Skip to the content
The Update Bureau
Exchange Server 2019 CU15 Exchange Server 2019 CU15 ESU only Security update
Patch Tuesday

KB5071875

KB5071875: Security update for Exchange Server 2019 CU15, December 2025. Mail-server elevation-of-privilege and spoofing flaws patched

In short

The update resolves two Exchange Server 2019 CU15 security vulnerabilities: an elevation-of-privilege flaw and a spoofing flaw. Only organizations enrolled in the Extended Security Update program are eligible.

Does it affect you? Applies to: Organizations running Exchange Server 2019 CU15 that are enrolled in the Extended Security Update (ESU) program are eligible for this update.

Build
15.2.1748.42
Servicing branch
CU15
Security
2 fixed

For servers running Cumulative Update 15: 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 an elevation-of-privilege vulnerability and a spoofing vulnerability in Exchange Server.1
    Addresses CVE-2025-64666 and CVE-2025-64667.

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
CU15
Replaces
KB5066367
Replaced by
KB5074993
VersionBuildChannel
2019 15.2.1748.42 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 PowerShell147
Transport93
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. CVE-2025-64666 - Microsoft Exchange Server Elevation of Privilege Vulnerability CVE-2025-64667 - Microsoft Exchange Server Spoofing Vulnerability 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 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