solspace/craft-freeform Vulnerable to XSS in `PhpSpreadsheet` HTML Writer Due to Unsanitized Styling Data

Summary

### Summary _Short summary of the problem. Make the impact and severity as clear as possible. For example: An unsafe deserialization vulnerability allows any unauthenticated user to execute arbitrary code on the server._ \PhpOffice\PhpSpreadsheet\Writer\Html doesn't sanitize spreadsheet styling information such as font names, allowing an attacker to inject arbitrary JavaScript on the page. ### Details _Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer._ See https://github.com/advisories/GHSA-wgmf-q9vr-vww6 ### PoC _Complete instructions, including specific configuration details, to reproduce the vulnerability._ Example target script: ``` <?php require 'vendor/autoload.php'; $reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReader("Xlsx"); $spreadsheet = $reader->load(__DIR__ . '/book.xlsx'); $writer = new \PhpOffice\PhpSpreadsheet\Writer\Html($spreadsheet); print($writer->generateHTMLAll()); ``` Save this file in the same directory: [book.xlsx](https://github.com/PHPOffice/PhpSpreadsheet/files/15212797/book.xlsx) Open index.php in a web browser. An alert should be displayed. ### Impact _What kind of vulnerability is it? Who is impacted?_ Full takeover of the session of users viewing spreadsheet files as HTML.