With Gumblar, more and more sites are now being infected. Landesman believes it's because Gumblar's creators have been good at obfuscating their attack code and making it harder to spot on infected sites. And because they've been stealing FTP login credentials, they've been able to use a few new tricks to get their software onto the sites. "They're doing things like changing folder permissions … and leaving behind multiple ways that they can get back into the server," she said.

Still, Web attacks have become so widespread that Gumblar remains a relatively small-scale phenomenon, according to Symantec Security Response Product Manager John Harrison. Last year, Symantec counted 18 million online attacks against its customers. With Gumblar, it has counted 10,000. "It's really just another day with drive-by downloads," he said. "There really are so many of these."

Security experts say that if you're using a fully-patched system with up-to-date security software, you should be protected from these attacks. To date, they've worked by hitting the victim with malicious PDF or Flash files.