A remote code execution vulnerability exists in the way that the MSHTML engine inproperly validates input.
An attacker could execute arbitrary code in the context of the current user. If the current user is logged on with administrative user rights, an attacker who successfully exploited the vulnerability could take control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.
In a HTML editing attack scenario, an attacker could trick a user into editing a specially crafted file that is designed to exploit the vulnerability.
The security update addresses the vulnerability by modifying how MSHTML engine validates input.