Microsoft has posted security advisory 2974294, "Vulnerability in Microsoft Malware Protection Engine Could Allow Denial of Service," indicating that there’s an update to the Microsoft Malware Protection Engine that addresses a security vulnerability that, if not patched, would allow denial of service attacks.

The software affected includes Microsoft Security Essentials, Windows Defender, and several other MS packages.

More information, including the complete list of affected software: MS Security Advisory 2974294