About This Article
This article was generated using an automated workflow powered by generative AI. On September 13, 2026, we reviewed the official information regarding the monthly security updates and Windows 11 KB5124008/KB5124012 released by Microsoft on September 8, 2026, and formulated read-only procedures to check whether your PC is in scope for the update.Verification Status: 📰 Official Microsoft information verified, physical target PC not yet verified
For the September 2026 Windows monthly security updates, rather than just reading the news, verifying your PC's OS build and update historyconnects directly to practical operations. Microsoft released security updates on September 8, 2026, US time, and recommends early application for affected products.
Check Your Windows First
Read the status via PowerShell without changing any settings.
Get-ComputerInfo | Select-Object WindowsProductName, WindowsVersion, OsBuildNumber Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 10 HotFixID, InstalledOn, Description
Get-HotFixdoes not necessarily represent the complete Windows Update history in full detail. For final confirmation, also use Settings -> Windows Update -> Update history.
Key Points This Month
The Microsoft Japan Security Team announced the release of security updates addressing vulnerabilities in Microsoft products on September 8, 2026 (US time).
OS builds 26100.9445 / 26200.9445 are provided for Windows 11 24H2/25H2 via KB5124008, and 28000.2954 is provided for 26H1 via KB5124012. Because the applicable KB changes depending on your edition and version, do not copy and rely on someone else's KB number.
What to Look At
自分のWindows version
↓
対象KBをMicrosoft公式ページで確認
↓
Windows Update履歴で適用状況を見る
↓
未適用なら組織の更新方針を確認
Corporate PCs may manage distribution schedules using WSUS, Intune, Windows Autopatch, or similar tools. The date Microsoft releases an update is not necessarily the date it is deployed to your PC.
Try Changing One Thing
Run the same command not only on your own PC but on a single managed test PC, comparing only the OS build. Rather than deploying scripts to all machines at once, first check the output format on a single machine.
Secure Boot is Another Item to Verify in This Update Information
Microsoft's KB5124008 also provides information regarding Secure Boot certificates. Since 2026 is an important period for Secure Boot certificate updates, it is worth viewing monthly updates not just as patches requiring a reboot, but as maintenance that includes updates to the OS trust foundation.
What Administrative and Office Staff Should Do
You do not need to apply patches manually. First, check whether you are ignoring reboot requests, whether failures persist in your update history, or if an update notice has been issued by IT.
On the IT department side, do not treat "unapplied" as an immediate anomaly; instead, evaluate it in combination with the target version, deployment ring, compatibility verification, and reboot grace period.
Don't Just Chase Numbers in the News
The vulnerability count for this month varies among media outlets due to different aggregation methods. What matters in operations is cross-referencing the Microsoft Security Update Guide with your organization's target products rather than focusing on large numbers alone. This article prioritizes official information and checking the state of your own PC over a numbers race.
Summary
The monthly updates for September 2026 were released on September 8. First, read your Windows version/build, cross-reference the applicable KB with official information, and verify the deployment status in your Windows Update history. For corporate PCs, prioritize your organization's distribution policy.
Official and Primary Sources
Microsoft Japan Security Team, "September 2026 Security Updates (Monthly):" https://www.microsoft.com/en-us/msrc/blog/2026/09/202609-security-update
Windows 11 24H2/25H2 KB5124008: https://support.microsoft.com/help/5124008
Windows 11 26H1 KB5124012: https://support.microsoft.com/help/5124012
