October 2026 security updates for Windows Server hosts have known issues that impacts SCVMM managed environments.
Impacted scenarios:
After you install Windows Server 2025, Windows Server 2022, and Windows Server 2019 October 2026 patches, KB5126646, KB5126655, and KB5126651 respectively,
- Content-transfer workflows like VM deployment by using VM templates that uses BITS transfers fail with HTTP 500 error. This impacts SCVMM 2025, 2022, and 2019 environments.
- [AMD processors only] Host and cluster refresh fails with UInt64-to-Int64 OverflowException affecting the ability to reliably manage the AMD hosts and clusters. This impacts SCVMM managed WS 2025, 2022, and 2019 hosts and clusters.
Solution:
IMPORTANT: Follow these steps ONLY when you have updated your WS hosts with the October 2026 patches. These issues will not impact WS hosts which are not updated with the October 2026 patches.
Issue #1:
The product team is working on publishing a fix as a top priority. The fix will be delivered on top of the latest Update Rollup (UR) versions - UR1 for SCVMM 2025 and UR4 for SCVMM 2022. While the team works on publishing the fix, update your SCVMM 2025 to UR1 and your SCVMM 2022 to UR4. As a temporary workaround, roll back the affected components by following these steps:
- Download the rollback MSI for Windows Server 2025 from here, for Windows Server 2022 from here and for Windows Server 2019 from here.
- Install the downloaded MSI files on all the affected hosts, SCVMM server, and SCVMM library.
- Disable the KIR Policy by using Group Policy Editor. Open the Local Group Policy Editor. Navigate to Local Computer Policy > Administrative Templates > (Respective MSI file name) > Windows Server 2025/2022/2019. Select the policy name.
- Double-click the policy setting on the right labeled with the respective MSI file name. This action opens the policy, and you see the default setting is Not Configured. Change it to Disabled and select OK.
- To validate the update, run the gpupdate /sync command from an administrator command prompt.
- Restart the machines in which you made these changes.
Issue #2:
This issue is isolated to hosts or clusters that use AMD processors. To mitigate this issue,
- Update your SCVMM 2025 and SCVMM 2022 instances to Update Rollup 1 and Update Rollup 4 versions respectively.
- Download the hotfix from here for SCVMM 2025 UR1 or from here for SCVMM 2022 UR4. File 2025UR1_HF or 2022UR4_HF contains the file required to fix the aforementioned issue.
- Back up the ImgLibEngine.dll file before running 2025UR1_HF or 2022UR4_HF.
- Run 2025UR1_HF or 2022UR4_HF to extract the hotfix.
- Stop the service System Center Virtual Machine Manager on SCVMM Server.
- Replace the files under your Virtual Machine Manager Install directory %VMMINSTALLPATH%\bin\ImgLibEngine.dll on the SCVMM Server with the new files extracted in the previous step.
Note: SCVMM 2019 hotfix for Issue #2 will be published after the permanent fix for Issue #1 is published.
If you need further help with these issues, please contact the Microsoft Support team by creating a support request.
Microsoft