GHSA-wgmf-q9vr-vww6

### Summary

`\PhpOffice\PhpSpreadsheet\Writer\Html` doesn't sanitize spreadsheet styling information such as font names, allowing an attacker to inject arbitrary JavaScript on the page.

### PoC

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

Full takeover of the session of users viewing spreadsheet files as HTML.