Because of the seriousness of a remote code execution vulnerability recently discovered in the Windows Server Service, Microsoft has released a fix outside of its normal "Patch Tuesday."According to Chris Budd of the Microsoft Security Response Center, the vulnerability was discovered during the reverse engineering process routinely accomplished by the folks in Microsoft's Malware Protection Center. Developers working the project discovered that the exploit was extremely "wormable," especially on Windows XP and older systems, and as a result a patch was distributed today as soon as it was ready for public distribution.
A specially crafted RPC request can trigger the exploit on systems unprotected by firewalls. Because most corporate and home users sit behind a firewall that filters in-bound connections to TCP ports 139 and 445, you will not be reachable from the Internet via RPC. As a result, attacks using this exploit will generally come from unpatched machines on the local LAN.
Systems running Vista and Windows 2008 Server enjoy an additional level of security, but still need to be patched.
More information on MS08-067 is available here.