After hackers temporarily shut down the websites of the one of world's biggest credit card companies, Mastercard, there is growing concern about the spreading cyber war of attacks and counter-attacks.
The Operation Payback campaign is targetting firms that have withdrawn services from Wikileaks, the whistle-blowing website which has been publishing leaked secret cables from US diplomats.
The hackers' top targets have included websites run by the financial groups MasterCard, Visa and PayPal.
Jeff Jarvis is a technology journalist in New York.
First broadcast 9 December 2010