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Exchange Server 2016 CU23 Exchange Server 2016 CU23 ESU only Out-of-band security update
Out-of-band

KB5129958

KB5129958: Security update for Exchange Server 2016 CU23, October 2026. Shared mailbox wrapper messages fixed; published calendars can fail

In short

Fixes wrapper messages appearing in shared mailboxes in hybrid environments. Published calendar subscriptions can still return HTTP 500; KB5128898 later resolves the issue for Exchange Server 2016 CU23.

Does it affect you? Applies to: Exchange Server 2016 CU23 organizations enrolled in the Period 2 Extended Security Update (ESU) program. Microsoft says Exchange Server 2016 is out of support, and security updates are available to eligible ESU organizations through the end of October 2026.

Build
15.1.2507.75
Servicing branch
CU23
Security
9 fixed

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

Shared mailbox

For: Organizations with the specified hybrid mailbox configuration

  • In certain hybrid deployments, messages sent as or on behalf of a shared mailbox no longer also arrive in its Inbox as attachments with a wrapper message.1
    Applies to the scenario where the shared mailbox is hosted in Exchange Online, the sending mailbox is on-premises, the sender has Send As or Send on Behalf permissions, and MessageCopyForSentAsEnabled or MessageCopyForSendOnBehalfEnabled is enabled.

Known issues

  1. Open Affects: Organizations and IT administrators

    Published calendar (.ics) returns HTTP 500 for calendar applications

    Subscriptions to anonymously published calendars can stop refreshing; the calendar URL returns HTTP 500. The linked report describes the issue after the August 2026 security update on Exchange Server Subscription Edition, build 15.2.2562.46. For Exchange Server 2016 CU23, KB5128898 or later resolves it.2

    How to apply the workaround IIS / URL Rewrite

    Add an IIS URL Rewrite rule to the Exchange Back End site on every server that serves the published calendar.2

    Step-by-step workaround instructions
    4 steps
    Show steps Hide steps
    Step-by-step procedure
    1. Confirm that the IIS URL Rewrite module is installed; the workaround requires it.
    2. Add the rule only to the Exchange Back End site, not Default Web Site or the server level.
    3. Match ^owa/calendar/.+\.ics$, leave requests with an existing layout parameter unchanged, and rewrite to {R:0}?layout=premium&{QUERY_STRING} with append query string disabled.
    4. Apply the rule on every server that serves the published calendar. No restart is required.

Technical details

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

Data sheet

Released
2026-10-02
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU23
Replaces
KB5121611
Replaced by
KB5128898
VersionBuildChannel
2016 15.1.2507.75 General availability

Security

9 vulnerabilities fixed
1 important 8 not rated
VulnerabilitySeverityCVSS
CVE-2026-96940
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.8
CVE-2026-55007
Microsoft Exchange Server Remote Code Execution Vulnerability
 Not rated —
CVE-2026-69355
Microsoft Exchange Server Remote Code Execution Vulnerability
 Not rated —
CVE-2026-69356
Microsoft Exchange Server Spoofing Vulnerability
 Not rated —
CVE-2026-69361
Microsoft Exchange Server Spoofing Vulnerability
 Not rated —
CVE-2026-69375
Microsoft Exchange Server Tampering Vulnerability
 Not rated —
CVE-2026-69378
Microsoft Exchange Server Denial of Service Vulnerability
 Not rated —
CVE-2026-69382
Microsoft Exchange Server Information Disclosure Vulnerability
 Not rated —
CVE-2026-69641
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Not rated —

Updated components

1,021 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell127
Transport94
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 (2)
  1. 1. To fix this issue, install the following security update or later updates for your version of Microsoft Exchange Server: support.microsoft.com
  2. 2. Description of the security update for Microsoft Exchange Server 2016 CU23 October 13, 2026 (KB5128898) support.microsoft.com
Checked automatically: 7 of 8 checks passed, 2 quoted passages · compared with Microsoft's page on Oct 21, 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 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