WikiLeaks supporters on Friday downloaded increasing amounts of the spam-shooting software used to attack companies seen as hostile a development that could challenge even Internet giants such as PayPal during the crucial Christmas shopping season.

U.S. data security company Imperva says downloads of the attack program used to bombard websites with bogus requests for data have jumped to more than 40,000, with thousands of new downloads reported overnight.

“It’s definitely increasing,” Imperva Web researcher Tal Be’ery said.

The freely available software is a critical part of the campaign by “hacktivists” seeking to take revenge on sites they think have betrayed WikiLeaks, the anti-secrecy group that has outraged American officials by publishing hundreds of thousands of classified U.S. diplomatic cables and military intelligence reports.

Users who download the software essentially volunteer their computers to be used as weapons that volley streams of electronic spam at targeted websites. The more computers, the greater the flow of data requests, and the better the chances of overwhelming the targeted website.

The cyberguerillas, who gather under the name Anonymous, have generally been successful in foiling their enemies. Attacks directed at the main pages of Visa Inc. and MasterCard Inc. succeeded in making them inaccessible. Attacks on online payment company PayPal Inc. have periodically rendered part of its website inoperative., another targeted site, was inaccessible Friday.

All four sites have severed their links to WikiLeaks, often citing suspected “terms of use” violations, hurting the group’s ability to accept donations. The moves angered WikiLeaks supporters and alarmed free speech advocates, who claim the companies are caving in to U.S. pressure to muzzle the controversial website.