CVE-2023-4879: Instantcms Stored XSS Vulnerability

CVE-2023-4879 Overview

CVE-2023-4879 is a Stored Cross-Site Scripting (XSS) vulnerability discovered in the InstantCMS content management system (icms2) affecting versions prior to 2.16.1.-git. This vulnerability exists in the admin panel where user-supplied input is not properly sanitized before being rendered in HTML output, allowing attackers with administrative privileges to inject malicious scripts that persist in the application and execute in the browsers of other users viewing the affected pages.

Critical Impact

Authenticated attackers with high privileges can inject persistent malicious scripts in the InstantCMS admin panel, potentially leading to session hijacking, credential theft, or further compromise of administrative accounts.

Affected Products

• InstantCMS (icms2) versions prior to 2.16.1.-git
• InstantCMS admin panel widget management functionality
• InstantCMS content type configuration forms

Discovery Timeline

• 2023-09-10 - CVE-2023-4879 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-4879

Vulnerability Analysis

This Stored XSS vulnerability resides in the InstantCMS admin panel, specifically within the widget management functionality and content type configuration forms. The vulnerability occurs because user-controlled input values are directly inserted into HTML attributes without proper encoding or sanitization.

When processing widget positions and block configurations, the application constructs HTML elements using variable values that are incorporated directly into rel and id attributes. An attacker with administrative access can craft malicious input containing JavaScript payloads that will be stored in the application and subsequently executed when other administrators access the affected admin pages.

The vulnerability requires the attacker to have high privileges (administrative access) and requires user interaction (another admin must view the affected page), but because the scope is changed, the impact extends beyond the vulnerable component to affect the confidentiality and integrity of other users' sessions.

Root Cause

The root cause of this vulnerability is the failure to apply proper HTML encoding to user-supplied values before inserting them into HTML element attributes. In the system/controllers/admin/actions/widgets.php file, the $value variable from user input was directly concatenated into HTML strings for the rel and id attributes without sanitization.

Additionally, the content type configuration form lacked the store_via_html_filter option for certain HTML fields, allowing potentially malicious content to be stored without filtering.

Attack Vector

The attack is network-based and requires an authenticated attacker with administrative privileges. The attacker would:

1. Access the InstantCMS admin panel with valid administrative credentials
2. Navigate to the widget management or content type configuration interface
3. Inject malicious JavaScript payloads into input fields that are processed without sanitization
4. The malicious payload is stored in the application database
5. When another administrator views the affected page, the stored XSS payload executes in their browser context

// Vulnerable code - widgets.php
list($type, $value) = explode(':', $block);

if ($type == 'position') {
    $replace_html = '<ul class="position" rel="' . $value . '" id="pos-' . $value . '"></ul>';
}

Source: GitHub Commit Security Fix

The fix applies the html() encoding function to sanitize the $value variable before inserting it into HTML attributes:

// Patched code - widgets.php
list($type, $value) = explode(':', $block);

if ($type === 'position') {
    $replace_html = '<ul class="position" rel="' . html($value, false) . '" id="pos-' . html($value, false) . '"></ul>';
}

Source: GitHub Commit Security Fix

Detection Methods for CVE-2023-4879

Indicators of Compromise

• Unusual JavaScript code or HTML entities appearing in widget position names or block identifiers in the database
• Unexpected script tags or event handlers in content type description fields
• Administrator session anomalies or unauthorized actions following admin panel access
• Browser developer tools showing inline script execution from admin panel pages

Detection Strategies

• Review InstantCMS database tables for stored XSS payloads in widget and content type configuration fields
• Implement Content Security Policy (CSP) headers to detect and block inline script execution
• Deploy web application firewall (WAF) rules to detect XSS patterns in POST requests to admin endpoints
• Monitor admin panel access logs for suspicious activity patterns following content modifications

Monitoring Recommendations

• Enable detailed logging for all admin panel configuration changes
• Implement real-time alerting for any modifications to widget positions or content type settings
• Configure browser-based XSS auditing where available
• Establish baseline behavior for administrative actions to identify anomalous activity

How to Mitigate CVE-2023-4879

Immediate Actions Required

• Upgrade InstantCMS to version 2.16.1.-git or later immediately
• Audit existing widget configurations and content type descriptions for potential stored XSS payloads
• Review admin panel access logs for any suspicious activity
• Consider temporarily restricting admin panel access to trusted IP addresses during the patching process

Patch Information

The vulnerability has been addressed in commit d0aeeaf5979fbdbf80dc3a3227d6c58442ab7487. The patch applies proper HTML encoding using the html() function to all user-supplied values before they are inserted into HTML attributes in the widget management functionality. Additionally, the store_via_html_filter option has been enabled for HTML description fields in content type configuration forms.

For detailed patch information, refer to the GitHub Commit Security Fix and the Huntr Bounty Report.

Workarounds

• Implement strict Content Security Policy (CSP) headers to prevent inline script execution
• Limit admin panel access to a whitelist of trusted IP addresses
• Enable HTTP-only and Secure flags on all session cookies to reduce the impact of potential session hijacking
• Regularly audit admin panel input fields for suspicious content until the patch can be applied

# Example Apache configuration to restrict admin access by IP
<Directory "/path/to/instantcms/system/controllers/admin">
    Require ip 192.168.1.0/24
    Require ip 10.0.0.0/8
</Directory>