Microsoft Exchange Server 2016 Cumulative Update 18

A spoofing vulnerability exists in Microsoft Exchange Server which could result in an attack that would allow a malicious actor to impersonate the user.

This update addresses this vulnerability.

To prevent these types of attacks, Microsoft recommends customers to download inline images from different DNSdomains than the rest of OWA. Please see further instructions in the FAQ to put in place this mitigations.

An information disclosure vulnerability exists in how Microsoft Exchange validates tokens when handling certain messages. An attacker who successfully exploited the vulnerability could use this to gain further information from a user.

To exploit the vulnerability, an attacker could include specially crafted OWA messages that could be loaded, without warning or filtering, from the attacker-controlled URL. This callback vector provides an information disclosure tactic used in web beacons and other types of tracking systems.

The security update corrects the way that Exchange handles these token validations.