An unpatched Microsoft Word 2000 vulnerability is being actively exploited by hackers to spread malware.
The MDropper-Q Trojan downloader, recently detected by Symantec, takes advantage of the unspecified zero-day vuln to load other malware onto compromised PCs, including a backdoor Trojan called Backdoor-Femo, which surrenders control of compromised PCs to hackers. The attack is dangerous but not, as yet, widespread.
Documents incorporating the exploit code must be opened with a vulnerable copy of Microsoft Word 2000 for the attack to succeed, so the exploit doesn't lend itself towards the creation of self-replicating network worms.
Users are advised not to open untrusted documents until Microsoft patches the vulnerable software, in this case Word and Office 2000. Symantec is holding back on releasing details of the vulnerability pending a fix from Microsoft.
A previous version of MDropper attempted to target users of Microsoft's Office 2003 suite of applications. As Symantec notes: "Microsoft Office vulnerabilities are a great platform for social engineering and email based attacks."
http://www.theregister.com/2006/09/05/ms_office_trojan/