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

KB5103215

KB5103215: Security update for Exchange Server 2016 CU23, July 2026. Critical mail-server spoofing flaw addressed

In short

Resolves 11 vulnerabilities in Exchange Server 2016 CU23, including one critical spoofing vulnerability. It is available only to organizations enrolled in the Extended Security Update program.

Does it affect you? Applies to: Exchange Server 2016 CU23 organizations enrolled in the Extended Security Update (ESU) program. Exchange Server 2016 has reached end of support, and this update is available only to ESU participants.

Build
15.1.2507.71
Servicing branch
CU23
Security
11 fixed · 1 critical

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

  • The update resolves security vulnerabilities in Exchange Server.1
    The KB lists CVE-2026-55005 (remote code execution), CVE-2026-55006 and CVE-2026-55009 (elevation of privilege), and CVE-2026-55008 (spoofing).

Technical details

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

Data sheet

Released
2026-07-14
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU23
Replaces
KB5094144
Replaced by
KB5121576
VersionBuildChannel
2016 15.1.2507.71 General availability

Security

11 vulnerabilities fixed
1 critical 10 important
VulnerabilitySeverityCVSS
CVE-2026-55008
Microsoft Exchange Server Spoofing Vulnerability
 Critical 9.6
CVE-2026-45504
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.8
CVE-2026-55005
Microsoft Exchange Server Remote Code Execution Vulnerability
 Important 8.8
CVE-2026-45503
Microsoft Exchange Server Information Disclosure Vulnerability
 Important 8.1
CVE-2026-47631
Microsoft Exchange Server Spoofing Vulnerability
 Important 8.1
CVE-2026-55006
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.8
CVE-2026-55009
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.8
CVE-2026-45583
Microsoft Exchange Server Remote Code Execution Vulnerability
 Important 7.5
CVE-2026-45501
Microsoft Exchange Server Spoofing Vulnerability
 Important 6.5
CVE-2026-45500
Microsoft Exchange Server Spoofing Vulnerability
 Important 6.1
CVE-2026-45502
Microsoft Exchange Server Information Disclosure Vulnerability
 Important 5.0

Updated components

1,350 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell131
Transport94
Client access (POP, IMAP, ActiveSync)33
Mailbox replication33
Search27
Compliance and eDiscovery25
Exchange Web Services and Autodiscover17
Outlook on the web9
Setup8

Sources

Updated
Cited passages from Microsoft's official notes (1)
  1. 1. This security update resolves vulnerabilities in Microsoft Exchange Server. 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 4/5, 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