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Exchange Server Subscription Edition RTM Exchange Server Subscription Edition RTM Security update
Patch Tuesday Zero-day

KB5094139

KB5094139: Security update for Exchange Server Subscription Edition RTM, June 2026. Critical exploited spoofing flaw patched; mitigation services recover

In short

The update addresses Exchange vulnerabilities, including the actively exploited critical spoofing flaw CVE-2026-42897. It lists eight CVEs, but does not include the fix for CVE-2026-45583; follow that CVE’s guidance.

Does it affect you? Applies to: Exchange Server Subscription Edition RTM systems (x64). The update brings them to build 15.2.2562.43.

Build
15.2.2562.43
Servicing branch
RTM
Security
8 fixed · 1 critical · 1 exploited

What changes

Fix 1 change

Exchange mitigation and flighting services

  • Exchange mitigation and flighting services no longer fail with an “Unknown Issuer” error.1

Technical details

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

Data sheet

Released
2026-06-09
Classification
Security Updates
Type
Cumulative update
Servicing branch
RTM
Replaces
KB5081755, KB5074992
Replaced by
KB5103212
VersionBuildChannel
SE 15.2.2562.43 General availability

Security

8 vulnerabilities fixed
1 exploited 1 critical 7 not rated
VulnerabilitySeverityCVSS
CVE-2026-42897 Exploited
Microsoft Exchange Server Spoofing Vulnerability
 Critical 8.1
CVE-2026-45500
Microsoft Exchange Server Spoofing Vulnerability
 Not rated —
CVE-2026-45501
Microsoft Exchange Server Spoofing Vulnerability
 Not rated —
CVE-2026-45502
Microsoft Exchange Server Information Disclosure Vulnerability
 Not rated —
CVE-2026-45503
Microsoft Exchange Server Information Disclosure Vulnerability
 Not rated —
CVE-2026-45504
Microsoft Exchange Server Elevation of Privilege Vulnerability
 Not rated —
CVE-2026-45583
Microsoft Exchange Server Remote Code Execution Vulnerability
 Not rated —
CVE-2026-47631
Microsoft Exchange Server Spoofing Vulnerability
 Not rated —

Updated components

1,354 files changed since the previous update for the same versions.

Top changed componentsFiles
Management and PowerShell157
Transport93
Client access (POP, IMAP, ActiveSync)33
Mailbox replication33
Search27
Compliance and eDiscovery24
Exchange Web Services and Autodiscover17
Outlook on the web8
Setup8

Sources

Updated
Cited passages from Microsoft's official notes (1)
  1. 1. Exchange mitigation and flighting services fail due to "Unknown Issuer" error support.microsoft.com
Checked automatically: 7 of 8 checks passed, 1 quoted passage · 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: not 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