There's a new zero-day vulnerability related to Windows' image rendering - namely WMF files (Windows Metafiles). Trojan downloaders, available from unionseek[DOT]com, have been actively exploiting this vulnerability. Right now, fully patched Windows XP SP2 machines machines are vulnerable, with no known patch.
The exploit is currently being used to distribute the following threats: Trojan-Downloader.Win32.Agent.abs Trojan-Dropper.Win32.Small.zp Trojan.Win32.Small.ga Trojan.Win32.Small.ev.
Some of these install hoax anti-malware programs the likes of Avgold.
Note that you can get infected if you visit a web site that has an image file containing the exploit. Internet Explorer users might automatically get infected. Firefox users can get infected if they decide to run or download the image file.
In our tests (under XP SP2) older versions of Firefox (1.0.4) defaulted to open WMF files with "Windows Picture and Fax Viewer", which is vulnerable. Newer versions (1.5) defaulted to open them with Windows Media Player, which is not vulnerable...but then again, Windows Media Player is not able to show WMF files at all so this might be a bug in Firefox. Opera 8.51 defaults to open WMF files with "Windows Picture and Fax Viewer" too. However, all versions of Firefox and Opera prompt the user first.
As a precaution, we recommend administrators to block access to unionseek[DOT]com and to filter all WMF files at HTTP proxy and SMTP level.
F-Secure Anti-Virus detects the offending WMF file as W32/PFV-Exploit with the 2005-12-28_01 updates.
We expect Microsoft to issue a patch on this as soon as they can.