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Exchange Server Subscription Edition RTM Exchange Server Subscription Edition RTM Security update
Patch Tuesday

KB5128895

KB5128895: Security update for Exchange Server Subscription Edition RTM, October 2026. Critical remote-code-execution flaw patched; calendar feeds restored

In short

The update fixes a Critical remote-code-execution vulnerability and restores published calendar feeds and Korean-language search processing affected by earlier updates.

Does it affect you? Applies to: Exchange Server Subscription Edition RTM deployments. The update package is for x64.

Build
15.2.2562.56
Servicing branch
RTM
Security
6 fixed · 1 critical

What changes

Fixes 2 changes

Published calendar (.ics)

  • Subscriptions to anonymously published calendars refresh again instead of returning an HTTP 500 error.1
    Resolves the failure of calendar application subscriptions to anonymously published Exchange calendars.

ContentEngine

Verified in package

For: Exchange administrators processing Korean-language email

  • Processing Korean-language email no longer triggers the deadlock described in the issue, which could cause missing search results, delayed delivery, or unresponsive or disconnected clients.2
    Resolves the deadlock associated with missing assembly files for the updated Korean WordBreaker.

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
RTM
Replaces
KB5129955, KB5121608
Replaced by
KB5133453
VersionBuildChannel
SE 15.2.2562.56 General availability

Security

6 vulnerabilities fixed
1 critical 5 important
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-2026-87692
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.8
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

Updated components

1,012 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell129
Transport93
Client access (POP, IMAP, ActiveSync)33
Mailbox replication33
Search27
Compliance and eDiscovery25
Exchange Web Services and Autodiscover17
Outlook on the web8
Setup8

Verified in package: changed files confirm ContentEngine.

Page history

  1. Superseded by KB5133453

Sources

Updated
Cited passages from Microsoft's official notes (2)
  1. 1. To fix this issue, install the following Security Update or later updates: support.microsoft.com
  2. 2. To fix this issue, install the following Security Update or later updates: support.microsoft.com
Checked automatically: 8 of 8 checks passed, 2 quoted passages · compared with Microsoft's page on Nov 14, 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 4/5. 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