CVE-2021-42298: Microsoft Defender Remote Code Execution Vulnerability

References Identification First version of the Microsoft Malware Protection Engine with this vulnerability addressed Version 1.1.18700.3

See Manage Updates Baselines Microsoft Defender Antivirus for more information.

Microsoft Defender is disabled in my environment, why are vulnerability scanners showing that I am vulnerable to this issue?

Vulnerability scanners are looking for specific binaries and version numbers on devices. Microsoft Defender files are still on disk even when disabled. Systems that have disabled Microsoft Defender are not in an exploitable state.

Why is no action required to install this update?

In response to a constantly changing threat landscape, Microsoft frequently updates malware definitions and the Microsoft Malware Protection Engine. In order to be effective in helping protect against new and prevalent threats, antimalware software must be kept up to date with these updates in a timely manner.

For enterprise deployments as well as end users, the default configuration in Microsoft antimalware software helps ensure that malware definitions and the Microsoft Malware Protection Engine are kept up to date automatically. Product documentation also recommends that products are configured for automatic updating.

Best practices recommend that customers regularly verify whether software distribution, such as the automatic deployment of Microsoft Malware Protection Engine updates and malware definitions, is working as expected in their environment.

How often are the Microsoft Malware Protection Engine and malware definitions updated?

Microsoft typically releases an update for the Microsoft Malware Protection Engine once a month or as needed to protect against new threats. Microsoft also typically updates the malware definitions three times daily and can increase the frequency when needed.

Depending on which Microsoft antimalware software is used and how it is configured, the software may search for engine and definition updates every day when connected to the Internet, up to multiple times daily. Customers can also choose to manually check for updates at any time.

What is the Microsoft Malware Protection Engine?

The Microsoft Malware Protection Engine, mpengine.dll, provides the scanning, detection, and cleaning capabilities for Microsoft antivirus and antispyware software.

Windows Defender uses the Microsoft Malware Protection Engine. On which products is Defender installed and active by default?

Defender runs on all supported version of Windows.

Are there other products that use the Microsoft Malware Protection Engine?

Yes, Microsoft System Center Endpoint Protection, Microsoft System Center 2012 R2 Endpoint Protection, Microsoft System Center 2012 Endpoint Protection and Microsoft Security Essentials.

Does this update contain any additional security-related changes to functionality?

Yes. In addition to the changes that are listed for this vulnerability, this update includes defense-in-depth updates to help improve security-related features.

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Suggested Actions

Verify that the update is installed

Customers should verify that the latest version of the Microsoft Malware Protection Engine and definition updates are being actively downloaded and installed for their Microsoft antimalware products.

• Open the Windows Security program. For example, type Security in the Search bar, and select the Windows Security program.
• In the navigation pane, select Virus & threat protection.
• Under Virus & threat protection updates in the main window, select Check for updates
• Select Check for updates again.
• In the navigation pane, select Settings, and then select About.
• Examine the Engine Version number. The update was successfully installed if the Malware Protection Engine version number or the signature package version number matches or exceeds the version number that you are trying to verify as installed.

For affected software, verify that the Microsoft Malware Protection Engine version is 1.1.18700.3 or later.

If necessary, install the update

Administrators of enterprise antimalware deployments should ensure that their update management software is configured to automatically approve and distribute engine updates and new malware definitions. Enterprise administrators should also verify that the latest version of the Microsoft Malware Protection Engine and definition updates are being actively downloaded, approved and deployed in their environment.

For end-users, the affected software provides built-in mechanisms for the automatic detection and deployment of this update. For these customers, the update will be applied within 48 hours of its availability. The exact time frame depends on the software used, Internet connection, and infrastructure configuration.

End users that do not wish to wait can manually update their antimalware software.