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

KB5074995

KB5074995: Security update for Exchange Server 2016 CU23, February 2026. Mail-server spoofing vulnerability addressed; ESU coverage ends in October

In short

Addresses an Important spoofing vulnerability in Exchange Server 2016. It is available only to organizations enrolled in the Extended Security Update program, which provides security updates through October 31, 2026.

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

Build
15.1.2507.66
Servicing branch
CU23
Security
1 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

Exchange Server

For: business

  • A spoofing vulnerability in Exchange Server is addressed.1
    Addresses CVE-2026-21527.

Technical details

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

Data sheet

Released
2026-02-10
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU23
Replaces
KB5071873
Replaced by
KB5094144
VersionBuildChannel
2016 15.1.2507.66 General availability

Security

1 vulnerability fixed
1 important
VulnerabilitySeverityCVSS
CVE-2026-21527
Microsoft Exchange Server Spoofing Vulnerability
 Important 6.5

Updated components

1,027 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell133
Transport94
Client access (POP, IMAP, ActiveSync)33
Mailbox replication33
Search27
Compliance and eDiscovery26
Exchange Web Services and Autodiscover17
Outlook on the web9
Setup8

Sources

Updated
Cited passages from Microsoft's official notes (1)
  1. 1. CVE-2026-21527 - Microsoft Exchange Server Spoofing Vulnerability support.microsoft.com
Checked automatically: 8 of 8 checks passed, 1 quoted passage · compared with Microsoft's page on Sep 26, 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 4/5, 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