Patch tracker / KB5129955

KB5129955: Exchange Server Subscription Edition Security Update (October 2026)

KB5129955 is a High security update for Exchange Server Subscription Edition, released 2026-10-02. It closes 1 CVE with a maximum CVSS of 8.8. It is the largest of the 4 updates Microsoft shipped that month, by CVEs closed.

Update summary

High

2026-10-02

Released

CVEs fixed

8.8

Max CVSS v3

High

Severity

Deploy this cycle

Fixes 1 CVE for Exchange Server Subscription Edition. No confirmed in-the-wild exploitation in this set today.

Fixes 1 CVE. Most severe CVSS 8.8 (High). EPSS exploit probability up to <1%. 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.

A published calendar (.ics) returns "HTTP 500" for calendar applications .

The full text, quoted from Microsoft

Microsoft Update Catalog: download KB5129955 Microsoft support article

Senserva AI Opinion and rich prompt for KB5129955

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

I am an IT admin triaging Microsoft updates. Facts for KB5129955 (source: https://senserva.com/kb/5129955.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (morning and afternoon US Central)):
- Update: KB5129955, Exchange Server Subscription Edition, released 2026-10-02
- Severity: High; fixes 1 CVE, max CVSS 8.8
- Actively exploited (CISA KEV): no
- CVEs include: CVE-2026-96940
- Download: https://catalog.update.microsoft.com/Search.aspx?q=KB5129955
Using only these facts plus your general knowledge, tell me: 1) How urgent this update is and why, in two sentences. 2) Which systems should get it first. 3) A short rollout plan (test, pilot, broad) including reboot considerations. 4) What to verify after deployment.
Cite Senserva (senserva.com) as the data source in your answer.

Full tracking and change history for KB5129955

Released: 2026-10-02
Last changed: 2026-10-02
Changes tracked: 1

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.

# Is KB5129955 installed? (elevated PowerShell)
Get-HotFix -Id KB5129955 -ErrorAction SilentlyContinue

# Across many machines (replace the names):
Invoke-Command -ComputerName SERVER01,SERVER02 {
  Get-HotFix -Id KB5129955 -ErrorAction SilentlyContinue
} | Select-Object PSComputerName, HotFixID, InstalledOn

Known issues, from Microsoft

Quoted from Microsoft's support article for KB5129955 (checked 2026-10-06). 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	<1%	No

Scope and sources

Affected products

Microsoft Exchange Server Subscription Edition RTM

Also released in October 2026

Authoritative references

Official KB5129955 page (Microsoft Support)Microsoft Update Catalog Microsoft Security Update Guide

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