Gumblar can steal FTP credentials as well as hijack Google searches, replacing results on infected computers with links to other malicious sites.

When the Gumblar malware was found in March, it looked for instructions on a server at gumblar.cn. That domain was taken offline at the time, but has been reactivated within the last 24 hours, wrote Mary Landesman, a senior security researcher with ScanSafe, on a company blog.

Web sites that are infected with Gumblar contain an iframe, which is a way to bring content from one Web site into another. Malware writers usually make those iframes invisible. When a victim visits the site, the iframe will launch a series of exploits hosted on a remote computer to try and hack the visiting machine.

Gumblar checks to see if the victim's PC is running unpatched versions of Adobe Systems' Reader and Acrobat programs. If so, the machine will be compromised by a so-called drive-by download.

Domain name registrars will often suspend domain names that have been used for malicious purposes, and malware writers will usually frequently change the domains their software looks to for instructions as those bad domains are blacklisted. For some reason, the gumblar.cn domain was released and is in use again.

Landesman wrote that Web sites still infected with Gumblar may now be able to call back to the newly activated domain. It would allow those infected PCs to get updated with new malware.

"It's a mess," Landesman wrote. "Stay tuned."