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Exchange Server 2019 CU15 Exchange Server 2019 CU15 ESU only Out-of-band security update
Out-of-band

KB5129956

KB5129956: Security update for Exchange Server 2019 CU15, October 2026. Published calendar feeds can fail with HTTP 500

In short

Addresses 9 vulnerabilities in Exchange Server 2019 CU15. A known issue can stop published calendar subscriptions from refreshing and return HTTP 500; an IIS workaround is available.

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

Build
15.2.1748.53
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

Fix 1 change

Hybrid shared mailboxes

For: Organizations with Exchange hybrid deployments

  • In hybrid setups, messages sent as or on behalf of a shared mailbox no longer also arrive in its Inbox as wrapper attachments.1
    The issue affected shared mailboxes hosted in Exchange Online when an on-premises mailbox had Send As or Send on Behalf permissions and MessageCopyForSentAsEnabled or MessageCopyForSendOnBehalfEnabled was 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 SE build 15.2.2562.46, subscriptions to anonymously published calendars might stop refreshing. Calendar applications report a server error, and the URL returns HTTP 500.2

    How to apply the workaround IIS / URL Rewrite

    Add the URL Rewrite rule in IIS on every server that serves the published calendar, applying it only to the Exchange Back End site.2

    Step-by-step workaround instructions
    6 steps
    Show steps Hide steps
    Step-by-step procedure
    1. Confirm the IIS URL Rewrite module is installed; both workaround methods require it.
    2. In IIS Manager, open URL Rewrite for the Exchange Back End site and add a blank inbound rule.
    3. Set the case-insensitive regular-expression pattern to ^owa/calendar/.+\.ics$.
    4. Add a condition for {QUERY_STRING} that does not match (^|&)layout=.
    5. Set the rewrite URL to {R:0}?layout=premium&{QUERY_STRING}, clear Append query string, and stop processing subsequent rules.
    6. Apply the rule on every server serving the calendar. Do not apply it to Default Web Site or at server level.

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
CU15
Replaces
KB5121609
Replaced by
KB5128896
VersionBuildChannel
2019 15.2.1748.53 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

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

Top changed componentsFiles
Management and PowerShell127
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. Add one URL Rewrite rule to the Exchange Back End site in IIS. The rule appends layout=premium to .ics requests below /owa/calendar/. 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 5/5, clarity 5/5, 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