Microsoft has updated its malware protection technology following the discovery of a bug which might, given a plausible but unlikely set of circumstances, allow a hacker to gain root access to vulnerable systems.
Would-be hackers would already need to have access to targeted systems in order to exploit the privilege escalation bug, which creates a means for a common or garden user to make himself a super user. The hack involves using a specially crafted registry key on vulnerable systems.
Microsoft Malware Protection Engine is a part of several Microsoft anti-malware products, including Microsoft Security Essentials and Microsoft Forefront Client Security (Redmond's endpoint client for businesses). Microsoft Malicious Software Removal Tool is also affected, which means users who don't rely on Microsoft's anti-virus technology but use the widely deployed clean up tool are brought into the mix. Server side anti-malware products are not affected.
Updating the engine to fix the vulnerability should happen automatically as new software definition files are applied, as explained in an advisory by Microsoft here. ®