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Vallor, appearing before Southwark Crown Court, will serve a minimum of one year, after admitting writing the Gokar virus.
According to the prosecution, Gokar infected 27,000 computers in 42 countries when it was unleashed in December last year. He was caught when the FBI detected the Gokar virus and passed details on to Scotland Yard's computer crime unit.
Vallor also admitted writing the Redisi and Admirer viruses, which experts have described as sophisticated mass-mailing worms written in assembler code.
He has since apologised for writing the viruses, telling a local newspaper, "I regret doing it, but I don't have anything to say to people except that I am extremely sorry for any inconvenience I may have caused them."
Despite bringing a successful prosecution, police have been critical of businesses for failing to help them gather evidence against Vallor.
Judge Rivlin said only a custodial sentence was appropriate and, were it not for the circumstances surrounding the case, Vallor would have got four years. It is often thought that people who write viruses are just nerds, but these people well know that they are criminals, he added.
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