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Exchange Server 2019 CU14 Exchange Server 2019 CU14 ESU only Out-of-band security update
Out-of-band

KB5129957

KB5129957: Security update for Exchange Server 2019 CU14, October 2026. Hybrid shared-mailbox wrapper messages fixed; calendar feeds may fail

In short

Fixes a hybrid shared-mailbox message issue and addresses 9 vulnerabilities. It is available only to organizations in the Period 2 ESU program; published calendar feeds can still return HTTP 500.

Does it affect you? Applies to: Exchange Server 2019 CU14 organizations enrolled in the Period 2 Extended Security Update (ESU) program. This update installs build 15.2.1544.48.

Build
15.2.1544.48
Servicing branch
CU14
Security
9 fixed
Review notes Additional details from the official release notes (1) Read note
  • Known issues This calendar issue is also listed for Exchange Server 2019 CU14. The issue article names KB5128897 as the CU14 update that fixes it.

The official sources are linked at the end of the page.

For servers running Cumulative Update 14: 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 a hybrid deployment, messages sent as or on behalf of a shared mailbox hosted in Exchange Online no longer also appear in that mailbox’s Inbox as attachments with a wrapper message.1
    The affected configuration has an on-premises Exchange mailbox with Send As or Send on Behalf permissions for the shared mailbox, and MessageCopyForSentAsEnabled or MessageCopyForSendOnBehalfEnabled enabled.

Known issues

  1. Open Affects: Organizations and IT administrators

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

    After the August 2026 security update on Exchange Server Subscription Edition build 15.2.2562.46, subscriptions to anonymously published calendars might stop refreshing. Calendar applications might report a server error, and the calendar URL returns HTTP 500.2

    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
    5 steps
    Show steps Hide steps
    Step-by-step procedure
    1. Confirm the IIS URL Rewrite module is installed.
    2. In IIS Manager, open Sites > Exchange Back End > URL Rewrite and add a blank inbound rule.
    3. Match ^owa/calendar/.+\.ics$ using case-insensitive regular expressions; add a condition for {QUERY_STRING} that does not match (^|&)layout=.
    4. Set the rewrite URL to {R:0}?layout=premium&{QUERY_STRING}, clear Append query string, and select Stop processing of subsequent rules.
    5. Apply the rule only to Exchange Back End, not Default Web Site or the server level, on every server serving the calendar. Verify that the published calendar still opens; no application pool recycle or 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
CU14
Replaces
KB5121610
Replaced by
KB5128897
VersionBuildChannel
2019 15.2.1544.48 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

1,002 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 eDiscovery26
Exchange Web Services and Autodiscover17
Outlook on the web8
Setup8

Sources

Updated
Cited passages from Microsoft's official notes (2)
  1. 1. Wrapper messages appear in a shared mailbox in hybrid environments after installing the June 2026 Security Update support.microsoft.com
  2. 2. The rule appends layout=premium to .ics requests below /owa/calendar/. This action selects the supported code path, and restores the feed. 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 3/5, 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