A remote code execution vulnerability exists in Microsoft Word software when it fails to properly handle objects in memory. An attacker who successfully exploited the vulnerability could use a specially crafted file to perform actions in the security context of the current user. The file could then take actions on behalf of the logged-on user with the same permissions as the current user.
To exploit the vulnerability, a user must open a specially crafted file with an affected version of Microsoft Word software.
Two possible email attack scenarios exist for this vulnerability:

With the first email attack scenario, an attacker could send a specially crafted email message to the user and wait for the user to click on the message. When the message renders via Microsoft Word in the Outlook Preview Pane, an attack could be triggered.
With the second scenario, an attacker could attach a specially crafted file to an email, send it to a user, and convince them to open it.

In a web-based attack scenario, an attacker could host a website (or leverage a compromised website that accepts or hosts user-provided content) that contains a specially crafted file that is designed to exploit the vulnerability. However, an attacker would have no way to force the user to visit the website. Instead, an attacker would have to convince the user to click a link, typically by way of an enticement in an email or other message, and then convince the user to open the specially crafted file.
The security update addresses the vulnerability by correcting how Microsoft Word handles files in memory.
For users who view their emails in Outlook, the Preview Pane attack vector can be mitigated by disabling this feature. The following registry keys can be set to disable the Preview Pane in Outlook on Windows, either via manual editing of the registry or by modifying Group Policy.
Note Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or view the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe.
Outlook 2010:

HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Options
DWORD: DisableReadingPane
Value: 1

Outlook 2013:

HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Options
DWORD: DisableReadingPane
Value: 1

Outlook 2016, Outlook 2019, and Office 365 ProPlus:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options
DWORD: DisableReadingPane
Value: 1