Any user who can edit a page in XWiki can use Live Data's edit REST API in XWiki to change the rights on that page. This allows the user to obtain script right on the page. Script right allows the user to execute potentially dangerous Velocity scripts and send unfiltered HTML and JavaScript to the client. If there are other security checks, e.g., in extensions, implemented as listeners to UserUpdatingDocumentEvent and similar User… events, these checks can be circumvented, too.

This vulnerability has been patched by properly calling all checks in Live Data in XWiki 16.10.17, 17.4.10, 17.10.4 and 18.1.0.

We're not aware of any workarounds apart from upgrading.