A new wave of spam that disguises itself as a Microsoft security bulletin contains a link to malicious software that gives attackers complete access to the infected machine, security researchers are reporting.

The e-mail, which began circulating late Tuesday, identifies itself as Microsoft Security Bulletin MS05-039, and offers a link to what it claims is a patch against the Sober Zafi and Mytob worms.

In fact, there is no such thing as Microsoft Security Bulletin MS05-039, and real Microsoft security bulletins offer links to a Microsoft download site, rather than to the patches themselves, said Mikko Hyppönen, director of anti-virus research at F-Secure.

The phony patch is a variant of the SDBot Trojan software, which is at present not detected by anti-virus software products, according to a report form security research firm WebSense.

The risk of someone downloading this Trojan appears to be very low right now, because the server hosting the Trojan downloads no longer appears to be active, Hyppönen said. That server, which appeared to be hosted by ThePlanet.com Internet Services, apparently has exceeded its allowed bandwidth, he said.

"I think this particular case is not going to be a problem anymore, but nevertheless I think it was a fairly interesting case," Hyppönen said. "I wouldn't be surprised to see more of this happening."

The Swen e-mail worm, which began circulating in 2003, used a similar technique, he added.

The IDG News Service is a Network World affiliate.