An information disclosure vulnerability exists when the Windows GDI component improperly discloses the contents of its memory. An attacker who successfully exploited the vulnerability could obtain information to further compromise a user’s system. There are multiple ways an attacker could exploit the vulnerability, such as by convincing a user to open a specially crafted document or by convincing a user to visit an untrusted webpage. The update addresses the vulnerability by correcting how the Windows GDI component handles objects in memory.
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2019-1158