Microsoft has released Out-of-Band security update KB4578013 (https://support.microsoft.com/en-us/help/4578013/security-update-for-windows-8-1-rt-8-1-and-server-2012-r2)* addressing CVE-2020-1530 (https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-1530) and CVE-2020-1537 (https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-1537) which could allow attackers to gain elevated privileges after successful exploitation.
There are no prerequisites for the update and a restart is not required. To get the standalone package for this update, go to the Microsoft Update Catalog (http://www.catalog.update.microsoft.com/Search.aspx?q=KB4578013) website.
Note: The August 11, 2020 security update addressed the vulnerabilities for all other supported operating systems.
Windows Update History: Windows 8.1 (https://support.microsoft.com/en-us/help/4009470/windows-8-1-windows-server-2012-r2-update-history)
*Edit Note: KB Corrected
I just installed the update on my Win8.1 HP Stream7 tablet, and (at least, in my case) a reboot WAS required.
EDIT: I see that KB4578013 was apparently installed at the same time... perhaps that's what required the reboot?
No, I inadvertently used the link from the Windows Update History page and edited my original post. KB4578013 is the correct KB number and that pages states a restart isn't required.
KB article: "You do not have to restart your device after you apply this update."
WU: "Restart your PC to finish installing updates"
On two different server installations (I've installed it on so far). This is the only update I applied. <sigh />