Microsoft has released Security Updates (SUs) for vulnerabilities found in:

• Exchange Server Subscription Edition (SE)
• Exchange Server 2019
• Exchange Server 2016

SUs are available for the following specific versions of Exchange Server:

• Exchange SE RTM
• Exchange Server 2019 CU14 and CU15 (to access, organization must be enrolled into the Period 2 ESU program)
• Exchange Server 2016 CU23 (to access, organization must be enrolled into the Period 2 ESU program)

The October 2026 SUs address vulnerabilities responsibly reported to Microsoft by security partners and found through Microsoft’s internal processes.

These vulnerabilities affect Exchange Server. Exchange Online customers are already protected from the vulnerabilities addressed by these SUs and do not need to take any action other than updating any Exchange servers or Exchange Management tools workstations in their environment.

More details about specific CVEs can be found in the Security Update Guide (filter on ‘Server Software’ under Product Family for Exchange SE and ‘ESU’ under Product Family for Exchange 2016 and 2019).

Exchange 2016 and 2019 Period 2 ESU program ends

Exchange Server 2016 and 2019 are out of support for a full year now. October 2026 is the last month when customers who enrolled in the Period 2 Extended Security Update (ESU) program are eligible to receive newly released Exchange Server 2016 and 2019 security updates.

After October 2026:

• Updates will be released for Exchange Subscription Edition (SE) only.
• Our update announcements and any CVEs released by MSRC will stop mentioning Exchange 2016 and 2019. This does not mean that those versions are not vulnerable to future security issues. We will stop testing Exchange 2016 and 2019 for vulnerabilities.
• For a limited time, the ability to purchase Period 2 ESU will remain in effect, but that will only get you current with security updates through this October 2026 release.

Please continue upgrading your organizations to Exchange SE.

Issues resolved in this release

• Published calendar (.ics) returns HTTP 500 for calendar applications | Microsoft Support
• ContentEngine deadlock because of missing Korean WordBreaker rule files | Microsoft Support

Update installation

The following update paths are available:

• Inventory your Exchange Servers to determine which updates are needed using the Exchange Server Health Checker script. Running this script will tell you if any of your Exchange Servers are behind on updates (CUs, SUs, or manual actions).
• Install the latest CU. Use the Exchange Update Wizard to choose your current CU and your target CU to get directions.
• After setup is completed, please reboot the server and check that all Exchange services have started properly. If some services are in a disabled state, that indicates that something interrupted installation of the update. Please see the Workaround 1 in this article.
• If you encounter errors during or after installation of Exchange Server, run the SetupAssist script. If something does not work properly after updates, see Repair failed installations of Exchange Cumulative and Security updates. Also please see File version error when you try to install Exchange Server updates.

FAQs

Our organization is in Hybrid mode with Exchange Online. Do we need to do anything?
Exchange Online is already protected, but this SU needs to be installed on your Exchange servers, even if they are used only for management purposes. If you change the auth certificate after installing an SU, you should re-run the Hybrid Configuration Wizard.

The last SU/HU we installed is a few months old. Do we need to install all SUs in order to install the latest one?
SUs are cumulative. If you are running a CU supported by the SU, you do not need to install all SUs or HUs in sequential order; simply install the latest SU. Please see this blog post for more information.

Do we need to install SUs on all Exchange Servers within our organization? What about ‘Management Tools only’ machines?
Our recommendation is to install SUs on all Exchange Servers and all servers and workstations running the Exchange Management Tools to ensure compatibility between management tools clients and servers. If you are trying to update the Exchange Management Tools in the environment with no running Exchange servers, please see this.

We installed an Exchange SU or HU update and are running Windows Server 2025. How do we uninstall updates as they are not listed on Windows Server 2025?
We do not recommend uninstalling Security Updates. But see Can't view or uninstall Exchange security or hotfix updates in Control Panel on Windows Server 2025 | Microsoft Support.

Our organization does not have the Exchange 2016 and 2019 Period 2 ESU. How can we get current Exchange 2016 or 2019 updates?
Since Exchange 2016 and 2019 are now out of support, only customers who have enrolled into the Period 2 ESU program (which is valid between May and October 2026) can obtain Exchange 2016 or 2019 updates released after May 2026. For all other customers still running Exchange 2016 or 2019, we recommend that you upgrade your organization to Exchange SE as soon as possible.

Documentation may not be fully available at the time this post is published.

This post might receive future updates; they will be listed here (if available).

The Exchange Server Team