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Exchange Server 2019 CU15 Exchange Server 2019 CU15 ESU only Security update
Patch Tuesday Final update for this version

KB5128896

KB5128896: Security update for Exchange Server 2019 CU15, October 2026. Critical remote-code flaw patched; calendar feeds restored

In short

Fixes a critical Exchange Server remote-code execution vulnerability and restores published calendar subscriptions that returned HTTP 500 errors.

Does it affect you? Applies to: Organizations enrolled in the Period 2 Extended Security Update (ESU) program that run Exchange Server 2019 CU15.

Build
15.2.1748.55
Servicing branch
CU15
Security
6 fixed · 1 critical

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

Fixes 2 changes

Security

For: Organizations enrolled in the Period 2 ESU program

  • Exchange Server is protected against six security vulnerabilities, including a critical flaw that could allow remote code execution.1
    The update includes build 15.2.1748.55 and addresses CVE-2026-70571, CVE-2026-77910, CVE-2026-87692, CVE-2026-87695, CVE-2026-87696, and CVE-2026-70279.

Published calendars

For: Organizations using published calendars

  • Subscriptions to published calendars work again instead of returning HTTP 500 errors.2
    Resolves the issue with anonymously published Exchange calendars in .ics format.

Technical details

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

Data sheet

Released
2026-10-13
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU15
Replaces
KB5129956
VersionBuildChannel
2019 15.2.1748.55 General availability

Security

6 vulnerabilities fixed
1 critical 4 important 1 not rated
VulnerabilitySeverityCVSS
CVE-2026-70571
Microsoft Exchange Server Remote Code Execution Vulnerability
 Critical 8.1
CVE-2026-70279
Microsoft Exchange Server Spoofing Vulnerability
 Important 9.3
CVE-2026-77910
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.8
CVE-2026-87695
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.8
CVE-2026-87696
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.5
CVE-2026-87692
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Not rated —

Updated components

1,019 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell130
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 (2)
  1. 1. This security update resolves vulnerabilities in Microsoft Exchange Server. support.microsoft.com
  2. 2. - Published calendar (.ics) returns HTTP 500 for calendar applications support.microsoft.com
Checked automatically: 7 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: not 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