Any user with an account on the main wiki could run scheduling operations on subwikis. To reproduce, as a user on the main wiki without any special right, view the document Scheduler.WebHome in a subwiki. Then, click on any operation (e.g., Trigger) on any job. If the operation is successful, then the instance is vulnerable.
By creating a document with a special crafted documented reference and an XWiki.SchedulerJobClass XObject, it is possible to execute arbitrary code on the server whenever an admin visits the scheduler page or the scheduler page is referenced, e.g., via an image in a comment on a page in the wiki.
It is possible to schedule/trigger/unschedule existing jobs by having an admin visit the Job Scheduler page through a predictable URL, for example by embedding such an URL in any content as an image. To reproduce in an XWiki installation, open <xwiki-host>:/xwiki/bin/view/Scheduler/?do=trigger&which=Scheduler.NotificationEmailDailySender as a user with admin rights. If there is no error message that indicates the CSRF token is invalid, the installation is vulnerable.
XWiki Platform is a generic wiki platform offering runtime services for applications built on top of it. It's possible to execute anything with the right of the Scheduler Application sheet page. A user without script or programming rights, edit your user profile with the object editor and add a new object of type XWiki.SchedulerJobClass, In "Job Script", groovy code can be added and will be executed in the server context …