An elevation of privilege vulnerability exists when reparse points are created by sandboxed processes allowing sandbox escape. An attacker who successfully exploited the vulnerability could use the sandbox escape to elevate privileges on an affected system.
To exploit the vulnerability, an attacker would first have to log on to the system, and then run a specially crafted application to take control over the affected system.
The security update addresses the vulnerability by preventing sandboxed processes from creating reparse points targeting inaccessible files.