Windows Server guidance to protect against L1 terminal fault

Applies to: Windows Server version 1803Windows Server 2016 Version 1709Windows Server 2016 More


Microsoft is aware of a new variant of the class of attack known as speculative execution side-channel vulnerabilities. The new variant is named L1 Terminal Fault (L1TF). An attacker who can successfully exploit L1TF may be able to read privileged data across trust boundaries.

Vulnerability overview

In environments in which resources are shared, such as virtualization hosts, an attacker who can run arbitrary code on one virtual machine may be able to access information from another virtual machine or from the virtualization host itself.

Workloads such as Windows Server Remote Desktop Services (RDS) and more dedicated workloads such as Active Directory domain controllers are also at risk. Attackers who can run arbitrary code (regardless of its level of privilege) may be able to access operating system or workload secrets such as encryption keys, passwords, and other sensitive data.

Note This vulnerability affects Intel Core processors and Intel Xeon processors only.

Mitigation overview

To resolve these issues, Microsoft is working together with Intel to develop software mitigations and guidance. Software updates to help mitigate the vulnerabilities have been released. To obtain all available protections, updates may be required that could also include microcode from device OEMs.

This article describes how to mitigate the following vulnerabilities:

  • CVE-2018-3620 – "L1 Terminal Fault – OS, SMM"
  • CVE-2018-3646 – "L1 Terminal Fault – VMM"

To learn more about the L1TF vulnerability see the following security advisory:

Determining actions necessary to mitigate the threat

The following sections can help you identify systems that are affected by the L1TF vulnerabilities, and also help you to understand and mitigate the risks.

Potential performance impact

In testing, Microsoft has seen some performance impact from these mitigations, depending on the configuration of the system and which mitigations are required.

Some customers may have to disable hyper-threading to fully address the risk from L1TF. Be aware that disabling hyper-threading can cause performance degradation. This situation applies to customers who use the following:

The impact can vary by hardware and the workloads that are running on the system. The most common server configuration is to have hyper-threading enabled. Therefore, the performance impact is gated on the user or administrator who is taking the action to disable hyper-threading on the system.

Note To determine whether your system is using VBS-protected security features, follow these steps:

  1. On the Start menu, type MSINFO32.

    Note The System Information window opens.
  2. In the Find what box, type security.
  3. In the right pane, locate the two rows that are selected in the screen shot, and check the Value column to see whether Virtualization-based Security is enabled and which virtualized-based security services are running.

    System Info window

The Windows Server 2016 and Windows Server 2019 Hyper-V core scheduler mitigates the L1TF attack vector against Hyper-V virtual machines while still allowing hyper-threading to remain enabled. This provides minimal performance impact.

The core scheduler does not mitigate the L1TF attack vector against VBS-protected security features. For more information, refer to Mitigation C and the following Virtualization Blog article:

For detailed information from Intel about the performance impact, go to the following Intel website:

Identifying affected systems and required mitigations

The flow chart in figure 1 can help you identify affected systems and determine the correct set of actions. 

Important You must consider and apply the flow chart to Hyper-V hosts and each affected VM guest individually because mitigations may apply to both. Specifically, for a Hyper-V host, the flow chart steps provide inter-VM protections and intra-host protections. However, applying these mitigations to only the Hyper-V host is not sufficient to provide intra-VM protection. To provide intra-VM protection, you must apply the flow chart to each Windows VM. In most cases, this means making sure that the registry keys are set in the VM.

As you navigate the flow chart, you will encounter lettered blue circles that map to an action or a series of actions that are required to mitigate L1TF attack vectors that are specific to your system configurations. Each action that you encounter has to be applied. When you encounter a green line, it indicates a direct path to the end, and there are no additional mitigation steps. 

A short-form explanation of each lettered mitigation is included in the legend on the right. Detailed explanations for each mitigation that include step-by-step installation and configuration instructions are provided in the "Mitigations" section.



Important The following section describes mitigations that should be applied ONLY under the specific conditions that are determined by the flow chart in Figure 1 in the previous section. Do NOT apply these mitigations unless the flowchart indicates that the specific mitigation is necessary.

In addition to software and microcode updates, manual configuration changes may also be required to enable certain protections. We further recommend that Enterprise customers register for the security notifications mailer in order to be alerted about content changes. (See Microsoft Technical Security Notifications.)

