# KB5126421: September 2026 security update

KB5126421, released September 8, 2026, fixes 2 CVEs across 9 products. None of them is reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/fix/microsoft/kb5126421
- Platform: Microsoft
- Released: Sep 8, 2026
- Priority: High (Closes CVE-2026-69522, high severity)
- CVEs: 2, exploited: 0, on CISA KEV: 0
- Vendor advisory: https://support.microsoft.com/help/5126421

## Applies to

| Product | Release | Fixed in |
|---|---|---|
| [Microsoft .NET Framework 3.5 AND 4.8 on Windows 10 Version 1809 for 32-bit Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8-on-windows-10-version-1809-for-32-bit-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8 on Windows 10 Version 1809 for ARM64-based Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8-on-windows-10-version-1809-for-arm64-based-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8 on Windows 10 Version 1809 for x64-based Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8-on-windows-10-version-1809-for-x64-based-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8.1 on Windows 10 Version 21H2 for 32-bit Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8.1-on-windows-10-version-21h2-for-32-bit-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8.1 on Windows 10 Version 21H2 for ARM64-based Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8.1-on-windows-10-version-21h2-for-arm64-based-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8.1 on Windows 10 Version 21H2 for x64-based Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8.1-on-windows-10-version-21h2-for-x64-based-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8.1 on Windows 10 Version 22H2 for 32-bit Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8.1-on-windows-10-version-22h2-for-32-bit-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8.1 on Windows 10 Version 22H2 for ARM64-based Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8.1-on-windows-10-version-22h2-for-arm64-based-systems.md) |  |  |
| [Microsoft .NET Framework 3.5 AND 4.8.1 on Windows 10 Version 22H2 for x64-based Systems](https://patchblox.com/security/data/product/microsoft/microsoft-.net-framework-3.5-and-4.8.1-on-windows-10-version-22h2-for-x64-based-systems.md) |  |  |

## CVEs this fix closes

| CVE | Priority | Component | CVSS | EPSS | Exploitation |
|---|---|---|---:|---:|---|
| [CVE-2026-69522](https://patchblox.com/security/data/cve/CVE-2026-69522.md) | High | Visual Studio | 8.8 | 0.0084 |  |
| [CVE-2026-62886](https://patchblox.com/security/data/cve/CVE-2026-62886.md) | High | .NET | 7.8 | 0.0047 |  |

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Data as of 2026-11-07T12:40:37Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo