Microsoft has announced that they will be pushing an out-of-band security patch today. The patch, which affects nearly all of the company's major platforms, is rated 'critical' and it is recommended that you install the patch immediately.
The patch is rated 'critical' because it allows for elevation of privileges and will require a restart. The platforms that are affected include: Windows Server 2003, Windows Server 2008, Windows Server 2012 and Windows Server 2012 R2. Client Windows versions are listed but with 'No severity rating' meaning that the vulnerability isn't present on it, but the patch will be offered as a required update all the same.
The patch will be released at 10 A.M. PST but details about the exploit are not yet listed on Microsoft's page.
Seeing that this is an out-of-band patch and is rated critical, it may mean that the exploit is out in the wild and Microsoft wants to patch it as quickly as possible. You can install the patch using Windows Update in about three hours from the time of publishing.