An elevation of privilege vulnerability exists when Windows Core Shell COM Server Registrar improperly handles COM calls. An attacker who successfully exploited this vulnerability could potentially set certain items to run at a higher level and thereby elevate permissions. To exploit this vulnerability, an attacker would first have to log on to the system. An attacker could then run a specially crafted application that could exploit the vulnerability and take control of an affected system. The update addresses this vulnerability by correcting unprotected COM calls.
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2019-1184