MasterCard Inc., operator of the second-largest electronic payments network, with card brands MasterCard MasterCard, Maestro and Cirrus, temporarily lost service on its website due to a cyber attack from hackers backing whistle-blowing site WikiLeaks.

MasterCard restored its site after facing "intermittent service disruption," James Issokson, a spokesman for the firm, said in an e-mailed statement to Bloomberg News. "It is important to note that no cardholder data has been impacted and that cardholders can continue to use their cards securely."

Messages circulated on Twitter claiming that hackers were responsible. WikiLeaks tweeted on Tuesday that "hacktivists" had taken down MasterCard "over the continuing WikiLeaks fiscal embargo." Ibom Hacktivist wrote, "thats what you get when you mess with @wikileaks @Anon_Central and the enter [sic] community of lulz loving individuals."

A hacker group called LulzSec, or Lulz Security hacked the sites of Sony Corp., the CIA, an FBI affiliate, the U.S. Senate, and the Arizona Department of Public Safety, among many others, but disbanded last week after 50 days of high-profile cyber attacks, saying that they are bored and they need to go on with their lives. The early exit came after alleged leader or member Ryan Cleary was arrested in Britain.

The group Anonymous, an "internet vigilante group," had led a campaign against companies that stopped providing services for WikiLeaks, which released the US diplomatic cables and other sensitive information. The U.S. government received the most serious blow when the "anti-secrecy" organization WikiLeaks published, among other things, Pentagon documents on the Afghan war and the Iraq War and 250,000 confidential U.S. diplomatic cables on diplomats' candid assessments of terrorist threats and the behaviour of world leaders.

MasterCard's Internet site was hacked and knocked offline by Anonymous last December after it blocked WikiLeaks from using its payment network. MasterCard, along with Visa, PayPal and Swiss Bank PostFinance had stopped processing payments for WikiLeaks after the latter released the confidential diplomatic cables in 2010. The hackers respondent by sending distributed denial-of-service (DDOS) attacks against the websites of Mastercard, et al., in order to knock them offline.

Anonymous recently partnered with LulzSec for Operation Anti-Security, where they hack into, steal, and publish classified government information from any source while leaving the term "Antisec" as evidence of their intrusion. Victims of their Anti-Sec this month where the governments of Colombia, Brazil, Peru and the United Kingdom.

A large U.S. bank, Citibank, said earlier this month that more than 360,000 credit- card accounts, or 1.5 percent of credit card holders, in North America may have been compromised by hackers in May. The cyber attack against Citibank though is likely not from Anonymous or LulzSec as the attack against Citi appears to have been motivated by financial profit. Citi has been confirmed that 3,400 customers lost about $2.7 million when their credit-card information was breached by hackers earlier this year. Citibank will reimburse the customers.

With the WikiLeaks payback attacks on a comeback, it will be sleepless nights again for the security experts at PayPal and other potential targets.