Outlook in Microsoft Office for Mac 2008 before 12.3.6 and Office for Mac 2011 before 14.3.2 allows remote attackers to trigger access to a remote URL and consequently confirm the rendering of an HTML e-mail message by including unspecified HTML5 elements and leveraging the installation of a WebKit browser on the victim's machine, aka "Unintended Content Loading Vulnerability."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A16082
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2013/ms13-026