The Exchange product group released the December 2025 Security Update for Exchange Server SE. Organizations that enrolled in the Extended Security Update program will also have access to December 2025 security updates for Exchange Server 2019 and Exchange Server 2016. These ESU updates will not be made available publicly.

The vulnerabilities addressed in these Security Updates for Exchange Server are:

Vulnerability	Category	Severity	Rating
CVE-2025-64666	Elevation of Privilege	Important	CVSS:3.1 7.5 / 6.5
CVE-2025-64667	Spoofing	Important	CVSS:3.1 5.3 / 4.6

The Security Updates for each supported Exchange Server build are linked below:

Exchange	SU/HU	Download	Build	KB	Supersedes
Exchange SE	4	Download	15.2.2562.29	KB5071876	KB5066366
Exchange 2019 CU15	6	ESU Program	15.2.1748.42	KB5071875	KB5066367
Exchange 2019 CU14	9	ESU Program	15.2.1544.37	KB5071874	KB5066368
Exchange 2016 CU23	20	ESU Program	15.1.2507.63	KB5071873	KB5066369

Fixed Issues

The issue addressed in these hotfixes is:

• Skype for Business OWA integration fails after enabling the Dedicated Hybrid Application

Notes

• Security updates are Cumulative Update level specific. You cannot apply the Exchange 2019 CU15 update to Exchange 2019 CU14. When downloading, the security update might carry the same name for different Cumulative Updates. Nowadays, Microsoft adds the KB article number as a reference, but I would still tag the file name with the CU level for archival purposes, e.g., Exchange2019-CU15-KBxxxxxxx-x64-en.exe.
• Like Cumulative Updates, Security Updates are cumulative, and you only need to install the latest SU for your CU.
• Suppose you have deployed Exchange Management Tools to manage your on-premises Exchange Servers or installed the tools after removing the Last Exchange Server for recipient management. In that case, it is recommended that you apply the Security Update.

On a final note, as with any patch or update, it is recommended that you apply it in a test environment before implementing it in production. However, it is not recommended to wait for regular maintenance cycles when it comes to security updates; a more agile approach is preferable, and the ratings indicate the level of urgency.