The flaw affects the engine that provides the scanning, detection, and cleaning capabilities for Microsoft antivirus and antispyware software (full list available here).
"An attacker who successfully exploited this vulnerability could prevent the Microsoft Malware Protection Engine from monitoring affected systems until the specially crafted file is manually removed and the service is restarted," the company explained in the advisory.
"There are many ways that an attacker could place a specially crafted file in a location that is scanned by the Microsoft Malware Protection Engine," they explained. "For example, an attacker could use a website to deliver a specially crafted file to the victim's system that is scanned when the website is viewed by the user. An attacker could also deliver a specially crafted file via an email message or in an Instant Messenger message that is scanned when the file is opened. In addition, an attacker could take advantage of websites that accept or host user-provided content, to upload a specially crafted file to a shared location that is scanned by the Malware Protection Engine running on the hosting server."
"If the affected antimalware software has real-time protection turned on, the Microsoft Malware Protection Engine will scan files automatically, leading to exploitation of the vulnerability when the specially crafted file is scanned. If real-time scanning is not enabled, the attacker would need to wait until a scheduled scan occurs in order for the vulnerability to be exploited."
The good news is that enterprise administrators or end users do not have to do anything about this - the default configuration in Microsoft antimalware software helps ensure that malware definitions and the engine are kept up to date automatically.
Another good news is that the vulnerability hasn't been spotted being exploited in the wild. Information about the flaw has been shared by Tavis Ormandy of Google Project Zero.