The code is CVE-2017-11937, this vulnerability is actually caused by the memory failure when the Malware Protection Engine scans for fake files to detect vulnerabilities.
Microsoft said the attacker placed the infected file in a location, then scanned the Malware Protection Engine for memory errors and allowed remote code execution on the LocalSystem account and took control of the target machine.
"There are many ways to place this file, such as using a website that users access," Microsoft explained. Other ways can be email, chat applications. The attacker can also "take advantage of the website to approve or store the content the user provides to upload the file to a common location, then the Malware Protection Engine scans the host server and gets an error."
Microsoft assured customers that the vulnerability was fixed before any attack. They have released security updates and recommend patching as soon as possible. Most users can receive automatic emergency patches.
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2017-11937
This vulnerability was commanded by British National Network Security Center (NCSC), GCHQ's network security organization, and discovered and reported by the Department. The patch also came a few days before Microsoft released a Patch Tuesday patch for December.
See also: Microsoft released an updated patch for 25 critical security holes