The vulnerability exists due to insufficient sanitization of user-supplied data when adding contacts from specially crafted emails. A remote attacker can trick the victim into adding a contact from a specially crafted email and execute arbitrary HTML and script code in user's browser in context of vulnerable website.
https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy
https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P41#Security_Fixes
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.1#Security_Fixes
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.9#Security_Fixes