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

KB5126421

KB5126421: Cumulative update for .NET Framework, versions 3.5 and 4.8.1, on Windows 10 21H2/22H2, September 2026. Elevation-of-privilege and remote-code flaws addressed

In short

Addresses two Important .NET Framework vulnerabilities, including one that could let an attacker run code remotely. It also includes reliability improvements.

Does it affect you? Applies to: Devices running Windows 10, version 21H2 or 22H2, with .NET Framework 3.5 or 4.8.1 installed.

File version
2.0.50727.9183 · 4.8.9346.0
Security
2 fixed
Restart
Required

What changes

Fixes 2 changes

.NET Framework

2 items
  • The update addresses a flaw that could let an attacker gain higher privileges.1
    Addresses the elevation-of-privilege vulnerability CVE-2026-62886.
  • The update addresses a flaw that could let an attacker run code remotely.2
    Addresses the remote code execution vulnerability CVE-2026-69522.

Improvement 1 change

.NET Framework

  • Reliability improves for .NET Framework 3.5 and 4.8.1.3
    The update includes cumulative reliability improvements.

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
KB5120709, KB5101000
Replaced by
KB5127689
VersionFile versionChannel
3.5 2.0.50727.9183 General availability
4.8.1 4.8.9346.0 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)

x64 Windows 10 Enterprise LTSC 2021 / Windows 10, version 22H2
77.5 MB
windows10.0-kb5126421-x64-ndp481.msu Main update · KB5126421 SHA-1: 4b3c37c4…83deec
x86 Windows 10 Enterprise LTSC 2021 / Windows 10, version 22H2
48.9 MB
windows10.0-kb5126421-x86-ndp481.msu Main update · KB5126421 SHA-1: 13ffd151…35d558

Updated components

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

Top changed componentsFiles
WPF33

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. The September 8, 2026 update for Windows 10 Version 21H2 and Windows 10 Version 22H2 includes security and cumulative reliability improvements in .NET Framework 3.5 and 4.8.1. support.microsoft.com
Checked automatically: 8 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: passed

A second AI read the text against the sources: correctness 5/5, clarity 5/5, 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