Microsoft released September security updates for Windows 11 on 8 September 2026, including KB5124008 for versions 24H2 and 25H2, and KB5124012 for version 26H1.[6][1] Our advice for UK small businesses is to check which release each PC runs, test the applicable update on a small group, and track installation through to a completed restart.
You do not need to turn a routine update into a major project. You do need someone responsible for the machines that fail, stay offline or keep postponing the restart.
Match the update to the Windows version
The September package is not the same for every Windows 11 device. Microsoft’s release notes identify KB5124008 with OS builds 26200.9445 and 26100.9445 for 25H2 and 24H2 respectively; KB5124012 takes 26H1 to build 28000.2954.[6][1]
Ask your IT provider for a device list that includes the Windows edition, version and current build. Use the applicable Microsoft release note to check the expected result. Avoid sending staff a download link for a particular KB before you know whether it belongs on their machine.
For a small office, we recommend recording the device name, person using it, update status and any agreed exception. Include spare PCs and machines used by remote staff in that check, rather than looking only at the computers switched on in the office.
There are practical fixes alongside security changes
Microsoft says these releases include security improvements and a fix for Microsoft Teams and Outlook unexpectedly closing on Arm64-based PCs.[6][1] The 26H1 notes also describe a fix for Remote Desktop audio redirection in certain configurations.[1]
Those are specific Windows fixes. They do not establish that every Outlook crash has the same cause, or that an unrelated Microsoft 365 problem will disappear after this update. If staff have reported either symptom, give your support provider the affected device details and test the same workflow after installation.
The releases also expand targeting data for devices eligible to receive new Secure Boot certificates automatically.[6][1] We recommend asking your provider to review the applicable Microsoft guidance rather than having staff change firmware settings themselves.
Test the work your staff actually do
Our preferred approach is a short pilot with people who can report problems promptly. Choose machines that represent the hardware and business software you use, rather than testing only on the IT administrator’s PC.
After installation and restart, ask the pilot users to try their normal work. That might mean opening the accounts package, printing an invoice, joining a Teams call or connecting to a remote desktop. Include any workflow that would stop trading if it failed.
Agree a recovery route before rolling the update out more widely. For a business-critical machine, ask who will handle a failed installation, how the user’s work is protected and what they can use while the device is unavailable. We would rather settle those questions before the office opens than during a payroll run.
Check completion, not just approval
For the wider rollout, tell staff when restarts are expected and who to contact if something looks wrong. Give remote workers a clear window to bring their devices online.
Ask for a completion report that separates:
- Devices with the applicable update installed and the restart completed.
- Devices waiting for a restart or a connection.
- Failed installations that need investigation.
- Deliberate exceptions, with an owner and a review date.
Treat an approved deployment as the start of the work. For your own records, request evidence of the installed build and follow up on the exceptions instead of accepting a blanket “updates are enabled” answer.
What to ask BMT
If you are unsure whether your PCs are covered, ask BMT to review your Windows device list and patching arrangements. Bring any recurring update errors and details of the applications the business cannot work without.
The useful outcome is a rollout plan you can understand, with a named owner for failed machines and a check that staff can still do their jobs afterwards. Keep that process for next month’s updates too.
Sources
[1] https://support.microsoft.com/en-us/servicing/os/windows-11/2026/09/kb5124012-windows-11-26h1-security-update
[6] https://support.microsoft.com/en-us/servicing/os/windows-11/2026/09/kb5124008-windows-11-24h2-25h2-security-update
Featured image: editorial illustration generated for this article.