An error in Microsoft’s July update prevents certain virtual machines from starting.
Microsoft has released an emergency update to fix a bug preventing some virtual machines on Azure from starting correctly. The error was accidentally introduced during the July 2025 Patch Tuesday and mainly affects VMs with “virtualization-based security” (VBS) enabled.
Only with Certain Configurations
The bug occurs in Azure VMs running version 8.0 with the default security type. Virtual machines with VBS enabled, “trusted boot” disabled, and guest operating systems like Windows Server 2025 or Windows 11 24H2 are particularly vulnerable.
According to reports to Bleeping Computer, some users also experience issues with VMs outside Azure, such as on local Windows Server 2016 hypervisors. However, Microsoft has not officially confirmed this yet.
Issue in Secure Kernel
Microsoft states that the error lies in the initialization of the “secure kernel”. This kernel is a crucial part of VBS and ensures that sensitive processes of the operating system are shielded. If it doesn’t start properly, the VM doesn’t function correctly. Users can check if this security is active via msinfo32.exe and verify the security type set in the Azure console.
It’s already the second virtualization issue for Microsoft this year, writes TechRepublic. In January, the company resolved three vulnerabilities in Hyper-V that potentially gave attackers system rights. The update has been available since Sunday.