XWiki Platform: Privilege escalation (PR) from user registration through PDFClass

Impact

Remote code execution is possible via PDF export templates.
To reproduce on an installation, register a new user account with username PDFClass if XWiki.PDFClass does not exist.
On XWiki.PDFClass, use the class editor to add a "style" property of type "TextArea" and content type "Plain Text".
Then, add an object of class PDFClass and set the "style" attribute to $services.logging.getLogger('PDFClass').error("I got programming: $services.security.authorization.hasAccess('programming')").
Finally, go to <host>/xwiki/bin/export/Main/WebHome?format=pdf&pdftemplate=XWiki.PDFClass. If the logs contain "ERROR PDFClass - I got programming: true", the instance is vulnerable.

Patches

This vulnerability has been patched in XWiki 14.10.20, 15.5.4 and 15.10-rc-1.

Workarounds

If PDF templates are not typically used on the instance, an administrator can create the document XWiki.PDFClass and block its edition, after making sure that it does not contain a style attribute.
Otherwise, the instance needs to be updated.

References

• https://jira.xwiki.org/browse/XWIKI-21337
• https://github.com/xwiki/xwiki-platform/commit/d28e21a670c69880b951e415dd2ddd69d273eae9