A US computer hacker was jailed for almost five years on Monday for hijacking around 400,000 computers and using them to send out spam, display pop-up ads and attack internet sites.

A federal judge sentenced Jeanson Ancheta to 57 months for infecting computers with malicious software and corralling them into a network of "bots".

In January 2006 Ancheta pleaded guilty to using the computers to email spam, display ads and launch crippling attacks on websites.

Judge Gary Klausner told the court the crime was "extensive, serious and sophisticated" as he handed down the longest-known sentence for this type of computer crime. "Your worst enemy is your own intellectual arrogance that somehow the world cannot touch you on this," he told Ancheta during sentencing.

Advertising companies

Ancheta admitted violating the US Computer Fraud and Abuse Act and anti-spam laws, and hacking into computers in order to commit fraud.

According to court documents, advertising companies would pay Ancheta for each computer he was able to infect with software that displayed unsolicited ads on screen.

Ancheta admitted downloading the software onto more than 400,000 computers. He collected $107,000 in commissions.

He also charged spammers to use his botnet to send out huge quantities of unwanted e-mails. In more than 30 separate transactions, he sold access to up to 10,000 infected machines at a time.

Defence computers

To continually grow his network of infected computers, Ancheta ordered his "zombie" machines to scan the internet for more vulnerable computers, court documents stated. Ancheta hijacked computers at the Weapons Division of the US Naval Air Warfare Center in California, US, and the Defense Information Systems Agency in Virginia, US.

In pleading guilty, Ancheta agreed to pay about $15,000 in restitution to the US government. In addition, he agreed to forfeit his proceeds from the crimes, which include more than $60,000 in cash, a 1993 BMW luxury car and computer equipment.