XWiki Platform is a generic wiki platform. Prior to 15.10.14, 16.4.6, and 16.10.0-rc-1, it's possible for an user to get access to private information through the REST API - but could also be through another API - when a sub wiki is using "Prevent unregistered users to view pages". The vulnerability only affects subwikis, and it only concerns specific right options such as "Prevent unregistered users to view pages". or "Prevent unregistered users to edit pages". It's possible to detect the vulnerability by enabling "Prevent unregistered users to view pages" and then trying to access a page through the REST API without using any credentials. The vulnerability has been patched in XWiki 15.10.14, 16.4.6 and 16.10.0RC1.
XWiki Platform is a generic wiki platform. Prior to 15.10.14, 16.4.6, and 16.10.0-rc-1, protected pages are listed when requesting the REST endpoints /rest/wikis/[wikiName]/pages even if the user doesn't have view rights on them. It's particularly true if the entire wiki is protected with "Prevent unregistered user to view pages": the endpoint would still list the pages of the wiki, though only for the main wiki. The problem has been patched in XWiki 15.10.14, 16.4.6, 16.10.0RC1. In those versions the endpoint can still be requested but the result is filtered out based on pages rights.
XWiki Platform is a generic wiki platform. Prior to 15.10.15, 16.4.6, and 16.10.0, any user can exploit the WikiManager REST API to create a new wiki, where the user could become an administrator and so performs other attacks on the farm. Note that this REST API is not bundled in XWiki Standard by default: it needs to be installed manually through the extension manager. The problem has been patched in versions 15.10.15, 16.4.6 and 16.10.0 of the REST module.