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Exchange Server 2019 CU15 Exchange Server 2019 CU15 ESU only Security update
Patch Tuesday

KB5074993

KB5074993: Security update for Exchange Server 2019 CU15, February 2026. Mail-server spoofing flaw addressed; ESU enrollment required

In short

The update resolves CVE-2026-21527, an Exchange Server spoofing vulnerability. Only organizations enrolled in the Extended Security Update program are eligible to receive it.

Does it affect you? Applies to: Exchange Server 2019 CU15 organizations enrolled in the Extended Security Update (ESU) program. This update installs build 15.2.1748.43.

Build
15.2.1748.43
Servicing branch
CU15
Security
1 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

For: Exchange Server 2019 CU15 administrators

  • The Exchange Server spoofing vulnerability CVE-2026-21527 is resolved.1
    Resolves CVE-2026-21527, a Microsoft Exchange Server Spoofing Vulnerability.

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
CU15
Replaces
KB5071875
Replaced by
KB5094140
VersionBuildChannel
2019 15.2.1748.43 General availability

Security

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

Updated components

1,004 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell119
Transport93
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 (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