KB5126052: Cumulative update for .NET Framework, versions 3.5 and 4.8.1, on Windows 11 24H2/25H2/26H2 and Windows Server 2025, September 2026. Two .NET vulnerabilities addressed; 64-bit crashes targeted

The update addresses two Important .NET Framework vulnerabilities and an issue that could cause intermittent crashes in some 64-bit .NET Framework scenarios.

Does it affect you? Applies to: Devices running Windows 11, versions 24H2, 25H2, or 26H2, and Windows Server 2025, with .NET Framework 3.5 or 4.8.1 installed.

Review notes Additional details from the official release notes (1) Read note

Does it affect you? The official KB page lists Windows 11 versions 24H2 and 25H2, and Microsoft server operating system 24H2, as the products for this update.

The official sources are linked at the end of the page.

What changes

Fixes 3 changes

.NET Framework

2 items

For: Devices with .NET Framework 3.5 or 4.8.1

Addresses an elevation-of-privilege vulnerability in .NET Framework.¹
CVE-2026-62886.
Addresses a remote-code-execution vulnerability in .NET Framework.²
CVE-2026-69522.

NET Fundamentals

Addresses an issue that could cause intermittent crashes in some 64-bit .NET Framework scenarios.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: KB5122385, KB5120708, KB5120710, KB5100998

+21 earlier
KB5101001, KB5092427, KB5092430, KB5087051, KB5087054, KB5082417, KB5082420, KB5074831, KB5074828, KB5067931, KB5066128, KB5066131, KB5066613, KB5064401, KB5056579, KB5054979, KB5050577, KB5049622, KB5048162, KB5045934, KB5044030
Replaced by: KB5127690

Version	File version	Channel
3.5	2.0.50727.9183	General availability
4.8.1	4.8.9347.0	General availability

Security

2 vulnerabilities fixed

2 important

Vulnerability	Severity	CVSS
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)

x64 Windows Server 2025 / Windows 11, version 24H2 / Windows 11, version 25H2 / Windows 11, version 26H2

92.3 MB

windows11.0-kb5126052-x64-ndp481.msu Main update · KB5126052 SHA-1: 082cfd58…fc32a8

Arm64 Windows Server 2025 / Windows 11, version 24H2 / Windows 11, version 25H2 / Windows 11, version 26H2

110.6 MB

windows11.0-kb5126052-arm64-ndp481.msu Main update · KB5126052 SHA-1: 5678046e…7839f7

Updated components

1,604 files changed since the previous update for the same versions.

Top changed components	Files
ASP.NET	60
WPF	50
WCF	46
.NET networking	28
.NET data access	22
.NET runtime (CLR)	20
Windows Forms	8

Sources

Microsoft Support: KB5126052

Updated Oct 26, 2026

Cited passages from Microsoft's official notes (3)

1. This security update addresses an elevation of privilege vulnerability detailed in CVE-2026-62886. support.microsoft.com
2. This security update addresses a remote code execution vulnerability detailed in CVE-2026-69522. support.microsoft.com
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 3/5, 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