Microsoft releases out-of-band KB5020387 update to fix TLS handshake issues in Windows 11

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It may be the middle of the month, but that hasn't stopped Microsoft releasing an important update for various versions of Windows, including the KB5020387 update for Windows 11.

The updates fix a TLS handshake issue that causes SEC_E_ILLEGAL_MESSAGE errors. In all, there are seven non-security updates available for Windows, Windows 8.1, Windows 10, Windows 11, Windows Server 2012 and Windows Server 2022.

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Despite the fact that this will be regarded by many as an important update -- albeit one unrelated to security -- Microsoft has opted to not push it out via Windows Update. Instead, those affected by the problems it addresses will have to manually download the update, and you will find the necessary links below.

Microsoft says of the update:

We address an issue that might affect some types of Secure Sockets Layer (SSL) and Transport Layer Security (TLS) connections. These connections might have handshake failures. For developers, the affected connections are likely to receive one or more records followed by a partial record with a size of less than 5 bytes within a single input buffer. If the connection fails, your app will receive the error, "SEC_E_ILLEGAL_MESSAGE".

Here are the links you need to grab the update from the Microsoft Update catalog:

  • Windows 11, version 21H2: KB5020387
  • Windows 10, version 20H2; Windows 10, version 21H1; Windows 10, version 22H1; Windows 10 Enterprise LTSC 2021: KB5020435
  • Windows Server 2022: KB5020436
  • Windows 10 Enterprise LTSC 2019; Windows Server 2019: KB5020438
  • Windows 8.1; Windows Server 2012 R2: KB5020447
  • Windows 7 SP1; Windows Server 2008 R2 SP1: KB5020448
  • Windows Server 2012: KB5020449

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