Microsoft has released new Intel microcode updates (KB4346084) for Windows 10 version 1803 (Windows 10 April 2018 Update) and Windows Server Version 1803 (Server Core).
The microcode updates fix multiple vulnerabilities to include CVE-2018-3640 (Spectre Variant 3a), CVE-2018-3639 (Spectre Variant 4), as well as CVE-2018-3615 and CVE-2018-3646 (Foreshadow vulnerabilities).
See the full Microsoft advisory for details on microcode updates for Intel processors.