A spoofing vulnerability exists when Office Online does not validate origin in cross-origin communications handlers correctly. An attacker could exploit the vulnerability by sending a specially crafted request to an affected site.
The attacker who successfully exploited the vulnerability could then perform cross-origin attacks on affected systems. These attacks could allow the attacker to read content that the attacker is not authorized to read, and use the victim's identity to take actions on the site on behalf of the victim. The victim needs to be authenticated for an attacker to compromise the victim.
The security update addresses the vulnerability by ensuring that Office Online properly validates origins.