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

KB5071873

KB5071873: Security update for Exchange Server 2016 CU23, December 2025. Privilege escalation and spoofing flaws addressed

In short

Resolves two important security flaws: one could give an attacker more access, and the other could let an attacker fake an identity.

Does it affect you? Applies to: Exchange Server 2016 CU23 installations at organizations enrolled in the Extended Security Update (ESU) program. Exchange Server 2016 has reached end of support.

Build
15.1.2507.63
Servicing branch
CU23
Security
2 fixed
Review notes Additional details from the official release notes (1) Read note
  • What to do Microsoft says to follow the instructions in the MSRC articles for these vulnerabilities to address them.

The official sources are linked at the end of the page.

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

Fixes 2 changes

Exchange Server

2 items
  • A flaw that could give an attacker more access is fixed.1
    CVE-2025-64666 — Microsoft Exchange Server Elevation of Privilege Vulnerability.
  • A flaw that could let an attacker fake an identity is fixed.2
    CVE-2025-64667 — Microsoft Exchange Server Spoofing Vulnerability.

Technical details

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

Data sheet

Released
2025-12-09
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU23
Replaces
KB5066369
Replaced by
KB5074995
VersionBuildChannel
2016 15.1.2507.63 General availability

Security

2 vulnerabilities fixed
2 important
VulnerabilitySeverityCVSS
CVE-2025-64666
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.5
CVE-2025-64667
Microsoft Exchange Server Spoofing Vulnerability
 Important 5.3

Updated components

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

Top changed componentsFiles
Management and PowerShell143
Transport94
Client access (POP, IMAP, ActiveSync)33
Mailbox replication33
Search27
Compliance and eDiscovery25
Exchange Web Services and Autodiscover17
Outlook on the web8
Setup8

Sources

Updated
Cited passages from Microsoft's official notes (2)
  1. 1. CVE-2025-64666 - Microsoft Exchange Server Elevation of Privilege Vulnerability support.microsoft.com
  2. 2. CVE-2025-64667 - Microsoft Exchange Server Spoofing Vulnerability support.microsoft.com
Checked automatically: 8 of 8 checks passed, 2 quoted passages · 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 4/5, clarity 4/5, completeness 4/5. 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