As I'm sure you've heard by now, attackers are taking advantage of an unpatched vulnerability which gets exploited by .wmf files.
Dozens of sites are already hosting malicious .wmf files. In addition to this, the sites are distributing so called 'anti-spyware applications' (which require the infected user to pay) and other malware, such as Trojan-Spy.Win32.Small.ee, which isn't directly related to these applications.
Naturally we've been doing some research on this vulnerability and we've come up with some interesting findings.
At first glance it seems that hardware-based Data Execution Protection, which is available only with XP/SP2 on NX-bit (AMD) and XD-bit (Intel) enabled CPUs, prevents successful exploitation of the vulnerability.
We've tested on AMD and Intel platforms and HW DEP seemed initially to prevent successful exploitation in Internet Explorer and Windows Explorer. However, when testing the latest builds of third party image viewers like Irfanview and XnView HW DEP didn't prevent exploitation, even with HW DEP enabled for all programs. This is because both Irfanview and XnView are packed with ASPack and Windows disables HW DEP for ASPack packed files.
This shows that although HW DEP can help, it's by no means a solution.
Perhaps the most worrying thing about this whole issue is that NTFS rights have no effect on whether or not the vulnerability will be exploited.
Some people run under a limited user account (which among other things restricts NTFS rights). This may make people feel that they are protected from malware. In this case, nothing could be much further from the truth.
The attackers seem very well aware of this fact and have already released malware which will be downloaded and executed in a directory where a limited user has execution rights.
Our testing has also revealed that although Windows 2000 is not vulnerable by default, it is potentially vulnerable. If the Windows 2000 system has an image viewer which supports .wmf files installed, there's a high chance that the system will be vulnerable.
Image viewers like Irfanview and XnView rely on the vulnerable file to show .wmf files. Exploitation also successfully occurs on Windows 2000, with testing carried out on 2000/SP4 with all the latest patches.
The good thing however is that Internet Explorer will ask you (at least once) if you want to open or save the .wmf file instead of opening it by default.
WinXP Pro64 bit edition is also vulnerable. However, as all shellcode is written for IA32 processors the exploits won't work. Specific x64 shellcode needs to be written for the exploit to work. The chances of this happening (on a large scale) is slim as only a small number of users run WinXP Pro64 bit edition.
We've released heuristic detection for malicious .wmf files which exploit the new vulnerability. Suspicious files will be detected as Exploit.Win32.IMG-WMF.
*Updated to add information regarding ASPack packed files and HW DEP + other small correction.