Patch tracker / KB5129955
KB5129955: Exchange Server Subscription Edition Security Update (October 2026)
KB5129955 is a High security update for Exchange Server Subscription Edition, released 2026-10-13. It closes 1 CVE with a maximum CVSS of 8.8. Microsoft documents 1 known issue for this update, quoted below.
Before you deploy: Microsoft lists 1 known issue for this update.
Running Exchange Server? A Windows update this month has a known issue Microsoft says affects Exchange.
“Users might be unable to sign in to OWA or send emails on Exchange Server”
Microsoft lists this on its October 2026 Windows updates: KB5126646 (Windows Server 2025), KB5126651 (Windows Server 2019), KB5126655 (Windows Server 2022), KB5126868 (Windows Server 2016).
Microsoft links: October 2026 Exchange server security updates · Exchange servers may become unstable after installation of October 2026 Windows updates
Quoted from Microsoft's support articles for those updates. The full text is on each update's page.
Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date. If you are unsure, check the linked source before you act, and tell us when we are wrong so we can fix it.
Update summary
Fixes 1 CVE. Most severe CVSS 8.8 (High). EPSS exploit probability up to 2.8%. Among the top half of tracked Microsoft updates by EPSS exploit probability.
Microsoft documents 1 known issue with this update.
- A published calendar (.ics) returns "HTTP 500" for calendar applications .
Senserva AI Opinion and rich prompt for KB5129955
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5129955
- Released
- 2026-10-02
- Last changed
- 2026-10-25
- 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 KB5129955 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5129955 (checked 2026-10-27). Microsoft lists 1 known issue. Verify against the article before acting.
A published calendar (.ics) returns "HTTP 500" for calendar applications .
Availability (free/busy) fails for delegated mailboxes in Exchange hybrid deployments that use Graph API only .
A ContentEngine deadlock occurs 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.
| CVE | Severity | CVSS | EPSS | Exploited |
|---|---|---|---|---|
| CVE-2026-96940 | High | 8.8 | 2.7% | No |
Scope and sources
- Affected products
- Also released in October 2026
KB5126646
1041 CVEs · Critical · exploitedKB5126651
918 CVEs · Critical · exploitedKB5126653
965 CVEs · Critical · exploitedKB5126655
977 CVEs · Critical · exploitedKB5126657
899 CVEs · Critical · exploitedKB5126836
649 CVEs · Critical · exploitedKB5126868
769 CVEs · Critical · exploitedKB5127646
997 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.