Secunia

Jesse Ruderman has reported a vulnerability in MSN Messenger and Microsoft Word, allowing access to the Windows "shell:" functionality.

The problem is that the programs fail to restrict access to the "shell:" URI handler. This allows malicious people to invoke various programs associated with specific extensions. It is not possible to pass parameters to these programs, only filenames, thus limiting the impact of launching applications.

The Windows "shell:" URI handler is inherently insecure and should only be accessed from a few trusted sources - it may even pose a threat through Word documents. Multiple exploits in Internet Explorer also utilise "shell:" functionality.

This is related to a similar issue in Mozilla:
SA12027

Solution
Do not follow links in MSN Messenger.
Further details available in Customer Area

Provided and/or discovered by
Jesse Ruderman

Other references
Further details available in Customer Area

Deep Links
Links available in Customer Area