Microsoft Server Message Block Redirector Driver (mrxsmb.sys)does not
verify user-mode buffer properly, allowing to any user to overwrite any
desired memory address. The successful exploitation results in Ring0
code execution.

+ Paper/Advisory: -Reversing mrxsmb.sys. Chapter I "Getting Ring0"-(pdf)
+ Exploit code (c source code)

Both two can be downloaded at www.reversemode.com

This issue has been addressed in the recent bulletin, MS06-030 .

Ruben Santamarta,
www.reversemode.com