XWiki Platform CSRF remote code execution through the realtime HTML Converter API

XWiki Platform is a generic wiki platform. Starting in version 13.9-rc-1 and prior to versions 4.10.19, 15.5.4, and 15.10-rc-1, when the realtime editor is installed in XWiki, it allows arbitrary remote code execution with the interaction of an admin user with programming right. More precisely, by getting an admin user to either visit a crafted URL or to view an image with this URL that could be in a comment, the attacker can get the admin to execute arbitrary XWiki syntax including scripting macros with Groovy or Python code. This compromises the confidentiality, integrity and availability of the whole XWiki installation. This vulnerability has been patched in XWiki 14.10.19, 15.5.4 and 15.9. As a workaround, one may update `RTFrontend.ConvertHTML` manually with the patch. This will, however, break some synchronization processes in the realtime editor, so upgrading should be the preferred way on installations where this editor is used.