A Republican Party Web site has been hacked, and for some time it has been spreading a variation of the long-running Storm Trojan horse to vulnerable visitors, a security researcher said Friday.

This is the first time that Storm has taken to the Web for its victims, said Dan Hubbard, head of research at San Diego-based Websense. "The big news is that Storm has added infecting sites to its arsenal," said Hubbard.

Storm debuted in January but only cracked the top malware lists early this summer, and has become infamous for its ability to adapt its infection strategies.

"They have a knack for latching onto the latest newsworthy events and capitalizing on the public interest in them," Symantec researcher Hon Lau said last month. "And if no newsworthy events are happening at the time, then they will just make them up."

Until now, Storm has infected users via files attached to e-mail or through links embedded in spam. The change noticed by Websense's scanners, however, means that Storm's backers have moved to other attack vectors -- in particular, compromised Web sites that sport malicious IFRAMEs. Users visiting such sites are instantly infected with the Trojan if their browsers are not patched against whatever exploit the IFRAME code is throwing out.

According to Hubbard, several hundred sites have been compromised by Storm's makers, then seeded with IFRAMES that can inject the Trojan into vulnerable PCs.

One such site was a Republican Party Web site for the 1st Congressional District of Wisconsin. Within hours after Websense notified the site's owners, however, it had been purged of the dangerous IFRAME code. By mid-morning Friday, it was safe to visit. Hubbard did not know how the site was compromised.

The motive behind Storm's continued attacks and its expansion into new areas like this, said Hubbard, is a never-ending appetite for bots -- compromised computers that can be used for spamming or other criminal activities either by the original attackers or by others who lease sections of the botnet.

"Storm's botnet is clearly the biggest around," said Hubbard, who estimated its size as "conservatively, in the hundreds of thousands, although some people have thrown out numbers like 1 million or 2 million or even 4 million." Earlier this month, in fact, MessageLabs pegged the botnet at 2 million machines.

In the last few weeks alone, Storm has spread through e-mails touting a real-time scoreboard site for National Football League games, spam hyping a Web site that wished Americans a happy Labor Day holiday, and more mail that used YouTube videos as bait.

Computerworld is an InfoWorld affiliate