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Exchange Server 2016 CU23 Exchange Server 2016 CU23 ESU only Security update
Patch Tuesday

KB5066369

KB5066369: Security update for Exchange Server 2016 CU23, October 2025. Three vulnerabilities addressed; Auth Certificate export issue fixed

In short

The update addresses three Exchange Server vulnerabilities and an Auth Certificate export issue. It is available only to organizations in the Extended Security Update program.

Does it affect you? Applies to: Exchange Server 2016 CU23 devices. Because Exchange Server 2016 is out of support, this update is available only to organizations enrolled in the Extended Security Update program.

Build
15.1.2507.61
Servicing branch
CU23
Security
3 fixed

For servers running Cumulative Update 23: each cumulative update gets its own update. Support for this version ended on October 14, 2025: Microsoft releases this update only to organizations enrolled in the Extended Security Update (ESU) program, not through Microsoft Update. The others should move to Exchange Server Subscription Edition. Which version am I on?

What changes

Fix 1 change

Auth Certificate export

  • The Auth Certificate export issue involving an Exchange cmdlet is addressed; export through that cmdlet is no longer supported.1

Technical details

Data sheet · Security · Downloads · Components Click to collapse or expand

Data sheet

Released
2025-10-14
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU23
Replaces
KB5066370, KB5063223
Replaced by
KB5071873
VersionBuildChannel
2016 15.1.2507.61 General availability

Security

3 vulnerabilities fixed
3 important
VulnerabilitySeverityCVSS
CVE-2025-59249
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.8
CVE-2025-53782
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.4
CVE-2025-59248
Microsoft Exchange Server Spoofing Vulnerability
 Important 7.5

Updated components

1,037 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell140
Transport94
Client access (POP, IMAP, ActiveSync)33
Mailbox replication33
Search27
Compliance and eDiscovery26
Exchange Web Services and Autodiscover17
Outlook on the web8
Setup8

Sources

Updated
Cited passages from Microsoft's official notes (1)
  1. 1. Auth Certificate export through Exchange cmdlet is no longer supported support.microsoft.com
Checked automatically: 8 of 8 checks passed, 1 quoted passage · compared with Microsoft's page on Oct 20, 2026
  • Quotes found in the official pages: passed
  • Numbers found in the sources: passed
  • Vulnerabilities from the dossier only: passed
  • Links to allowed domains only: passed
  • Every official change covered: passed
  • Readable plain text: passed
  • Changes confirmed by the file list: passed
  • Sources agree: passed

A second AI read the text against the sources: correctness 5/5, clarity 5/5, completeness 5/5. How we work

Published by The Update Bureau. Written with AI from Microsoft's own documents, checked by rules and a second AI. How we work · Report a mistake