The websites for Visa and MasterCard were inaccessible for parts of Wednesday, likely the result of attacks by WikiLeaks supporters who are angry that the credit card companies had stopped processing donations to the organisation. Both MasterCard and Visa said that cardholders' accounts were not at risk and that people could continue using their credit cards throughout the day.
Supporters of the WikiLeaks, which has released thousands of classified government documents in recent weeks, said they would attack companies and groups hostile to site and its founder. An Internet group operating under the label "Operation Payback" claimed responsibility for the MasterCard and Visa problems in messages on Twitter and elsewhere.
Spokesman Ted Carr said Visa's processing network, which handles cardholder transactions, was working normally.
The hacking group Anonymous is distributing software tools to allow anyone with a computer and an Internet connection to join in the attacks as part of Operation Payback. Such tools are widely available on the Internet and can easily launch a large number of attacks on targeted websites, said Dean Turner from the computer security firm Symantec.
MasterCard acknowledged "a service disruption" involving its Secure Code system for verifying online payments, but spokesman James Issokson said consumers could still use their credit cards for secure transactions. Consumers can use credit card companies' websites to find information about the cards, but applying for one and accessing existing accounts are done through the banks that issue the cards.
Besides Visa and MasterCard, Amazon.com Inc and eBay Inc's PayPal cut ties to WikiLeaks amid intense US government pressure. In a statement, PayPal confirmed its website has been attacked, which at times slowed the site down but did not "significantly" affect payments. MasterCard said its systems have not been compromised. PayPal has admitted it suspended payments to WikiLeaks after an intervention from the US State Department.