Adam Mudd admitted creating malware in 2013 which was used to carry out 1.7 million cyber attacks.
The 20-year-old of Toms Lane in Kings Langley was jailed for two years at the Old Bailey in April.
But on Thursday, judges at the Court of Appeal found the original punishment had not given enough credit for his guilty plea.
Mudd, who made more than Â£380,000 worth of US dollars and Bitcoins from victims including gaming websites Minecraft and Xbox Live, had his sentence reduced by three months.

An astronomy lecturer told the BBC the "little old rock" was not to blame.
Rocks are poor conductors of heat and although they cause fireballs when they streak through the Earth's atmosphere from space, even recent large meteor falls have not started fires.
The Bowie Volunteer Fire Department apologised and said the Twitter post "should have never been made".
The blaze took 15 firefighters four hours to put out and left a crater 12-15 ft (3.7-4.6m) wide and 5-6 ft deep.
Afterwards, a tweet was sent out from the Bowie Volunteer Fire Department account showing a photograph of a crater and a rock, calling it a "possible meteorite strike".
But Greg Redfern, an astronomy lecturer and Nasa ambassador, told the BBC a rock could not be called a meteorite before it is tested by a specialist laboratory.
And, he said, people's perceptions of meteorites were not quite right.
"People have this vision that a meteorite is going to be hot and flaming and it is such a wrong notion," he said.
"When a meteorite has come through the atmosphere it is cooled off way up high. I can't give you a temperature but it would be cool to touch, maybe even cold."
Three years ago, space rocks fell over Russia, injuring nearly 1,000 people but there was no ground fire.
Rocks that enter the Earth's atmosphere from space partially burn up in the air but when they hit the ground they are cold.
Mr Redfern said: "The bottom line is that the fire in Maryland was not caused by that little old rock."
Jonathan Howard of the Bowie Volunteer Fire Department promised in a statement that measures would be taken to ensure there was no repeat of the mistaken report.

The Twilight star is one of 12 female creatives making films for women-focused media company Refinery29.
Fellow actresses Gabourey Sidibe and Chloe Sevigny will join Stewart under the umbrella of Refinery29's ShatterBox Anthology.
Oscar nominee Sidibe will direct her own short film titled A Tale of Four Women, according to Variety.
Meanwhile, Sevigny's Kitty, the first film of the anthology, will premiere at Cannes Film Festival later this month.
Speaking during Refinery29's presentation at the Digital Content NewFronts event in New York, Sidibe said it was important to tell a story through her own voice.
She said: "[In my acting career] everything I do, my image and who I am and what I feel represents me is in the hands of a man, usually a white man... I realised very recently that I want to be the one that decides what image I show the public."
It was also announced that Ugly Betty star America Ferrera will produce a multimedia project titled Behind the Headlines, and a documentary series titled Only Girl.

It will be the 20-year-old's second spell at the club, having joined on loan from Rochdale in 2016.
Bell made nine appearances in 2016-17 for Gateshead, scoring twice.
Earlier on Thursday, Chester - who finished 19th in the National League - announced new deals for midfielder Wade Joyce and utility player Evan Horwood on one-year and six-month contracts respectively.
A computer hacker who masterminded global online attacks from his bedroom has had his prison sentence reduced.
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Firefighters in Maryland have had to backtrack after blaming a meteorite for a brush fire.
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Kristen Stewart is to make her directorial debut with a short film about the power of women.
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Chester have signed Gateshead striker Nyal Bell on loan until 7 January.