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Hi, When I upgrade Exchange 2016 CU21 to CU22, I have error because KB2999226 is not installed. 
But Windows 2016 Server is not applied to this KB2999226.
Any idea for my upgrade ?
Thks
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Could you please check if there is any detailed information recorded in Exchange setup log? According to the official document we only see the prerequisites for Exchange Emergency Mitigation (EM) service
IIS URL Rewrite Module
Universal C Runtime in Windows (KB2999226) for Windows Server 2012 and Windows Server 2012 R2
In addition, check for Windows Updates in your Windows Server 2016
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
I may take a while to test whether your problem can be reproduced in my environment, you could also share your issue or feedback below this official link: New security feature in September 2021 Cumulative Update for Exchange Server
I have updated Exchange 2016 on Windows 2016 and this update was not required. Sounds crazy but are you sure you are running W2016?
My colleague did indeed use the wrong OS. he confused Exchange 2016 and Windows 2016 except that it is a Windows 2012.
Thks for help