GHSA-vxwr-wpjv-qjq7

### 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