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.NET Framework 3.5 / 4.8.1 .NET Framework 3.5 / 4.8.1 Security update
Patch Tuesday

KB5126053

KB5126053: Cumulative update for .NET Framework, versions 3.5 and 4.8.1, on Windows 11 26H1, September 2026. Elevation-of-privilege and code-execution flaws addressed

In short

Addresses two Important .NET Framework vulnerabilities, including remote code execution, and intermittent crashes in some 64-bit scenarios.

Does it affect you? Applies to: Devices running Windows 11, version 26H1, with .NET Framework 4.8.1 installed.

File version
4.8.9347.0
Security
2 fixed
Restart
Required

What changes

Fixes 3 changes

.NET Framework

2 items
  • An elevation-of-privilege flaw is addressed.1
    CVE-2026-62886 is an elevation of privilege vulnerability.
  • A remote code execution flaw is addressed.2
    CVE-2026-69522 is a remote code execution vulnerability.

NET Fundamentals

  • Intermittent crashes that could occur in some 64-bit .NET Framework scenarios are addressed.3

Technical details

Data sheet · Security · Downloads · Components Click to collapse or expand

Data sheet

Released
2026-09-08
Classification
Security Updates
Type
Cumulative update
Restart
Required
Replaces
KB5122386, KB5120711, KB5101002, KB5092431
+3 earlier KB5087055, KB5082421, KB5074837
Replaced by
KB5127691
VersionFile versionChannel
3.5 General availability
4.8.1 4.8.9347.0 General availability

Security

2 vulnerabilities fixed
2 important
VulnerabilitySeverityCVSS
CVE-2026-69522
.NET and Visual Studio Remote Code Execution Vulnerability
 Important 8.8
CVE-2026-62886
.NET Elevation of Privilege Vulnerability
 Important 7.8

Downloads (Microsoft Update Catalog)

Arm64 Windows 11, version 26H1
58.5 MB
windows11.0-kb5126053-arm64-ndp481.msu Main update · KB5126053 SHA-1: b8ad7338…fa23e4

Updated components

93 files changed since the previous update for the same versions.

Top changed componentsFiles
WPF33
.NET runtime (CLR)18

Sources

Updated
Cited passages from Microsoft's official notes (3)
  1. 1. This security update addresses an elevation of privilege vulnerability detailed in CVE-2026-62886. support.microsoft.com
  2. 2. This security update addresses a remote code execution vulnerability detailed in CVE-2026-69522. support.microsoft.com
  3. 3. - Addresses an issue where intermittent crashes could occur in some 64-bit .NET Framework scenarios. support.microsoft.com
Checked automatically: 7 of 8 checks passed, 3 quoted passages · compared with Microsoft's page on Nov 8, 2026
  • Quotes found in the official pages: passed
  • Numbers found in the sources: passed
  • Vulnerabilities from the dossier only: passed
  • Links to allowed domains only: passed
  • Every official change covered: passed
  • Readable plain text: passed
  • Changes confirmed by the file list: passed
  • Sources agree: not passed

A second AI read the text against the sources: correctness 5/5, clarity 4/5, completeness 5/5. How we work

Published by The Update Bureau. Written with AI from Microsoft's own documents, checked by rules and a second AI. How we work · Report a mistake