If you run any virtual machines in Azure, you should recently have received an email with the subject Accelerated maintenance to address CPU vulnerability in your subscription.
This is part of Microsoft’s response to the recent Meltdown and Spectre CPU vulnerabilities.
Other cloud providers, including Amazon Web Services and Google Compute Platform, are affected as well and are rolling out similar responses to the vulnerabilities.
What's the Plan: Cascading Reboots
Microsoft has already deployed fixes behind the scenes for its other Azure services, but because the fix for virtual machines requires a reboot, Microsoft has sent you that notice to let you know a reboot of your Azure VM is coming.
If you have multiple VMs in a redundant configuration within an Availability Set or VM scale set, the VMs will be rebooted at different times so that your overall service remains available. Individual VMs in non-redundant configurations will experience a brief downtime.
The Meltdown vulnerability is currently only known to affect Intel processors, while Spectre is known to affect almost all modern CPUs, including processors by ARM and AMD.
In addition to cloud computing platforms, these vulnerabilities can affect desktops, laptops, and smartphones.
Stick to the Plan: Installing Security Updates
If you are running any systems outside of Azure, it is important that you install security updates to address this vulnerability as soon as possible.
For more information on these vulnerabilities, see meltdownattack.com.