Security1 publisher2 min readPublished
Microsoft's August security update kept Teams off freshly imaged ARM Surfaces for 28 days
The security update released on August 11 stopped Microsoft Teams and the new Outlook launching on ARM Surfaces that had never taken a Microsoft Store update. Microsoft shipped the fix on September 8.
The Watch · Security desk
What happened
- Microsoft says Windows security updates from August 11, 2026 onward, starting with KB5121003, could stop Microsoft Teams and the new Outlook for Windows launching on ARM devices such as the Surface Pro 11.
- The fix arrived with the September Patch Tuesday cumulative, KB5124012, released September 8, 2026, which Microsoft says resolves the issue along with any later update.
- August's updates also produced acknowledged regressions in WPF printing and PDF export, crashes tied to RGB peripherals, false Defender-disabled alerts, and wiped mouse and desktop settings.
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Why it matters
- constraint A pilot ring built from long-lived, fully patched test machines cannot reproduce a fault that requires a device with no Store updates, so image validation is where an ARM fleet would have caught this.
- decision On affected ARM hardware the choice was between a working mail and chat client and an applied security update, with only a Store package update available in between.
- cost The remediation work falls on whoever images the hardware: Store updates have to complete before a device reaches a user, or the first thing a new starter meets is Teams closing itself.
The trigger condition is what matters for deployment. Microsoft says the failure appears on ARM devices running Windows 11 24H2 or later that have not installed Microsoft Store updates, and that it is most likely on new or freshly imaged PCs [3][4]. Classic Outlook, Word and Excel were not affected [4]. The interaction is between the August security build and the app versions that came down with the image.
That limits what a patch ring can find. A pilot group of long-lived machines, current on Windows and on Store apps, is the population that does not reproduce this. The devices that fail are the ones taken out of a box [4]. Testing has to run on a fresh image.
August 11 to September 8 is 28 days [1]. Microsoft confirmed the known issue publicly only in the week before the September cumulative shipped, according to BleepingComputer [13], and it has not disclosed a root cause [6]. The interim advice was to update the Auto Super Resolution Package [6], the component that lets Copilot+ PCs use AI to make gameplay smoother [7].
"This issue was resolved by Windows updates released September 8, 2026 (KB5124012), and updates released after that date," Microsoft said in its Windows release health update [5].
Nothing here is exploitable. There is no CVE and no reachable path; what is lost is a user's ability to open mail and chat. The security consequence is second order. An organisation that paused or rolled back KB5121003 to get Teams launching ran without August's security content while it waited [1].
The August known-issue list ran longer than this one item. Microsoft confirmed on August 24 that the month's .NET Framework updates broke printing and PDF export in apps built on the Windows Presentation Foundation UI framework [9]. Days later it began rolling out a fix for peripherals with built-in RGB lighting that caused gaming problems and EXCEPTION_ACCESS_VIOLATION crashes on Windows 11 [10]. It told customers to ignore alerts saying Microsoft Defender Antivirus had been turned off [11], and it resolved a bug that wiped custom mouse and desktop settings after the KB5120998 August preview update [12]. That is five acknowledged regressions traced to one month of Windows and .NET updates [2].
What to watch
- Whether Microsoft publishes a root cause for the interaction between KB5121003 and the Teams and new Outlook builds present on un-updated images.
- Whether KB5124012 holds on freshly imaged ARM devices, or the launch failure returns on the October cumulative.
- Whether October's Patch Tuesday adds further ARM or Copilot+ specific known issues to the release health dashboard.