Today, Microsoft released a critical security advisory warning customers of a serious new zero day vulnerability that affects Windows, Office, and Lync.
In a nutshell, the vulnerability has to do with how certain versions of Windows, Office, and Lync handle specially crafted TIFF images. If an attacker can trick you into viewing a malicious image, including ones embedded in Office documents, he can exploit this flaw to execute code on your computer, with your privileges. If you have local administrative permissions, as most Windows users do, they attacker gains complete control of your computer.
McAfee researchers first discovered this flaw being exploited in the wild, and they share some interesting details about the issue on their blog (Microsoft also shares some extra technical detail here). While the flaw lies in Microsoft's image handling components (GDI+), the public attack actually arrives as a malicious Word document with an embedded TIFF, which the attackers send via email. Microsoft claims attackers are only exploiting the flaw in limited, targeted cases.
Since they just learned about the flaw recently, Microsoft hasn't had time to patch it yet. However, they have released a FixIt which mitigates the issue. FixIts are not considered full patches, but they can protect you until Microsoft releases their final update. If you use any of the affected versions of Windows, Office, or Lync, I highly recommend you apply the FixIt as soon as you can. Microsoft does also offers a few other workarounds, such as disabling the TIFF codec, or using the EMET tool (something I suggest you do in general), but I think the FixIt is the quickest and most reliable solution.