On August 21st, Microsoft released an update to the May KB4100347 security update for Windows 10 and Windows Server 2016, which contain Intel CPU microcode updates to mitigate Spectre vulnerabilities. After installing this latest version, Windows users are reporting that they are unable to properly boot Windows 10 or are having performance issues. To further complicate the issue, this Intel CPU update is also being pushed to users of AMD processors.
It is not known what has been changed in the August 21st KB4100347 revision as Microsoft has not updated the support article since July 24th 2018. What we do know, is that since being offered via Windows update there have been numerous reports of Windows 10 machines no longer being able to boot, being stuck in a infinite automatic repair loops, or performance issues when playing music, using Chrome, or other applications.
According to a Reddit post, an administrator has seen these boot problems mostly affecting workstations using Xeon processors, but other reports indicate it is also causing issues in Intel i3, i5, and i7 processors as well.
Reddit post about the KB4100347 update
The above Reddit post also offers a method that can be used to remove this update using the Windows Recovery Environment on systems that are unable to boot up. For those who are able to boot into Windows and are having performance issues, you can uninstall the update and see if that helps.
To make this microcode update mess even more confusing, users who already installed the May update and then install the latest August version are seeing the KB4100347 update listed twice in the update history.
KB4100347 showing twice (Source: Microsoft Forums)
Users with AMD processors offered Intel microcode update
AMD users are not being left out of the fun. Even though the KB4100347 is designed for Intel processors, according to post on Reddit Windows Update is also installing the update on computers using AMD processors.
While it appears that AMD users are not suffering the same issues as those using Intel processors, it does appear that Microsoft made a mistake by pushing out the update to these users and those who are using unsupported Intel processors.
Bleeping Computer has contacted Microsoft with questions regarding this update, but have not heard back at the time of this publication.