Patch tracker / KB5121608

KB5121608: Exchange Server Subscription Edition Security Update (September 2026)

KB5121608 is a Critical security update for Exchange Server Subscription Edition, released 2026-09-08. It closes 9 CVEs with a maximum CVSS of 9.3. It is the 27th largest of the 69 updates Microsoft shipped that month, by CVEs closed.

Update summary

Critical
2026-09-08
Released
9
CVEs fixed
9.3
Max CVSS v3
Critical
Severity
Deploy this cycle
Fixes 9 CVEs for Exchange Server Subscription Edition. No confirmed in-the-wild exploitation in this set today.

Fixes 9 CVEs. Most severe CVSS 9.3 (Critical). EPSS exploit probability up to 1.2%. Among the lower half of tracked Microsoft updates by EPSS exploit probability.

Senserva found thisread these before you deploy broadly

Microsoft documents 1 known issue with this update.

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

The full text, quoted from Microsoft

Microsoft Update Catalog: download KB5121608Microsoft support article
Senserva AI Opinion and rich prompt for KB5121608

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

Full tracking and change history for KB5121608

Released
2026-09-08
Last changed
2026-09-26
Changes tracked
3

Senserva began recording day-to-day change history on 2026-07-07, and refreshes it several times a day. Release dates are historical where Microsoft publishes them; revision history is tracked from that date forward, so it does not include changes made before then.

Check if KB5121608 is installed

Elevated PowerShell. No output means it is not installed.

Known issues, from Microsoft

Quoted from Microsoft's support article for KB5121608 (checked 2026-09-27). Verify against the article before acting.

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

Availability (free/busy) fails for delegated mailboxes in Exchange hybrid deployments using Graph API only

ContentEngine deadlock because of missing Korean WordBreaker rule files

CVEs fixed by this update

Sorted exploited-first, then by CVSS. Every CVE links to its Senserva page with the full risk facts, references, and a ready-to-paste AI prompt.

CVESeverityCVSSEPSSExploited
CVE-2026-69356Critical9.31.2%No
CVE-2026-69382Critical9.31.2%No
CVE-2026-69641Critical9.11.2%No
CVE-2026-69355High8.81.2%No
CVE-2026-55007High8.11.2%No
CVE-2026-69380High8.11.2%No
CVE-2026-69378High7.51.2%No
CVE-2026-69361Medium6.51.2%No
CVE-2026-69375Medium6.51.2%No

Scope and sources

Affected products
Also released in September 2026

KB5122871

679 CVEs · Critical · exploited

KB5122876

616 CVEs · Critical · exploited

KB5122878

568 CVEs · Critical · exploited

KB5122880

602 CVEs · Critical · exploited

KB5122882

641 CVEs · Critical · exploited

KB5123065

393 CVEs · Critical · exploited

KB5123066

418 CVEs · Critical · exploited

KB5123099

555 CVEs · Critical · exploited

The full month, summarized: Microsoft Patch Tuesday.

Authoritative references
Official KB5121608 page (Microsoft Support)Microsoft Update CatalogMicrosoft Security Update Guide

See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.

New to a term on this page? CVE, KB, KEV, CVSS, and EPSS are each explained once, on the Senserva security lexicon. Reference: Microsoft CVE and vulnerability management and the Microsoft patching guide.
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