https://azure.microsoft.com/en-us/blog/securing-azure-customers-from-cpu-vulnerability/
An industry-wide, hardware-based security vulnerability was disclosed yesterday . 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.