December Patch Tuesday has broken Network Adapters in Hyper-V hosts on Windows Server

Microsoft rolled out Patch Tuesday updates to various Windows versions just a couple of days ago. While some of them have brought along new features and known issues, some other problems have also popped up. Today, Microsoft has confirmed that the process to create Network Adapters on some Hyper-V hosts is broken in Windows Server.

In an update on its health dashboard, Microsoft notes that customers who install the KB5021249 update might be treated to issues concerning Hyper-V hosts on Windows Server installations. Basically, an error will be thrown when creating a new Network Adapter or Network Interface Card (NIC) joined to a virtual machine (VM) network. This may also occur when creating a new VM with a Network Adapter joined to a VM network. Therefore the bug only affects new NICs and existing ones should work fine post-KB5021249 update. The issue will interrupt the workflow on Hyper-V hosts managed by Software Defined Networking (SDN) configured System Center Virtual Machine Manager (VMM).

You may encounter one of the following errors:

• When creating a new VM or a new network adapter on an existing VM, you might receive, "vmName failed to modify device 'Ethernet Connection"
• Software defined networking (SDN) software load balancer service might fail, and you might receive, "SLBVMName failed to modify device 'Ethernet Connection' error"
• SDN RAS Gateway service might fail, and you might receive, "GatewayVMName failed to modify device 'Ethernet Connection' error"

For now, the mitigation that Microsoft has offered is to run the following commands on PowerShell running with administrator privileges:

$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

For large-scale installations, these commands can also be integrated as a post-install script through this guide.

Microsoft has assured customers that it is working on a fix but has not hinted at how long it will take to patch the problem yet. The issue only affects Windows Server 2019 and Windows Server 2022. Client installations of Windows are unaffected.