Microsoft has issued a security advisory on the WMF vulnerability.

Details are available here

Update by Scott Fendley:
Microsoft has updated their security advisory tonight(December 30 UTC) with more information
and frequently asked questions with answers.

Some noteable things that I read in it.

"
** Windows Metafile (WMF) images can be embedded in other files such as Word documents. Am I vulnerable to an attack from this vector?

No. While we are investigating the public postings which seek to utilize specially crafted WMF files through IE, we are looking thoroughly at all instances of WMF handling as part of our investigation. While we're not
aware of any attempts to embed specially crafted WMF files in, for example Microsoft Word documents, our advice is to accept files only from trusted source would apply to any such attempts.

** It has been reported that malicious files indexed by MSN Desktop Search could lead to exploitation of the vulnerability. Is this true?

We have received reports and are investigating them thoroughly as part of our ongoing investigation. We are not aware at this time of issues around the MSN Desktop Indexer, but we are continuing to investigate.

** Is this issue related to Microsoft Security Bulletin MS05-053 - Vulnerabilities in Graphics Rendering Engine Could Allow Code Execution (896424) which was released in November?*

No, these are different and separate issues.

** Are there any third party Intrusion Detection Systems (IDS) that would help protect against attempts to exploit this vulnerability?

While we don't know of specific products or services that currently scan or detect for attempts to render specially crafted WMF files, we are working with our partners through industry programs like VIA to provide information as we have it. . Customers should contact their IDS provider to determine if it offers protection from this vulnerability.
"

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Scott Fendley
Handler on Duty