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.