XWiki Platform is a generic wiki platform. A vulnerability in versions from 5.0 to 16.7.1 affects users with Message Stream enabled and a wiki configured as closed from selecting "Prevent unregistered users to view pages" in the Administrations Rights. The vulnerability is that any message sent in a subwiki to "everyone" is actually sent to the farm: any visitor of the main wiki will be able to see that message through the Dashboard, even if the subwiki is configured to be private. This issue will not be patched as Message Stream has been deprecated in XWiki 16.8.0RC1 and is not maintained anymore. A workaround for this issue involves keeping Message Stream disabled by default. It's advised to keep it disabled from Administration > Social > Message Stream.