CVE-2026-4882: Unauthenticated File Upload in WordPress User Registration Advanced Fields

Executive Summary

A critical arbitrary file upload vulnerability (CVE-2026-4882) has been disclosed in the User Registration Advanced Fields plugin for WordPress, affecting all versions up to and including 1.6.20. The flaw carries a CVSS score of 9.8 and requires no authentication to exploit.

The vulnerability stems from missing file type validation in the URAF_AJAX::method_upload function. An unauthenticated attacker can upload arbitrary files — including PHP webshells — directly to the web server, potentially achieving full remote code execution.

Site owners running affected versions should update immediately or disable file upload functionality until a patch is applied.

Vulnerability Overview

Affected Versions

Technical Analysis

Root Cause

The User Registration Advanced Fields plugin provides enhanced field types for WordPress user registration forms, including file upload fields. The upload handler is exposed via an AJAX action that processes requests through the URAF_AJAX::method_upload function.

The critical flaw: the function performs no validation on the file type or extension of uploaded files. There is no allowlist of safe MIME types, no blocklist of dangerous extensions (.php, .phtml, .php5, etc.), and no use of WordPress core file type checking functions such as wp_check_filetype_and_ext().

An attacker can therefore:

1. Identify a WordPress site running the vulnerable plugin version
2. Send a crafted AJAX POST request without any authentication tokens
3. Upload a PHP file (e.g., a webshell or reverse shell) to the server's upload directory
4. Access the uploaded file via a direct URL to achieve remote code execution

Attack Flow

1. Attacker identifies WordPress target with User Registration Advanced Fields <= 1.6.20
2. Attacker crafts multipart/form-data POST request:
   - action: uraf_upload (or equivalent AJAX action name)
   - file: malicious.php (PHP webshell content)
   - No nonce, authentication cookie, or capability check required
3. Server receives request, passes to URAF_AJAX::method_upload
4. Function saves file to wp-content/uploads/ (or configured path)
5. Attacker requests the uploaded PHP file via browser/curl
6. PHP webshell executes with web server privileges (www-data / apache)
7. Attacker achieves remote code execution on the hosting server

Exploitation Conditions

• User Registration Advanced Fields version 1.6.20 or earlier must be installed and active
• No user interaction, authentication, or privileges required
• WordPress must have the AJAX endpoint accessible (standard on all WordPress installs)
• The web server must execute PHP files in the uploads directory (common in shared hosting environments)

Impact Assessment

Immediate Remediation

Step 1: Update the Plugin

Update User Registration Advanced Fields to a version beyond 1.6.20 through the WordPress admin:

# Via WP-CLI
wp plugin update user-registration-advanced-fields
 
# Verify installed version
wp plugin get user-registration-advanced-fields --field=version

Or navigate to WordPress Admin > Plugins > Installed Plugins and update the plugin.

Step 2: Disable the Plugin if Update Is Not Immediately Possible

# Via WP-CLI
wp plugin deactivate user-registration-advanced-fields

Or navigate to WordPress Admin > Plugins > Installed Plugins and deactivate.

Step 3: Scan for Uploaded Webshells

# Search for recently uploaded PHP files in wp-content/uploads/
find /path/to/wordpress/wp-content/uploads/ -name "*.php" -type f
 
# Also check for common webshell extensions
find /path/to/wordpress/wp-content/uploads/ \( -name "*.php" -o -name "*.phtml" -o -name "*.php5" -o -name "*.phar" \) -type f
 
# Check file modification times (last 7 days)
find /path/to/wordpress/ -name "*.php" -newer /path/to/wordpress/wp-config.php -type f | grep uploads

Step 4: Block PHP Execution in Uploads Directory

Add to your server configuration or .htaccess in wp-content/uploads/:

# Block PHP execution in uploads directory
<Files "*.php">
    deny from all
</Files>
 
# Block additional dangerous extensions
<FilesMatch "\.(php|phtml|php3|php4|php5|php7|phar|shtml|pl|py|cgi)$">
    deny from all
</FilesMatch>

For Nginx:

location ~* /wp-content/uploads/.*\.php$ {
    deny all;
}

Detection Indicators

Post-Remediation Checklist

1. Update User Registration Advanced Fields beyond version 1.6.20
2. Scan wp-content/uploads/ for PHP files and remove any found
3. Block PHP execution in the uploads directory via server config or .htaccess
4. Review web server access logs for signs of prior exploitation
5. Audit all WordPress administrator accounts for unauthorized additions
6. Reset all credentials if compromise is suspected (DB passwords, wp-config.php salts)
7. Deploy a WAF with WordPress rulesets (Wordfence, Cloudflare, Sucuri)
8. Monitor file system for new PHP files in non-plugin directories

References

• NVD — CVE-2026-4882
• Wordfence — Vulnerability Database