CVE-2026-6279: Avada Builder Unauthenticated RCE via PHP Function Injection

Executive Summary

A critical remote code execution vulnerability (CVE-2026-6279) has been discovered in the Avada Builder (fusion-builder) plugin for WordPress, one of the most widely used page builder plugins powering the Avada theme ecosystem. The flaw carries a CVSS score of 9.8 and affects all versions up to and including 3.15.2.

The vulnerability resides in the wp_conditional_tags case within Fusion_Builder_Conditional_Render_Helper::get_value(), where attacker-controlled input is passed directly into a PHP function call without proper sanitization. An unauthenticated attacker can exploit this to execute arbitrary PHP code on the affected server.

All sites using Avada Builder versions 3.15.2 or earlier should update immediately.

Vulnerability Overview

Affected Versions

Technical Analysis

Root Cause

The vulnerability exists in the Fusion_Builder_Conditional_Render_Helper::get_value() function, specifically in the handling of the wp_conditional_tags case. When processing shortcode attributes for conditional rendering, the method accepts a function name from attacker-controlled input and invokes it via PHP's dynamic function calling mechanism — without validating that the supplied value is a legitimate, allow-listed callback.

This constitutes a PHP Function Injection vulnerability: an attacker can supply any callable PHP function or user-defined function as the tag name, causing arbitrary code execution in the context of the web server.

Attack Flow

1. Attacker identifies a WordPress site running Avada Builder <= 3.15.2
2. Attacker crafts a malicious HTTP request targeting a public-facing endpoint
   that invokes the fusion-builder shortcode renderer
3. Attacker injects an arbitrary PHP function name (e.g., system, exec, passthru)
   as the wp_conditional_tags value
4. get_value() passes the input directly to a PHP call_user_func() or equivalent
5. The server executes the attacker's chosen PHP function with attacker-supplied arguments
6. Attacker achieves Remote Code Execution (RCE) on the server

Exploitation Conditions

• Avada Builder version 3.15.2 or earlier must be installed and active
• No authentication is required — the vulnerable endpoint is accessible to unauthenticated users
• PHP functions like system, exec, passthru, or shell_exec must not be individually disabled via disable_functions in php.ini (though alternative exploitation paths may exist even with restrictions)

Impact Assessment

Immediate Remediation

Step 1: Update Avada Builder to 3.15.3 or Later

Update via the WordPress admin panel:

Plugins > Installed Plugins > Avada Builder > Update Now

Or via WP-CLI:

# Update fusion-builder plugin
wp plugin update fusion-builder
 
# Verify the installed version
wp plugin get fusion-builder --field=version

If using the Avada theme package, update the full theme, which bundles the plugin.

Step 2: Audit for Exploitation Indicators

# Check for recently modified PHP files in the uploads directory (common webshell target)
find /var/www/html/wp-content/uploads/ -name "*.php" -type f
 
# Scan for recently modified files across the WordPress installation
find /var/www/html/ -name "*.php" -newer /var/www/html/wp-config.php -not -path "*/cache/*" -type f
 
# Review recent WordPress error logs for suspicious function calls
grep -i "system\|exec\|passthru\|shell_exec\|eval" /var/log/apache2/error.log

Step 3: Harden PHP Configuration

If immediate patching is not possible, disable dangerous PHP functions:

# php.ini — add to restrict dangerous function execution
disable_functions = system,exec,shell_exec,passthru,popen,proc_open,eval,assert

Restart the web server after applying changes.

Step 4: Deploy a Web Application Firewall

Temporarily block exploitation attempts with a WAF rule targeting the vulnerable parameter pattern. Wordfence, Cloudflare WAF, and Sucuri all offer WordPress-specific rule sets that should detect PHP Function Injection attempts.

Detection Indicators

Post-Remediation Checklist

1. Update Avada Builder to version 3.15.3 or later
2. Audit wp-content/uploads/ for PHP files — remove any found
3. Review recently modified PHP files site-wide
4. Check WordPress user accounts for unauthorized admins
5. Reset WordPress secret keys and all admin passwords
6. Invalidate active sessions to force re-authentication
7. Enable a Web Application Firewall with WordPress rule sets
8. Restrict PHP dangerous functions via disable_functions as defense-in-depth
9. Monitor access logs for continued exploitation attempts
10. Verify database integrity for unauthorized content modifications

References

• NVD — CVE-2026-6279
• Wordfence — Avada Builder Plugin Vulnerability