The hackers behind the attacks on a range of WikiLeaks opponents took down parts of MasterCard's online presence yesterday, and have now turned their attention to Visa.
Other targeted firms have included PayPal and Swiss Bank Post Finance. All of the companies were attacked because of their refusal to process donations for WikiLeaks.
MasterCard said this morning that it is making "significant progress" in restoring services to its corporate web site, and insisted that its core payments business had not been affected despite being targeted by the hackers for most of yesterday.
"Our core processing capabilities have not been compromised and cardholder account data has not been placed at risk," MasterCard said in a statement.
"While we have seen limited interruption in some web-based services, cardholders can continue to use their cards for secure transactions globally."
However, the card giant's 3-D Secure authentication system MasterCard SecureCode also appears to have been affected, although the cause has yet to be determined.
A posting by online processor Secure Trading yesterday read: "MasterCard SecureCode is currently down. This means that all MasterCard and Maestro transactions cannot be processed via 3-D Secure. This is affecting all payment service providers and is not SecureTrading specific."
The issue also affected Visa's 3-D Secure system, Verified by Visa, today, the payment processor said.
A hacker claiming to be from the Anonymous group calling himself 'Coldblood' said on the BBC's Today programme that the campaign will continue, and that a tool made available for use in denial-of-service attacks is still being downloaded by new users.
"We are trying to keep the internet open and free but, in recent years, governments have been trying to limit the freedom we have on the internet," he said.
However, a Twitter account with the name Op_Payback, that also purports to be part of the Anonymous group, is displaying this post today: 'Attention all media: 1. We don't have a spokesperson named Coldblood.'
Yesterday the group was using a different Twitter account, which appears to have been suspended.
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