An elevation of privilege vulnerability exists in the Microsoft XmlDocument class that could allow an attacker to escape from the AppContainer sandbox in the browser. An attacker who successfully exploited this vulnerability could gain elevated privileges and break out of the Edge AppContainer sandbox.
The vulnerability by itself does not allow arbitrary code to run. However, this vulnerability could be used in conjunction with one or more vulnerabilities (for example a remote code execution vulnerability and another elevation of privilege vulnerability) to take advantage of the elevated privileges when running.
The security update addresses the vulnerability by modifying how the Microsoft XmlDocument class enforces sandboxing.