The Microsoft Malware Protection Center has a post noting security bulletin MS10-087, which addresses a number of critical vulnerabilities in how Microsoft Office parses various office file formats. This was addressed in November, in the MS10-087 update.
One of them is CVE-2010-3333, "RTF Stack Buffer Overflow Vulnerability," which could lead to remote code execution via specially crafted RTF data. A few days before Christmas, we received a new sample (sha1: cc47a73118c51b0d32fd88d48863afb1af7b2578) that reliably exploits this vulnerability and is able to execute malicious shellcode which downloads other malware.
If you use Microsoft Office, you may install the update via Windows Update (aka WU): go to http://windowsupdate.microsoft.com/ to learn more about how to use WU. You can launch WU by clicking the Start button on your Windows computer, then click All Programs and select Windows Update.
Also available via http://bit.ly/fSIIoH