Patch tracker / KB5121000
KB5121000: Windows 11 version 26H1 Security Update (August 2026)
KB5121000 is a Critical security update for Windows 11 version 26H1, released 2026-08-11. It closes 183 CVEs with a maximum CVSS of 9.8. At least one of them is actively exploited (CISA KEV), so it is urgent, and Microsoft lists 1 known issue: test it before broad deployment. The most likely of them to be attacked is CVE-2026-68820, which FIRST.org rates at 9% probability of exploitation in the next 30 days.
Before you deploy: Microsoft lists 1 known issue for this update.
Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date.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 183 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 9.8 (Critical). EPSS exploit probability up to 8.6%. Among the top half of tracked Microsoft updates by EPSS exploit probability.
A newer security update for Windows 11 version 26H1 for x64-based Systems has shipped since this one: KB5127646 (2026-10-13). For cumulative update families the newer package includes these fixes, so installing that one is usually the shorter path. Verify against your own update rings before you skip this package.
Microsoft documents 1 known issue with this update.
- Microsoft Teams and Outlook might fail to launch on ARM-based devices
Senserva AI Opinion and rich prompt for KB5121000
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5121000
- Released
- 2026-08-11
- Last changed
- 2026-10-30
- Changes tracked
- 15
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.
- 2026-08-12 CVEs fixed 196 to 197
- 2026-08-20 CVEs fixed 197 to 198
Recorded by Senserva on the dates shown, from the MSRC release feed. This is what moved and when, which neither the vendor advisory nor NVD publishes.
Check if KB5121000 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5121000 (checked 2026-10-29). Microsoft lists 1 known issue. Verify against the article before acting.
Microsoft Teams and Outlook might fail to launch on ARM-based devices
Symptoms
After installing Windows security updates released on or after August 11, 2026, (KB5121000) , Microsoft Teams and the new Outlook for Windows might fail to launch or might close unexpectedly on ARM-based devices, such as Surface Pro 11 and Surface Laptop 7. Classic Outlook, Word, Excel, and other applications are not known to be affected. This issue is most likely to occur on new or freshly imaged PCs that have not yet installed any Microsoft Store updates.
Workaround
To work around this issue:
Open Microsoft Store.
Select Downloads > Check for updates.
Install the latest update for the Auto Super Resolution Package (version 1.0.19.0 or later).
Resolution
This issue is resolved in Windows updates released on and after September 8, 2026 such as KB5124012 . We recommend you install the latest Windows update for your device as it contains important improvements and issue resolutions, including this one.
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.
Scope and sources
- Affected products
- Also released in August 2026
KB5120228
204 CVEs · Critical · exploitedKB5120229
184 CVEs · Critical · exploitedKB5120233
204 CVEs · Critical · exploitedKB5120238
174 CVEs · Critical · exploitedKB5120240
160 CVEs · Critical · exploitedKB5120242
184 CVEs · Critical · exploitedKB5120385
136 CVEs · Critical · exploitedKB5120386
131 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.