Authentication is not required to exploit this vulnerability.
The specific flaw exists within the nrouter.exe service while processing a malformed calendar meeting request. The process copies the contents of the name parameter within the Content-Type header into a fixed size stack buffer. By providing enough data this buffer can overflow leading to arbitrary code execution under the context of the SYSTEM user.
Disclosure Timeline:
2008-08-26 - Vulnerability reported to vendor
2011-02-07 - Coordinated public release of advisory