💣 XWiki Platform XSS with edit right in the create document form for existing pages (xwiki xwiki-platform)

Description

XWiki Platform is a generic wiki platform offering runtime services for applications built on top of it. `org.xwiki.platform:xwiki-platform-web` starting in version 3.1-milestone-2 and prior to version 13.4-rc-1, as well as `org.xwiki.platform:xwiki-platform-web-templates` prior to versions 14.10.12 and 15.5-rc-1, are vulnerable to cross-site scripting. When trying to create a document that already exists, XWiki displays an error message in the form for creating it. Due to missing escaping, this error message is vulnerable to raw HTML injection and thus XSS. The injected code is the document reference of the existing document so this requires that the attacker first creates a non-empty document whose name contains the attack code. This has been patched in `org.xwiki.platform:xwiki-platform-web` version 13.4-rc-1 and `org.xwiki.platform:xwiki-platform-web-templates` versions 14.10.12 and 15.5-rc-1 by adding the appropriate escaping. The vulnerable template file `createinline.vm` is part of XWiki's WAR and can be patched by manually applying the changes from the fix.