Online retailing was disrupted yesterday when Visa and Mastercard were targeted by hackers following the firms' refusal to process Wikileaks payments.
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The group of "hacktivists" launched Operation Payback in a revenge attack against the websites. "A hacktivist organisation known as Anonymous is coming to Wikileaks' defence with a botnet DDoS (distributed denial of service) attack against PayPal and others," said a Twitter post.
The Mastercard website was forced offline for several hours, while Visa also experienced problems.
The hackers have also targeted PayPal, Amazon and a Swiss bank over the Wikileaks row.
A Mastercard spokeswoman said the firm had made significant progress in restoring full service to its corporate website. "Our core processing capabilities have not been compromised and cardholder account data has not been placed at risk," she said.
Visa said it would release a statement this morning once the situation had been fully assessed.
Twitter is thought to be next in line, following accusations that it has blocked Wikileaks from trending on its microblogging site.
The company denied it had done so in a blog post yesterday. "This week, people are wondering about Wikileaks, with some asking if Twitter has blocked #wikileaks, #cablegate or other related topics from appearing in the list of top trends," said the firm.
"The answer: absolutely not. In fact, some of these terms, including #wikileaks and #cablegate, have previously trended either worldwide or in specific locations."