Windows Server 2019/2022: December 2022 security updates cause Hyper-V issues

[German]The security updates rolled out by Microsoft on the December 2022 patchday lead to problems with Hyper-V in certain constellations. New VMs can no longer be created, existing VM have problems with Ethernet connections, etc. Microsoft has admitted to these problems, although only Windows Server 2019 as well as Windows Server 2022 are said to be affected.

I covered the security updates for December 2022 in the articles Patchday: Windows 11/Server 2022-Updates (December 13, 2022) for Windows Server 2022 and Patchday: Windows 10-Updates (December 13, 2022) for Windows Server 2016 and 2019. Microsoft mentioned issues with Hyper-V in the Know Issues sections of the support articles of various security updates for Windows Server. Affected, according to the Windows Server 2022 Release Health status page are:

• Update KB5021249 (Windows Server 2022)
• Update KB5021237 (Windows Server 2019)

In the list of known issues, Microsoft says about these updates:

After installing this update on Hyper-V hosts managed by an SDN-configured System Center Virtual Machine Manager (VMM), you might see a workflows error.

This affects the creation of a new network adapter (NIC) connected to a VM network or a new virtual machine (VM) with a network adapter connected to a VM network.

Existing VMs with existing network adapters should have no issues connecting through network adapters following the installation of these updates, according to Microsoft. So it only affects new network adapters created after this update is installed. Affected systems may exhibit the following errors:

• When creating a new VM or a new network adapter on an existing VM, the error "<vmName> could not change device 'Ethernet Connection'" is displayed.
• The Load Balancer service (Software Defined Networking) might fail and the message "<SLBVMName> could not change device 'Ethernet Connection'" is displayed.
• The SDN RAS Gateway service may fail and the message "<GatewayVMName> could not change the device 'Ethernet connection'" is displayed.

Microsoft is working on a solution and plans to provide an update in one of the next releases. Until this solution is rolled out, Microsoft suggests a workaround. To do this, run the following PowerShell commands in a PowerShell window running with administrative privileges (Run as Administrator):

$lang = (Get-WinSystemLocale).Name
C:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem$lang \VfpExt.mfl
C:\Windows\system32\wbem\mofcomp.exe C:\Windows\system32\wbem\VfpExt.mof

A script with this workaround for large deployments and a post-installation script that can be integrated into patching tools are available under Win12B issue in System Center Virtual Machine Manager. The colleagues at Bleeping Computer have published a table here that shows which combinations of the aforementioned servers are affected by the problem.

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