Microsoft made available update KB4091666 (opens in new tab) that includes microcode updates to mitigate Spectre Variant 2 on additional Intel processors. This expands protection coverage achievable with Microsoft updates to include most consumer Skylake, Broadwell, and Haswell processors. We explained the difference between the Microsoft-provided, in-Windows microcode updates, and Intel’s BIOS-applied microcode updates in this article.
In short, if either you don’t want to update the BIOS of your device or no BIOS update was created for your device, you can apply Microsoft’s Windows update to achieve equivalent protection within Windows. However, if you boot into another OS or reinstall Windows without re-applying the patch, you will lose protection.
Applying the microcode patch for your device via a BIOS patch is the preferred method because it gives your device permanent and universal protection. Intel concluded its patching efforts earlier this month, but it could take significantly longer for Intel’s partners to deploy BIOS updates for all their devices.
There should be no harm to installing Microsoft’s patch and then later applying a BIOS patch.