An elevation of privilege vulnerability exists in the way that the rpcss.dll handles objects in memory. An attacker who successfully exploited the vulnerability could execute code with elevated permissions. To exploit the vulnerability, a locally authenticated attacker could run a specially crafted application. The security update addresses the vulnerability by ensuring the rpcss.dll properly handles objects in memory.
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2019-1177