Microsoft Issues Patches for Defender Zero-Day and 82 Other Windows Flaws

For the first patch Tuesday of 2021, Microsoft released security updates addressing a total of 83 flaws spanning as many as 11 products and services, including an actively exploited zero-day vulnerability.

The latest security patches cover Microsoft Windows, Edge browser, ChakraCore, Office and Microsoft Office Services, and Web Apps, Visual Studio, Microsoft Malware Protection Engine, .NET Core, ASP .NET, and Azure. Of these 83 bugs, 10 are listed as Critical, and 73 are listed as Important in severity.

The most severe of the issues is a remote code execution (RCE) flaw in Microsoft Defender (CVE-2021-1647) that could allow attackers to infect targeted systems with arbitrary code.

Microsoft Malware Protection Engine (mpengine.dll) provides the scanning, detection, and cleaning capabilities for Microsoft Defender antivirus and antispyware software. The last version of the software affected by the flaw is 1.1.17600.5, before it was addressed in version 1.1.17700.4.

The bug is also known to have been actively exploited in the wild, although details are scarce on how widespread the attacks are or how this is being exploited. It’s also a zero-click flaw in that the vulnerable system can be exploited without any interaction from the user.

Microsoft said that despite active exploitation, the technique is not functional in all situations and that the exploit is still considered to be at a proof-of-concept level, with substantial modifications required for it to work effectively.

What’s more, the flaw may already be resolved as part of automatic updates to the Malware Protection Engine — which it typically releases once a month or as when required to safeguard against newly discovered threats — unless the systems are not connected to the Internet.

“For organizations that are configured for automatic updating, no actions should be required, but one of the first actions a threat actor or malware will try to attempt is to disrupt threat protection on a system so definition and engine updates are blocked,” said Chris Goettl, senior director of product management and security at Ivanti.

Tuesday’s patch also rectifies a privilege escalation flaw (CVE-2021-1648) introduced by a previous patch in the GDI Print / Print Spooler API (“splwow64.exe”) that was disclosed by Google Project Zero last month after Microsoft failed to rectify it within 90 days of responsible disclosure on September 24.

Other vulnerabilities fixed by Microsoft include a memory corruption flaws in Microsoft Edge browser (CVE-2021-1705), a Windows Remote Desktop Protocol Core Security feature bypass flaw (CVE-2021-1674, CVSS score 8.8), and five critical RCE flaws in Remote Procedure Call Runtime.

To install the latest security updates, Windows users can head to Start > Settings > Update & Security > Windows Update, or by selecting Check for Windows updates.