Another new wave of spam that disguises itself as a Microsoft Corp. 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 Hypponen, director of antivirus research at F-Secure Corp.

The phony patch is a variant of the SDBot Trojan software, which is at present not detected by antivirus software products, according to a report from security research firm WebSense Inc.

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.