Windows Hotpatch Overview

Key Concepts

• Hot Patching: A technology allowing security updates to be applied to a running operating system without requiring a reboot, primarily for Windows Server and Windows 11.
• Cumulative Updates: Monthly updates from Microsoft that bundle feature, quality, and security updates, traditionally requiring a system reboot.
• Baseline Reboot: A mandatory quarterly reboot that incorporates all feature, quality, and security updates, including those that cannot be hot-patched.
• Forward Attach: A mechanism within hot patching that modifies an in-memory process to redirect calls from an old function to a newly loaded, updated function.
• Reverse Attach: A mechanism allowing a newly loaded, updated function (via hot patch) to access existing resources, handles, or global variables from the original process.
• Metadata (in Hot Patch): A section within a hot patch file that defines the "plumbing" for forward and reverse attaches, instructing how to modify running processes.
• Rings of Deployment: A safe deployment practice involving rolling out changes to small, controlled groups first, then gradually expanding to larger populations.
• Azure Arc for Servers: A service that extends Azure management capabilities to on-premises or multi-cloud servers, enabling features like hot patching for non-Azure OS scenarios.
• Azure Update Manager: A service used in conjunction with Azure Arc to manage updates, including hot patching, for hybrid environments.

Introduction to Traditional Patching Challenges

Traditionally, Microsoft releases cumulative updates monthly, which bundle feature, quality, and security updates. These updates typically require a system reboot to fully apply. The necessity for reboots stems from the fact that core operating system files (executables, dynamic link libraries) are loaded into memory and actively used by running processes. Replacing these in-use files or their functions requires restarting the entire operating system to ensure the new versions are loaded correctly and all inter-process calls are properly re-established.

This reboot requirement poses significant challenges:

• Deployment Delays: Critical servers or user environments often resist reboots due to potential impact on workloads or user productivity.
• Increased Attack Surface: Delays in deploying critical security updates leave systems vulnerable to malicious actors for longer periods.
• Orchestration Complexity: Managing reboots across large organizations adds complexity to the patching process.

What is Hot Patching?

Hot patching is a technology designed to apply security updates without requiring a system reboot. Its primary goal is to accelerate the deployment of security fixes, thereby improving the security posture of environments faster and with minimal disruption. A hot patch typically takes less than 30 seconds to deploy.

Benefits of Hot Patching

• Accelerated Security: Rapid deployment of security updates reduces the window of vulnerability.
• Minimized Disruption: Eliminates the need for reboots for most security updates, ensuring continuous availability of critical services and user productivity.
• Simplified Orchestration: Reduces the complexity associated with planning and executing reboots.
• Smaller Update Size: Hot patches are significantly smaller than full cumulative updates as they only contain security-related function updates.

Deployment Best Practices

Despite the ability to deploy hot patches instantly, the video emphasizes that safe deployment practices (e.g., using rings of deployment) should still be followed. Changes should be rolled out to small test populations first to gain confidence that they will not negatively impact workloads before broader deployment.

Scope and Limitations

Hot patching supports updates for various Windows components, including:

• Kernel components
• Shell team components
• Hypervisor

However, it has limitations:

• Unsupported Today: .NET binaries and some preboot files are not currently supported. Updates to preboot files are rare but would necessitate a full reboot.
• Security Updates Only: Hot patching is exclusively for security updates; feature and quality updates still require a reboot.

Hot Patching Cadence and Lifecycle

Microsoft has aligned the patching process around a quarterly cycle when hot patching is enabled:

1. First Month of the Quarter (Baseline): This month (e.g., January, April, July, October) serves as the baseline. During this cycle, a full reboot is required. All types of updates are applied:
   • Feature updates
   • Quality updates
   • Security updates
   • Updates to components like .NET and Defender that require reboots.
   • All changes from the previous quarter's hot patches are rolled into the base files.
2. Second and Third Months of the Quarter (Hot Patch Only): In these months (e.g., February, March, May, June), only security hot patches are delivered. No reboot is required. Feature and quality updates that would normally be released in these months are deferred and rolled into the baseline update of the next quarter.

