Several changes were applied to improve security. See ADV-2017-11 through ADV-2017-15 in Security Bulletins for more details.
Installation Guide
Install and Configure Tableau Server
Planning to upgrade?
Tableau Server Upgrade Page
Release Notes for Tableau 9.0.23
Note: Tableau Desktop customers in environments with Product Updates enabled will be prompted to upgrade a few days after a maintenance release is made available. For more information, see the Tableau Knowledge Base.