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

Summary

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\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

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See https://github.com/advisories/GHSA-wgmf-q9vr-vww6

PoC

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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. 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.