PhpSpreadsheet allows bypass XSS sanitizer using the javascript protocol and special characters

Bypass XSS sanitizer using the javascript protocol and special characters

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: an attacker can use special characters, so that the library processes the javascript protocol with special characters and generates an HTML link Impact: executing arbitrary JavaScript code in the browser Vulnerable component: class PhpOffice\PhpSpreadsheet\Writer\Html, method generateRow 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 Bypass XSS sanitizer using the javascript protocol and special characters 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 6. 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 use special characters so that this library processes the javascript protocol with special characters and generates a HTML link. The Excel file is unpacked and a hyperlink in the file is inserted into the xl/worksheets/sheet1.xml file.

!fig11

*Figure 11. Using the javascript protocol with special characters*

Some payloads help bypass the security system and carry out a XSS attack.

*Listing 7. HTML form that demonstrates the exploitation of the XSS vulnerability*

jav	ascript:alert()
jav
ascript:alert()
jav
ascript:alert()

It's clear that the javascript protocol with special characters is used.

!fig12

*Figure 12. Using the javascript protocol with special characters*

Due to the special characters, the execution stream ends up on line 1543, and the link is built in HTML form with the javascript protocol.

<img width="373" alt="fig13" src="https://github.com/user-attachments/assets/3ca0c3c6-daa9-4502-ad9e-b803f308fd26" />

*Figure 13. Executing arbitrary JavaScript code*

Credit

This vulnerability was discovered by Aleksey Solovev (Positive Technologies)