A student from Northampton has been convicted of being part of the hacking group "Anonymous".
Anonymous's cyber attacks wreaked havoc on the websites of MasterCard, Visa, Ministry of Sound, the British Recorded Music Industry organisation (BPI) and the International Federation of the Phonographic Industry (IFPI).
Christopher Weatherhead, of Holly Road, Northampton, remained impassive as the unanimous verdict was returned at London's Southwark Court.
The 22 year-old took part in distributed denial of service, or DDoS, attacks - one of which cost the website PayPal £3.5m.
Weatherhead was studying at Northampton University when he joined the cyber campaign.
Victims of attacks found their website suddenly crashed - the DDoS attacks paralysed computer systems by flooding them with an intolerable number of online requests.
Victims' websites would be directed to a page displaying the message: "You've tried to bite the Anonymous hand. You angered the hive and now you are being stung."
Southwark Crown Court heard that PayPal was attacked after it decided not to process payments on behalf of the Wau Holland Foundation, an organisation involved in raising funds for WikiLeaks.
Weatherhead had denied a charge of conspiring to impair the operation of computers between August 1, 2010 and January 22 last year.
The jury of six men and five women returned the guilty verdict after little more than two hours of deliberations.
The amount the hacking cost MasterCard and Visa was not given but the defendant, in an internet relay chat (IRC) channel conversation with someone called Tred, boasted: "We have probably done some million pound of dmg (damage) to mc (MasterCard)."
The BPI was attacked on September 19 and 20 2010, costing it £3,996 for online security and hundreds of pounds in other costs.
Four websites run by the Ministry of Sound were targeted in two separate attacks between October 2 and 6 2010, which cost the company around £9,000 in additional staffing, software and loss of sales.
The financial cost to the IFPI was more than £20,000 as its website was down for nine days when it was hacked between November 27 and December 6 2010.
Three other defendants, Ashley Rhodes, 27, of Bolton Crescent, Camberwell, south London; Peter Gibson, 24, of Castletown Road, Hartlepool; and Jake Birchall, 18, from Chester. had already pleaded guilty to the charge.
No date was fixed for sentencing but pre-sentencing reports were ordered for the four.
Judge Peter Testar said: "I am not making any promises at all by allowing bail and ordering a pre-sentencing report. This is a serious offence and I hope the defendant understands that.''