KB5126104: what it fixes and who gets it
KB5126104 is a security update from Microsoft's September 2026 release for .NET 8.0 installed on Linux, .NET 8.0 installed on Mac OS, .NET 8.0 installed on Windows and other products.
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Should you install it?
Yes. It is a routine security update; none of its fixes are rated Critical or reported as exploited. Windows Update installs it automatically on supported home PCs.
If the installation fails or the PC misbehaves afterwards, note the exact error code first: the fix depends on it. Our update troubleshooting guides are linked on this page.
Security fixes in KB5126104
| CVE | Type | CVSS | Severity |
|---|---|---|---|
| CVE-2026-69439.NET and Visual Studio Elevation of Privilege Vulnerability | Elevation of Privilege | 8.8 | Important |
| CVE-2026-69522.NET and Visual Studio Remote Code Execution Vulnerability | Remote Code Execution | 8.8 | Important |
| CVE-2026-71328.NET and Visual Studio Remote Code Execution Vulnerability | Remote Code Execution | 8.8 | Important |
| CVE-2026-58649.NET Information Disclosure Vulnerability | Information Disclosure | 6.5 | Important |
| CVE-2026-69304ASP.NET Core Denial of Service Vulnerability | Denial of Service | 5.9 | Important |
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