Microsoft released several Security Updates (SUs) for Microsoft Exchange Server to address vulnerabilities. Due to the critical nature of these vulnerabilities, we recommend that customers apply the Dcember 2025 Exchange Server Security Updates to affected systems immediately to protect the environment.
Note: These vulnerabilities affect Microsoft Exchange Server. Exchange Online is not affected.
Exchange Server Security Updates
Microsoft has released Security Updates for vulnerabilities found in:
- Exchange Server 2016
- Exchange Server 2019
- Exchange Server Subscription Edition (SE)
These Security Updates are available for the following specific versions of Exchange:
- Exchange Server 2016 (CU23) > available in the Extended Security Update (ESU) program
- Exchange Server 2019 (CU14, CU15) > available in the Extended Security Update (ESU) program
- Exchange Server Subscription Edition (SE) RTM
Read more on how to Install Exchange Security Update.
If you are not at these Exchange Server CU versions, please update right now and apply the above patch.
Read more on how to Install Exchange Cumulative Update.
Issues that are fixed in this update
The following issues have been resolved in this update:
FAQs
How does this SU relate to Extended Protection feature?
If you already enabled Extended Protection on your servers, install the SU as usual. If you did not enable Extended Protection yet, our recommendation is to enable it after installing January (or any later) SU. Running Health Checker script will always help you validate exactly what you might need to do after SU installation.
Is Windows Extended Protection a prerequisite that needs to be activated before or after applying the SU, or is that an optional but strongly recommended activity?
Extended Protection is not a prerequisite for this Security Update. You can install it without having to activate the Extended Protection feature. However, configuring Extended Protection is strongly recommended, which can help you protect your environments from authentication relay or “Man in the Middle” (MITM) attacks.
The last SU that we installed is (a few months old). Do we need to install all SUs in order, to install the latest one?
The Exchange Server Security Updates are cumulative. If you are running the CU that the SU can be installed on, you do not need to install all the SUs in sequential order but can install the latest SU only.
My organization is in Hybrid mode with Exchange Online. Do I need to do anything?
While Exchange Online customers are already protected, the December 2025 Security Update needs to be installed on your on-premises Exchange Servers, even if they are used only for management purposes. You do not need to re-run the Hybrid Configuration Wizard (HCW) after applying updates.
Do I need to install the updates on “Exchange Management Tools only” workstations?
Install Security Updates on all Exchange Servers as well as servers or workstations running Exchange Management Tools only, which will ensure that there is no incompatibility between management tools clients and servers.