When document names are validated according to a name strategy (disabled by default), XWiki is vulnerable to a reflected XSS attack in the page creation form. To reproduce, make sure that "Validate names before saving" is enabled in the administration under "Editing" -> "Name strategies" and then open <xwiki-host>/xwiki/bin/create/Main/%3Cscript%3Ealert%28%27Test%20Test%20Test%20Test%20Test%27%29%3C%2Fscript%3E where <xwiki-host> is the URL of your XWiki installation. This displays an alert if the installation is vulnerable. This allows an attacker to execute arbitrary actions with the rights of the user opening the malicious link. Depending on the rights of the user, this may allow remote code execution and full read and write access to the whole XWiki installation.
Patches
This has been patched in XWiki 14.10.12 and 15.5-RC-1 by adding appropriate escaping.
Workarounds
The vulnerable template file createinline.vm is part of XWiki's WAR and can be patched by manually applying the changes from the fix.