XmlScanner bypass leads to XXE

Summary

The XmlScanner class has a scan method which should prevent XXE attacks.

However, the regexes used in the scan method and the findCharSet method can be bypassed by using UCS-4 and encoding guessing as described in https://www.w3.org/TR/xml/#sec-guessing-no-ext-info.

Details

The scan method converts the input in the UTF-8 encoding if it is not already in the UTF-8 encoding with the toUtf8 method. Then, the scan method uses a regex which would also work with 16-bit encoding.

However, the regexes from the findCharSet method, which is used for determining the current encoding can be bypassed by using an encoding which has more than 8 bits, since the regex does not expect null bytes, and the XML library will also autodetect the encoding as described in https://www.w3.org/TR/xml/#sec-guessing-no-ext-info.

A payload for the workbook.xml file can for example be created with CyberChef. If you open an Excel file containing the payload from the link above stored in the workbook.xml file with PhpSpreadsheet, you will receive an HTTP request on 127.0.0.1:12345. You can test that an HTTP request is created by running the nc -nlvp 12345 command before opening the file containing the payload with PhpSpreadsheet.

PoC

• Create a new folder.
• Run the composer require phpoffice/phpspreadsheet command in the new folder.
• Create an index.php file in that folder with the following content:

<?php
require 'vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Writer\Xlsx;

$spreadsheet = new Spreadsheet();

$inputFileType = 'Xlsx';
$inputFileName = './payload.xlsx';

/**  Create a new Reader of the type defined in $inputFileType  **/
$reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReader($inputFileType);
/**  Advise the Reader that we only want to load cell data  **/
$reader->setReadDataOnly(true);

$worksheetData = $reader->listWorksheetInfo($inputFileName);

foreach ($worksheetData as $worksheet) {

$sheetName = $worksheet['worksheetName'];

echo "<h4>$sheetName</h4>";
/**  Load $inputFileName to a Spreadsheet Object  **/
$reader->setLoadSheetsOnly($sheetName);
$spreadsheet = $reader->load($inputFileName);

$worksheet = $spreadsheet->getActiveSheet();
print_r($worksheet->toArray());

}

• Run the following command: php -S 127.0.0.1:8080
• Add the payload.xlsx file, which contains a payload similar to the payload from the details section, but with the URL https://webhook.site/65744200-63d2-43a2-a6a0-cca8d6b0d50a instead of the http://127.0.0.1:12345/ext.dtd URL, in the folder and open https://127.0.0.1:8080 in a browser. You will see an HTTP request on https://webhook.site/#!/view/65744200-63d2-43a2-a6a0-cca8d6b0d50a.

Impact

An attacker can bypass the sanitizer and achieve an XXE attack.