This quarterly baseline reboot ensures that all components are eventually updated and reloaded cleanly, while the intervening months benefit from reboot-less security updates.

How Hot Patching Works (Technical Details)

The "magic" behind hot patching involves specific file structures and in-memory modifications:

1. Special Hotfix Files: For months two and three of a quarter, special _hotfix files are delivered (e.g., a_hotpatch.exe). These files contain only the updated functions with security fixes.
2. Updated Functions: Instead of replacing the entire executable, the hot patch file contains only the new or modified functions (e.g., FunctionTwo with security fixes).
3. Metadata for Plumbing: Each hot patch file includes a section of metadata at its beginning. This metadata acts as "plumbing" instructions, detailing how to modify existing running processes in memory to use the new functions.
4. Forward Attaches: If an existing function (e.g., FunctionOne) normally calls another function (e.g., FunctionTwo), and FunctionTwo has been updated in the hot patch, a forward attach modifies the in-memory call instruction of FunctionOne to now point to and call the newly loaded, updated version of FunctionTwo.
5. Reverse Attaches: If the newly updated function (e.g., FunctionTwo from the hot patch) needs to interact with other existing functions (e.g., FunctionThree) or use global variables and handles from the original process, a reverse attach mechanism allows it to do so. This ensures the updated function can seamlessly integrate with the existing process context.
6. In-Memory Injection: The key is that the process itself is not unloaded from memory. Instead, the hot patch injects these new plumbing instructions (forward and reverse attaches) into the running process, redirecting calls and enabling access to necessary resources without interruption.
7. Cumulative Nature: Hot patches are cumulative. If FunctionTwo is updated in month two and FunctionThree in month three, the month three hot patch will contain both the updated FunctionTwo and FunctionThree, along with updated metadata to re-plumb any previous forward attaches to the latest cumulative hot patch.

History and Availability

Hot patching technology originated in Azure to allow Azure hosts to update without interrupting customer VMs and containers, thereby improving Azure's security posture faster.

Currently, hot patching is available in several scenarios:

• Azure Environments:

  • Windows Server 2022 and 2025 Data Center Azure Edition: When deploying these OS versions in Azure, users can enable hot patching via guest OS update options or by selecting a dash hotpatch version of the image SKU name (e.g., via CLI).
  • Windows 365 Enterprise: For hosted desktop environments.
  • Azure Virtual Desktop: For virtual desktop infrastructure.

• Non-Azure OS Scenarios (On-premises/Hybrid):

  • Windows Server 2025 Standard and Data Center Editions (Arc-enabled): For on-premises servers, hot patching requires Azure Arc for Servers to be enabled. Additionally, the Azure Update Manager must be used, which incurs a per-server, per-core charge for the hot patch feature.

• Client Platform:

  • Windows 11 Enterprise or Education (24H2 and above, including 25H2): Hot patching is available for devices managed by Intune. As part of the Intune Autopatch configuration or quality update policy, there is an option to enable hot patching, provided the necessary licenses/SKUs are in place.

Synthesis/Conclusion

Hot patching represents a significant advancement in how security updates are delivered, addressing the long-standing challenge of reboots impacting critical workloads and user productivity. By enabling reboot-less deployment of security fixes for two out of three months in a quarter, it allows organizations to accelerate their security posture improvements while maintaining high availability. While a quarterly baseline reboot is still necessary to incorporate all types of updates and refresh the system, the intervening hot patches provide a seamless, in-memory update mechanism through sophisticated "forward" and "reverse attaches." This technology, initially developed for Azure's infrastructure, is now extending its benefits to on-premises servers via Azure Arc and to Windows 11 client devices managed by Intune, offering a powerful tool for modern patch management.