XWiki Platform is a generic wiki platform offering runtime services for applications built on top of it. Any user who can edit their own user profile and notification settings can execute arbitrary script macros including Groovy and Python macros that allow remote code execution including unrestricted read and write access to all wiki contents. This has been patched in XWiki 14.10.6 and 15.2RC1. Users are advised to update. As a workaround the main security fix can be manually applied by patching the affected document `XWiki.Notifications.Code.NotificationRSSService`. This will break the link to the differences, though as this requires additional changes to Velocity templates as shown in the patch. While the default template is available in the instance and can be easily patched, the template for mentions is contained in a `.jar`-file and thus cannot be fixed without replacing that jar.
The product constructs all or part of a command, data structure, or record using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify how it is parsed or interpreted when it is sent to a downstream component.