Microsoft released the update KB5020387 to Windows 11 in an out-of–band update to fix a critical vulnerability. The company also released the update KB5020435 to Windows 10 21H2 and 21H1 HTML1_, respectively, at the same time.
Update KB5020387 increases the version number of Windows 11 to build 22000.1100. The update KB5020435 raises the version to 19042.2132 (20H2) and 19043.2132 (2H1) for Windows 10.
The announcement ( Link 2) states that it only addresses an issue that may affect certain types of Secure Sockets Layer and Transport Layer Security connections that might experience handshake failures. Developers will most likely see the affected connections receive one or more records, followed by a partial record that is less than five bytes in a single input buffer. If the connection fails, the app will receive the error “SEC_E_ILLEGAL_MESSAGE.”
This problem affects only the original Windows 11 (version 21H2), and not version 22H2 or the three most recent Windows 10.
Update KB5020387 available for Windows 11
You can download the update immediately and it will automatically install. You can force an update by going to Settings> Windows update. Click the Check For Updates button and choose the option to download the update. The Settings> Security Windows 10 Update page.