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Exchange Server 2019 CU15 Exchange Server 2019 CU15 ESU only Security update
Patch Tuesday

KB5121609

KB5121609: Security update for Exchange Server 2019 CU15, September 2026. Published calendar feeds can fail; hybrid sent-mail copies fixed

In short

Security vulnerabilities are addressed, and a hybrid shared-mailbox message-copy problem is fixed. Published calendar subscriptions can still return HTTP 500; a workaround is available.

Does it affect you? Applies to: Exchange Server 2019 CU15, build 15.2.1748.51. Only organizations enrolled in the Period 2 Extended Security Update (ESU) program are eligible to receive this update.

Build
15.2.1748.51
Servicing branch
CU15
Security
9 fixed

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

  • Security vulnerabilities in the server are addressed.1

Shared mailboxes

For: Hybrid Exchange organizations that meet these conditions

  • In some hybrid setups, sent-as or sent-on-behalf messages no longer also appear as attachments in a shared mailbox's Inbox.2
    Applies when the shared mailbox is hosted in Exchange Online, another mailbox is on an on-premises server, 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

    Published calendar subscriptions might stop refreshing, and calendar apps can show an HTTP 500 server error.3

    How to apply the workaround IIS / URL Rewrite

    Add an IIS URL Rewrite rule to the Exchange Back End site so calendar .ics requests use the premium layout.3

    Step-by-step workaround instructions
    6 steps
    Show steps Hide steps
    Step-by-step procedure
    1. Confirm the IIS URL Rewrite module is installed; the workaround requires it.
    2. In IIS Manager, open Sites > Exchange Back End > URL Rewrite. Do not apply the rule to Default Web Site or at server level.
    3. Add a blank inbound rule named OWA published calendar force premium layout; match ^owa/calendar/.+\.ics$ using case-insensitive regular expressions.
    4. Add a condition on {QUERY_STRING} that does not match (^|&)layout=.
    5. Set the action to Rewrite with {R:0}?layout=premium&{QUERY_STRING}. Clear Append query string and select Stop processing.
    6. Apply the rule. No application pool recycle is required.

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
CU15
Replaces
KB5121574
Replaced by
KB5129956
VersionBuildChannel
2019 15.2.1748.51 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

1,175 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell123
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. messages that are Sent As, or Sent on Behalf of, the shared mailbox may also be delivered to the Inbox folder of the shared mailbox as attachments support.microsoft.com
  3. 3. After you install the August 2026 security update (Exchange Server SE, build 15.2.2562.46), subscriptions to an anonymously published Exchange calendar stop refreshing. 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 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