to target profits derived from use of botnets. Under a plea agreement, which still must be approved by a judge, Ancheta will receive from four years to six years in prison, forfeit a 1993 BMW and more than $58,000 (£32,500) in profit and pay $19,000 (£11,000) in restitution to the federal government, according to court documents.

Ancheta has been in custody since he was charged in November. The 52-page indictment listing the charges gives an unusually detailed glimpse into a world where young hackers often brag in chatrooms about their armies of junk mail robots and the arsenals they can muster for denial of service attacks.

Ancheta signed up as an affiliate in programmes maintained by online advertising companies, that pay people each time they get a computer user to install software that displays adverts and collects information about the sites a user visits.

Prosecutors say Ancheta and another hacker, identified as SoBe, then installed ad software from two companies – Gamma Entertainment of Montreal, Quebec, and Loudcash, whose parent company was acquired last year by 180Solutions of Bellevue, Washington – on the bots they controlled, pocketing more than $58,000 (£32,500) in 13 months.

"It’s immoral, but the money makes it right," Ancheta told SoBe during one online chat, according to the indictment.

The prosecution wouldn’t say whether authorities plan to charge SoBe or any of the people accused of renting out Ancheta’s bots.

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