Mitigation A

Obtain and apply the latest Windows updates

Apply all available Windows operating system updates, including the monthly Windows security updates. You can see the table of affected products at

Mitigation B

Obtain and apply the latest microcode or firmware updates

In addition to installing the latest Windows security updates, a processor microcode update is also required. Installation of these updates is provided by the device OEM.

Note If you’re using nested virtualization (including running Hyper-V containers in a guest VM), you must expose the new microcode enlightenments to the guest VM. This might require upgrading the VM configuration to version 8. Version 8 includes the microcode enlightenments by default. For more information and the required steps, see the following article Microsoft Docs article:

Run Hyper-V in a Virtual Machine with Nested Virtualization

Mitigation C

Should I disable hyper-threading (HT)?

The L1TF vulnerability introduces risk that the confidentiality of Hyper-V virtual machines and the secrets that are maintained by Microsoft Virtualization Based Security (VBS) could be compromised by using a side-channel attack. When hyper-threading (HT) is enabled, the security boundaries provided by both Hyper-V and VBS are weakened.

The Windows Server 2016 and Windows Server 2019 Hyper-V core scheduler mitigates the L1TF attack vector against Hyper-V virtual machines while still allowing hyper-threading to remain enabled. This provides minimal performance impact.

The Windows Server 2016 and Windows Server 2019 Hyper-V core scheduler does not mitigate the L1TF attack vector against VBS-protected security features. The L1TF vulnerability introduces risk that the confidentiality of VBS secrets could be compromised via a side-channel attack when Hyper-Threading (HT) is enabled, weakening the security boundary provided by VBS. Even with this increased risk, VBS still provides valuable security benefits and mitigates a range of attacks with HT enabled. Hence, we recommend that VBS continue to be used on HT-enabled systems. Customers who want to eliminate the potential risk of the L1TF vulnerability on the confidentiality of VBS should consider disabling HT to mitigate this additional risk. 

Customers who want to eliminate the risk that the L1TF vulnerability poses, whether to the confidentiality of Hyper-V versions that are earlier than Windows Server 2016 or to VBS security capabilities, must weigh the decision and consider disabling HT to mitigate the risk. In general, this decision can be based upon the following guidelines:

The steps that are required to disable HT differ from OEM to OEM. However, they are typically part of the BIOS or firmware setup and configuration tools.

Mitigation D

Enable Hyper-V core scheduler and set the VM hardware thread count per core to 2

Note These mitigation steps apply only to Windows Server 2016.  The core scheduler is enabled by default on Windows Server 2019.

Using the core scheduler is a two-stage process that requires you to first enable the scheduler on the Hyper-V host and then configure each VM to take advantage of it by setting their hardware thread count per core to two (2).

The Hyper-V core scheduler that was introduced in Windows Server 2016 is a new alternative to the classic scheduler logic. The core scheduler offers decreased performance variability for workloads inside VMs that are running on an HT-enabled Hyper-V host. 

For a detailed explanation of Hyper-V’s core scheduler and the steps to enable it, see the following Windows IT Pro Center article:

Understanding and using Hyper-V hypervisor scheduler types

To enable the Hyper-V core scheduler on Windows Server 2016, enter the following command:

bcdedit /set HypervisorSchedulerType core

Next, decide whether to configure a given VM’s hardware thread count per core to two (2). If you expose the fact that virtual processors are hyper-threaded to a guest virtual machine, you enable the scheduler in the VM operating system, and also the VM workloads, to use HT in their own work scheduling. To do this, enter the following PowerShell command, in which <VMName> is the name of the virtual machine:

Set-VMProcessor -VMName <VMName> -HwThreadCountPerCore 2

Mitigation E

Enable mitigations for advisories CVE-2017-5715 and CVE-2017-5754

Note These mitigations are enabled by default on Windows Server 2019.

To enable mitigations for advisories CVE-2017-5715 and CVE-2017-5754, use the guidance in the following Office article:

Windows Server guidance to protect against speculative execution side-channel vulnerabilities

Note These mitigations include and automatically enable the safe page frame bits mitigation for the Windows kernel and also for the mitigations that are described in CVE-2018-3620. For a detailed explanation of the safe page frame bits mitigation, see the following Security Research & Defense Blog article:

Analysis and mitigation of L1 Terminal Fault (L1TF)