"This one has massive financial implications if someone exploits it," Litchfield said.

The TNEF vulnerability, which Microsoft spelled out in the MS06-003 security bulletin, is a flaw in how Microsoft's Outlook client and older versions of its Exchange server software decode the TNEF MIME attachment. TNEF is used by Exchange and Outlook when sending and processing messages formatted as Rich Text Format (RTF), one of the formatting choices available to Outlook users.

"All that's required to exploit this is an e-mail message," said Litchfield. No user interaction is needed to compromise an Exchange 5.0, 5.5, or 2000 server; all that's necessary is to deliver a maliciously-crafted e-mail to the server.
from: http://www.personaltechpipeline.com/news/175803807

Looks like MS has more problems on their hands now...