UPDATE: MasterCard says a list of purported credit card numbers released by pro-WikiLeaks hacktivists is not genuine.

“Those are not MasterCard numbers,” company spokesman Chris Montero told Raw Story.

Montero said that MasterCard numbers all begin with a “5.” Raw Story’s copy of the list does not appear to include any serial number that begins with a five.

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Earlier in the day, Operation Payback — an anonymous group of hackers seeking revenge on organizations and individuals seen to be enemies of WikiLeaks — had claimed that the list represented 10,000 valid MasterCard numbers.

It is unknown whether the numbers are real credit card numbers. About one quarter of the serial numbers begin with a “4,” which is the number all Visa cards begin with.

ORIGINAL STORY FOLLOWS BELOW

Operation Payback, the hacktivist group taking revenge on groups and individuals seen as enemies of WikiLeaks, has published a list of what it says are 10,000 MasterCard credit card numbers.

The anonymous hackers earlier in the day attacked the websites of both MasterCard and Visa, in retaliation for the companies’ freezing of WikiLeaks’ account.

“To the people of the industrial world, dismiss your MasterCard now!” the group declared on a Twitter account that has since been taken down.

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The list includes what appear to be card numbers, as well as expiry dates. However, the names of the card-holders are not included.

The numbers were posted to a file-sharing site that has evidently since taken them down. Raw Story has obtained a copy of the list, but has decided not to publish it for privacy reasons.

It has not been confirmed that these are actual credit card numbers. MasterCard spokespeople were unavailable to comment Wednesday evening.

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The security breach could pose a serious public relations problem for the credit card company, which may find itself defending its customer privacy practices and the security of its computer networks. News reports earlier Wednesday indicated that it took all of a minute to shut down MasterCard’s website once hackers launched an attack.

Operation Payback’s Twitter account was shut down Wednesday. Speculation abounds on the Internet that the publishing of the credit card numbers may have prompted the shut-down.

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On Wednesday evening, the group –now armed with a new Twitter account — boasted of its successful attempts at shutting down websites, which — aside from the credit card companies — included Sen. Joe Lieberman’s site, the site of the Swedish prosecutor handling the sexual assault investigation into WikiLeaks founder Julian Assange, and Swiss bank PostFinance, which froze Assange’s legal defense fund.

“Bloody hell! @anon_operation takes down mastercard, visa, a Swiss bank, the Swedish prosecution, and Lieberman’s site. In 24 hours? Whoa!” the hacktivists declared.

Meanwhile, the company that processes payments to WikiLeaks says it will sue Visa over its decision to shut out WikiLeaks.

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“DataCell, who facilitates those payments towards WikiLeaks, has decided to take up immediate legal action to make donations possible again,” the company said in a statement flagged by TalkingPointsMemo.

“Visa is hurting WikiLeaks and DataCell in high figures. … Visa users have explicitly expressed their will to send their donations to Wikileaks and Visa is not fulfilling this wish.”