Patch tracker / KB5121609
KB5121609: Exchange Server 2019 Cumulative Update Security Update (September 2026)
KB5121609 is a Critical security update for Exchange Server 2019 Cumulative Update, released 2026-09-08. It closes 9 CVEs with a maximum CVSS of 9.3. Microsoft documents 1 known issue for this update, quoted below.
Before you deploy: Microsoft lists 1 known issue for this update.
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Update summary
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.
A newer security update for Microsoft Exchange Server 2019 Cumulative Update 15 has shipped since this one: KB5128896 (2026-10-13). For cumulative update families the newer package includes these fixes, so installing that one is usually the shorter path. Verify against your own update rings before you skip this package.
Microsoft documents 1 known issue with this update.
- Published calendar (.ics) returns HTTP 500 for calendar applications
Senserva AI Opinion and rich prompt for KB5121609
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5121609
- 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 KB5121609 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5121609 (checked 2026-10-16). Microsoft lists 1 known issue. Verify against the article before acting.
Published calendar (.ics) returns HTTP 500 for calendar applications
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.
| CVE | Severity | CVSS | EPSS | Exploited |
|---|---|---|---|---|
| CVE-2026-69356 | Critical | 9.3 | 1.2% | No |
| CVE-2026-69641 | Critical | 9.1 | 1.2% | No |
| CVE-2026-69355 | High | 8.8 | 1.2% | No |
| CVE-2026-55007 | High | 8.1 | 1.2% | No |
| CVE-2026-69380 | High | 8.1 | 1.2% | No |
| CVE-2026-69378 | High | 7.5 | 1.2% | No |
| CVE-2026-69361 | Medium | 6.5 | 1.2% | No |
| CVE-2026-69375 | Medium | 6.5 | 1.2% | No |
| CVE-2026-69382 | Medium | 5.9 | 1.2% | No |
Scope and sources
- Affected products
- Also released in September 2026
KB5122871
684 CVEs · Critical · exploitedKB5122876
620 CVEs · Critical · exploitedKB5122878
572 CVEs · Critical · exploitedKB5122880
608 CVEs · Critical · exploitedKB5122882
645 CVEs · Critical · exploitedKB5123065
398 CVEs · Critical · exploitedKB5123066
423 CVEs · Critical · exploitedKB5123099
559 CVEs · Critical · exploitedThe full month, summarized: Microsoft Patch Tuesday.
- Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.