Summary

\PhpOffice\PhpSpreadsheet\Writer\Html does not sanitize "javascript:" URLs from hyperlink href attributes, resulting in a Cross-Site Scripting vulnerability.

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

Open index.php in a web browser and click on both links. The first demonstrates the vulnerability in a regular hyperlink and the second in a HYPERLINK() formula.