November 12, 2019 update: Please refer to the guidance for mitigating speculative execution side-channel vulnerabilities here.

An industry-wide, hardware-based security vulnerability was disclosed today. Keeping customers secure is always our top priority and we are taking active steps to ensure that no Azure customer is exposed to these vulnerabilities. At the time of this blog post, Microsoft has not received any information to indicate that these vulnerabilities have been used to attack Azure customers.

The majority of Azure infrastructure has already been updated to address this vulnerability. Some aspects of Azure are still being updated and require a reboot of customer VMs for the security update to take effect. Many of you have received notification in recent weeks of a planned maintenance on Azure and have already rebooted your VMs to apply the fix, and no further action by you is required.

With the public disclosure of the security vulnerability today, we are accelerating the planned maintenance timing and will begin automatically rebooting the remaining impacted VMs starting at 3:30pm PST on January 3, 2018. The self-service maintenance window that was available for some customers has now ended, in order to begin this accelerated update.

During this update, we will maintain our SLA commitments of Availability Sets, VM Scale Sets, and Cloud Services. This reduces impact to availability and only reboots a subset of your VMs at any given time. This ensures that any solution that follows Azure’s high availability guidance remains available to your customers and users. Operating system and Data disks on your VM will be retained during this maintenance. You can see the status of your VMs and if the reboot completed within the Azure Service Health Planned Maintenance Section in your Azure Portal.

The majority of Azure customers should not see a noticeable performance impact with this update. We’ve worked to optimize the CPU and disk I/O path and are not seeing noticeable performance impact after the fix has been applied. A small set of customers may experience some networking performance impact. This can be addressed by turning on Azure Accelerated Networking (Windows, Linux), which is a free capability available to all Azure customers. We will continue to monitor performance closely and address customer feedback.

This Azure infrastructure update addresses the disclosed vulnerability at the hypervisor level and does not require an update to your Windows or Linux VM images. However, as always, you should continue to apply security best practices for your VM images.

Please consult with the vendor of your operating systems for updates and instructions, as needed. For Windows Server VM customers, guidance has now been published and is available here.