Cross-Site Scripting (XSS) vulnerability in custom properties

Product: Phpspreadsheet Version: version 3.6.0 CWE-ID: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') CVSS vector v.3.1: 5.4 (AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N) CVSS vector v.4.0: 4.8 (AV:N/AC:L/AT:N/PR:L/UI:A/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N) Description: the HTML page is generated without clearing custom properties Impact: executing arbitrary JavaScript code in the browser Vulnerable component: class PhpOffice\PhpSpreadsheet\Writer\Html, method generateMeta Exploitation conditions: a user viewing a specially generated Excel file Mitigation: additional sanitization of special characters in a string Researcher: Aleksey Solovev (Positive Technologies)

Research

The researcher discovered zero-day vulnerability Cross-Site Scripting (XSS) vulnerability in custom properties in Phpspreadsheet. The following code is written on the server, which translates the XLSX file into a HTML representation and displays it in the response.

Listing 9. Source code on the server

<?php

require __DIR__ . '/vendor/autoload.php';

$inputFileName = './doc/Book1.xlsx';
$spreadsheet = \PhpOffice\PhpSpreadsheet\IOFactory::load($inputFileName);
$writer = new \PhpOffice\PhpSpreadsheet\Writer\Html($spreadsheet);
print($writer->generateHTMLAll());

An attacker can embed a payload in a file property that will result in the execution of arbitrary JavaScript code. The Excel file is unpacked and a custom property in the file is inserted into the docProps/custom.xml file.

Figure 17. Embedding the payload

After making the changes, a new archive with the xlsx extension was created. At the moment of converting the xlsx file into an HTML representation, a property is obtained that participates in the formation of a string without sanitization.

Figure 18. Getting a custom property

When calling the static generateMeta method, you can see that the key of the custom property is displayed without sanitization.

Figure 19. Getting a custom property

As a result, when viewing the excel file as the HTML representation, arbitrary JavaScript code will be executed.

Figure 20. Executing arbitrary JavaScript code

Credit

This vulnerability was discovered by Aleksey Solovev (Positive Technologies)