An information disclosure vulnerability exists on ARM implementations that use speculative execution in control flow via a side-channel analysis, aka "straight-line speculation." To exploit this vulnerability, an attacker with local privileges would need to run a specially crafted application. The security update addresses the vulnerability by bypassing the speculative execution.
https://www.tenable.com/blog/microsoft-s-august-2020-patch-tuesday-addresses-120-cves-cve-2020-1337
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-1459