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Exchange Server Subscription Edition RTM Exchange Server Subscription Edition RTM Security update
Patch Tuesday

KB5103212

KB5103212: Security update for Exchange Server Subscription Edition RTM, July 2026. Critical Exchange spoofing flaw addressed; mailbox issue remains

In short

A critical Exchange spoofing vulnerability is addressed. Hybrid shared mailboxes might still receive wrapper messages after the June security update; KB5121608 or later resolves the issue.

Does it affect you? Applies to: Exchange Server Subscription Edition RTM servers, x64.

Build
15.2.2562.45
Servicing branch
RTM
Security
11 fixed · 1 critical

What changes

Fix 1 change

Exchange Server

  • Exchange Server's critical spoofing vulnerability is addressed.1
    Addresses CVE-2026-55008.

Known issues

  1. Open Affects: Organizations and IT administrators

    Wrapper messages appear in shared mailbox in hybrid environments after installing the June 2026 Security Update

    After the June 2026 Security Update, hybrid shared mailboxes might receive sent messages as attachments in the Inbox. This affects setups with an Exchange Online shared mailbox, an on-premises sender mailbox, and message-copy settings enabled; KB5121608 or later resolves it.2

    How to apply the workaround Exchange Management Shell

    Check whether the setting override exists, and remove it if present. If the check returns the specified not-found error, no action is needed.2

    Step-by-step instructions, command & download kit
    2 steps Exchange Management Shell Instructions (.txt)
    Show steps & kit Hide steps & kit
    Step-by-step procedure
    1. In Exchange Management Shell, check for DisableBlockSharedAndUserMailboxHeaders.
    2. If the command returns the specified not-found error, take no further action. If the override is present, remove it.
    Exchange Management Shell 
    > Get-SettingOverride "DisableBlockSharedAndUserMailboxHeaders"> Remove-SettingOverride "DisableBlockSharedAndUserMailboxHeaders"

Technical details

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

Data sheet

Released
2026-07-14
Classification
Security Updates
Type
Cumulative update
Servicing branch
RTM
Replaces
KB5094139, KB5081755
Replaced by
KB5121573
VersionBuildChannel
SE 15.2.2562.45 General availability

Security

11 vulnerabilities fixed
1 critical 10 important
VulnerabilitySeverityCVSS
CVE-2026-55008
Microsoft Exchange Server Spoofing Vulnerability
 Critical 9.6
CVE-2026-45504
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 8.8
CVE-2026-55005
Microsoft Exchange Server Remote Code Execution Vulnerability
 Important 8.8
CVE-2026-45503
Microsoft Exchange Server Information Disclosure Vulnerability
 Important 8.1
CVE-2026-47631
Microsoft Exchange Server Spoofing Vulnerability
 Important 8.1
CVE-2026-55006
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.8
CVE-2026-55009
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Important 7.8
CVE-2026-45583
Microsoft Exchange Server Remote Code Execution Vulnerability
 Important 7.5
CVE-2026-45501
Microsoft Exchange Server Spoofing Vulnerability
 Important 6.5
CVE-2026-45500
Microsoft Exchange Server Spoofing Vulnerability
 Important 6.1
CVE-2026-45502
Microsoft Exchange Server Information Disclosure Vulnerability
 Important 5.0

Updated components

1,448 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell129
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. CVE-2026-55008 - Microsoft Exchange Server Spoofing Vulnerability support.microsoft.com
  2. 2. If no error is returned and the override is present, run the following command to remove it: support.microsoft.com
Checked automatically: 8 of 8 checks passed, 2 quoted passages · compared with Microsoft's page on Sep 26, 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