Four UK youths acting in the name of activist hacking collective Anonymous cost PayPal £3.5m as a result of cyber attacks carried out 2010 and 2011, a court has heard.
The cyber attacks were made using a free tool downloaded from the internet called Low Orbit Ion Canon (LOIC), prosecutors told Southwark crown court.
More than 100 workers from PayPal's parent company, eBay, spent three weeks working on issues related to the attacks.
Southwark crown court heard PayPal also had to pay for more software and hardware to defend against similar attacks in the future, at a cost estimated at £3.5m.
Read more about DDoS attacks
Under the guise of Operation Payback, the four bombarded websites belonging to companies including PayPal and Ministry of Sound to make them inaccessible.
MasterCard, Visa, the British Recorded Music Industry and the International Federation of the Phonographic Industry were also hit.
People who tried to visit the sites were greeted with the message: "You've tried to bite the Anonymous hand. You angered the hive and now you are being stung,” according to The Guardian.
Anonymous's Operation Payback originally targeted companies involved in the music industry and opponents of internet piracy, but was later broadened to include revenge for WikiLeaks after the backlash against the site for publishing thousands of US diplomatic cables.
Ashley Rhodes, 28, of south London; Peter Gibson, 24, of Hartlepool; and an 18-year-old from Chester have pleaded guilty to conspiracy to impair the operation of computers between 1 August 2010 and 27 January 2011.
But Christopher Weatherhead, a 22-year-old student at Northampton University, has denied the charges. He is accused of being "part of a small cabal of leaders" of the cyber attacks.
The trial continues.