A group of hackers simply going by the name “Anonymous” is targeting companies that have blocked whistleblowing site WikiLeaks from using their services.
The latest target is MasterCard, whose site mastercard.com has currently been knocked offline thanks to a series of DDOS (distributed denial of service) attacks against it. On a very basic level, DDOS attacks consist of automated, high-volume visits to a particular site so great in number and frequency that the site’s servers overload and shut down.
The group is calling its actions “Operation: Payback” and has set up a site of its own, complete with a list of organizations that have hindered WikiLeaks’ progress in some manner or another. The list currently includes Amazon.com, the French government, EveryDNS and PayPal.
An executive from payment processing site, PayPal, recently told attendees at a web conference that the State Department informed PayPal that WikiLeaks was engaged in illegal activities. PayPal then blocked donations to WikiLeaks, citing a desire to “comply with regulations around the world, making sure that we protect our brand,” said the executive.
The Guardian reports that the hacking group has also set its sights on Twitter, which it believes has censored certain tweets containing #wikileaks and #cablegate hashtags. Hashtags are used on Twitter to group various areas of discussion together, so any tweet that contained “#wikileaks” in its text would show up under a search for “#wikileaks” on Twitter.
The complaint was that the #wikileaks hashtag wasn’t showing up in Twitter’s list of popular, “trending” topics. Twitter has denied censoring WikiLeaks news, saying that its algorithm for detecting trending topics measures quick bursts of commonly-used hashtags rather than a high-volume of commonly-used hashtags over longer periods.
UPDATE: The hacking group has now turned its attention to Visa, knocking visa.com offline at around 4:00 Eastern time this afternoon.
More on TIME.com: