It appears that there is a very serious bug which will affect users of Windows 7 and Windows Server 2008 R2.
This vulnerability affects SMB (Windows Server Message Block) this is the part of the system which deals with Network resources such as access remote printers and access to remote computers.

Microsoft confirmed that currently there is no fix for the problem. Only one solution is to disable SMB and await for update from Microsoft.

Apparently this attack can be performed even if on the machine firewall is enabled.

more informations on author blog.