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Windows 10 Build 19041.21 (Slow Ring)

Microsoft is releasing a new build to Insiders in the Slow ring. It doesn't include anything new besides a security fix for a critical vulnerability recently discovered in Windows 10.

The new build is released in form of a cumulative update with the patch number KB4535550. The change log mentions security updates to Windows App Platform and Frameworks, Windows Input and Composition, Windows Graphics, Microsoft Scripting Engine, .NET Framework, Windows Cryptography, Windows Subsystem for Linux, Windows Peripherals, Windows Storage and Filesystems, and Windows Server.

Here are some important details related this update:


A spoofing vulnerability exists in the way Windows CryptoAPI (Crypt32.dll) validates Elliptic Curve Cryptography (ECC) certificates.

An attacker could exploit the vulnerability by using a spoofed code-signing certificate to sign a malicious executable, making it appear the file was from a trusted, legitimate source. The user would have no way of knowing the file was malicious, because the digital signature would appear to be from a trusted provider.

A successful exploit could also allow the attacker to conduct man-in-the-middle attacks and decrypt confidential information on user connections to the affected software.

The security update addresses the vulnerability by ensuring that Windows CryptoAPI completely validates ECC certificates.

Also, check out the following document.

If you have configured your device to receive updates from the Slow Ring ring, open Settings - > Update & recovery and click on the Check for Updates button on the right. This will install the latest available Insider Preview of Windows 10.

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2 thoughts on “Windows 10 Build 19041.21 (Slow Ring)

  1. Doug Wallace

    Are you aware of any network connectivity issues with build 19041.21. Shortly after installing it, I have an error that I can’t resolve. The error reads ‘you do not have permission to access…’, and the machine I’m trying to reach is running windows 10 1909 (latest build). Any thoughts would be appreciated.



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