Skip to the content
The Update Bureau
Exchange Server 2019 CU14 Exchange Server 2019 CU14 ESU only Security update
Patch Tuesday

KB5121610

KB5121610: Security update for Exchange Server 2019 CU14, September 2026. Exchange security flaws addressed; hybrid mailbox wrappers fixed

In short

Security flaws are addressed, and a hybrid shared-mailbox message issue is fixed under specific mailbox and permission settings. Published calendar subscriptions stop refreshing and return HTTP 500 errors.

Does it affect you? Applies to: Organizations running Exchange Server 2019 CU14 and enrolled in the Period 2 Extended Security Update program. Exchange Server 2019 has reached end of support.

Build
15.2.1544.46
Servicing branch
CU14
Security
9 fixed

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

Fixes 2 changes

Exchange Server

  • Security flaws in Exchange Server are addressed.1
    Addresses 9 Important vulnerabilities.

Shared mailboxes

For: Hybrid Exchange organizations with the stated mailbox setup

  • In hybrid setups, sent-as or sent-on-behalf messages may also reach a shared mailbox as wrapper attachments. The shared mailbox is in Exchange Online; the sender's mailbox is on-premises, with matching rights and copy settings enabled.2
    The affected scenario has Send As or Send on Behalf permissions and the corresponding MessageCopyForSentAsEnabled or MessageCopyForSendOnBehalfEnabled parameter enabled.

Known issues

  1. Open Affects: Organizations and IT administrators

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

    Subscriptions to anonymously published Exchange calendars stop refreshing, and calendar apps receive HTTP 500 errors.3

    How to apply the workaround IIS / URL Rewrite

    Add the URL Rewrite rule to the Exchange Back End site on every server that serves the published calendar. Confirm the URL Rewrite module is installed first.3

    Step-by-step workaround instructions
    4 steps
    Show steps Hide steps
    Step-by-step procedure
    1. Confirm the IIS URL Rewrite module is installed; Exchange Server Setup does not install it.
    2. On each server serving the calendar, add the rule to Exchange Back End only. Do not apply it to Default Web Site or at server level.
    3. Match ^owa/calendar/.+\.ics$ and add a condition to leave requests alone when the query string already contains layout=.
    4. Rewrite to {R:0}?layout=premium&{QUERY_STRING} and clear the Append query string option.

Technical details

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

Data sheet

Released
2026-09-08
Classification
Security Updates
Type
Cumulative update
Servicing branch
CU14
Replaces
KB5121575
Replaced by
KB5129957
VersionBuildChannel
2019 15.2.1544.46 General availability

Security

9 vulnerabilities fixed
9 important
VulnerabilitySeverityCVSS
CVE-2026-69356
Microsoft Exchange Server Spoofing Vulnerability
 Important 9.3
CVE-2026-69641
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 9.1
CVE-2026-69355
Microsoft Exchange Server Remote Code Execution Vulnerability
 Important 8.8
CVE-2026-55007
Microsoft Exchange Server Remote Code Execution Vulnerability
 Important 8.1
CVE-2026-69380
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.1
CVE-2026-69378
Microsoft Exchange Server Denial of Service Vulnerability
 Important 7.5
CVE-2026-69361
Microsoft Exchange Server Spoofing Vulnerability
 Important 6.5
CVE-2026-69375
Microsoft Exchange Server Tampering Vulnerability
 Important 6.5
CVE-2026-69382
Microsoft Exchange Server Information Disclosure Vulnerability
 Important 5.9

Updated components

1,173 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell124
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 (3)
  1. 1. This security update resolves vulnerabilities in Microsoft Exchange Server. support.microsoft.com
  2. 2. To fix this issue, install the following security update or later updates for your version of Microsoft Exchange Server: support.microsoft.com
  3. 3. Add one URL Rewrite rule to the Exchange Back End site in IIS. 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: 8 of 8 checks passed, 3 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: 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