A security feature bypass exists when Windows incorrectly validates CAB file signatures. An attacker who successfully exploited this vulnerability could inject code into a CAB file without invalidating the file's signature. To exploit the vulnerability, an attacker could modify a signed CAB file and inject malicious code. The attacker could then convince a target user to execute the file. The update addresses the vulnerability by correcting how Windows validates file signatures.
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2019-1163