A new, zero-day email worm is circulating in the wild that exploits an undisclosed vulnerability in Microsoft Word.

According to handlers at SANS Internet Storm Center, victims receive an email with a Microsoft Word attachment that contains a Trojan horse (Trojan.Mdropper.H). When the document is opened, a second Trojan (Backdoor.Ginwui) is launched and tries to connect to a Web server on the Internet to wait for a command.

In a statement, Microsoft said that it is working on a patch for the vulnerability that will be released on June 13 "or sooner as warranted." Microsoft also offered the helpful advice that users should "exercise extreme caution when opening unsolicited attachments from both known and unknown sources."

The exploit requires a user to try to open the Word document, so the chances of automated infection are low. Once the Trojan is launched, it overwrites the infected Word document with a clean copy in an attempt to hide. Symantec's DeepSight threat management team reports that Word 2000 simply crashed, and the Trojan didn't launch. But the exploit was successful in Windows 2003.

SANS handlers said the Trojan also exhibits rootkit functionality by hiding its files from Windows Explorer. The running process and startup registry key is also hidden, according to security company F-Secure.

Experts aren't yet sure which Word vulnerability is being exploited, but analysts at Symantec think it may be a buffer overflow in Word on an OLE component.

Dave Cole, director of Symantec's Security Response Team, doesn't see any need for widespread concern yet. "The attack seemed fairly targeted from one organization to another, and there isn't any exploit code circulating, unlike the Windows Metafile vulnerability circulating over Christmas," Cole says.

"This could be an example of a black market exploit," he adds. "We are not overstating when we say that zero-day [exploits] are being used in targeted attacks. In this case, we have no reason to believe that there will be a widespread attack." If exploit code starts circulating, however, the problem could be more pervasive.

According to the SANS report, the person who discovered the problem noticed discrepancies in an email that appeared to originate from his own domain, and was written in the fashion of an internal email, complete with signature.

— Mike Fratto, Editor at Large, Dark Reading

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