Microsoft has acknowledged the Windows 7 zero-day vulnerability reported last week with a Security Advisory. The advisory from Microsoft provides some additional details about the scope and nature of the threat, as well as some steps you can take immediately to protect vulnerable systems.

The Security Advisory explains that this vulnerability cannot be used by an attacker to gain control of a vulnerable system, or to install malicious software. It also notes that, while functional exploit code has been published, there are no reported incidents of this flaw being exploited in the wild at this time.
The identified flaw affects the SMB networking protocol and can be exploited on vulnerable systems to cause a denial-of-service (DoS) condition. The issue is confirmed to exist on Windows 7 and Windows Server 2008 R2 systems--both 32-bit and 64-bit platforms.
The Microsoft Security Advisory notes that standard firewall best practices should mitigate this threat in most instances. The number of ports open through the firewall should be minimized, and the ports used by SMB ought to be blocked at the firewall by default as a function of normal security policy.
Microsoft is developing a security update which will resolve this issue, but the earliest you can expect to see that update is probably Microsoft's Patch Tuesday for December, which isn't until December 8. In the meantime, there are some workarounds or extra steps you can take to safeguard your systems against having this vulnerability exploited.
(Source: The Washington Post)