CVE-2023-4771: CKSource CKEditor XSS Vulnerability

CVE-2023-4771 Overview

A Cross-Site Scripting (XSS) vulnerability has been discovered in CKSource CKEditor affecting versions 4.15.1 and earlier. This vulnerability exists in the /ckeditor/samples/old/ajax.html file, which allows an attacker to inject and execute malicious JavaScript code in the context of an authorized user's browser session. Successful exploitation could enable attackers to steal sensitive user information, session tokens, or perform actions on behalf of authenticated users.

Critical Impact

Attackers can inject malicious JavaScript through the ajax.html sample file to steal authorized user information and potentially hijack user sessions.

Affected Products

• CKSource CKEditor versions 4.15.1 and earlier
• CKSource CKEditor WordPress plugin (all versions up to and including 4.15.1)
• Web applications integrating vulnerable CKEditor versions

Discovery Timeline

• November 16, 2023 - CVE-2023-4771 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-4771

Vulnerability Analysis

This Cross-Site Scripting vulnerability (CWE-79) exists within the sample files distributed with CKEditor. The vulnerable endpoint at /ckeditor/samples/old/ajax.html fails to properly sanitize user-supplied input before reflecting it in the browser response. This allows attackers to craft malicious URLs containing JavaScript payloads that execute when a victim visits the link.

The vulnerability requires user interaction—a victim must click on a malicious link or visit a page controlled by the attacker that redirects to the vulnerable endpoint. When successfully exploited, the injected script runs with the same privileges as the authenticated user, potentially allowing the attacker to access sensitive data, modify page content, or perform unauthorized actions.

Root Cause

The root cause of this vulnerability is improper input validation and output encoding in the ajax.html sample file. The file fails to sanitize user-controlled input before including it in the HTML response, allowing arbitrary JavaScript injection. Sample files are often overlooked during security reviews but can present significant attack surfaces when deployed to production environments.

Attack Vector

The attack is conducted over the network and requires no prior authentication. An attacker must craft a malicious URL containing a JavaScript payload targeting the vulnerable ajax.html endpoint. The attacker then needs to convince an authenticated user to click the malicious link through social engineering techniques such as phishing emails, malicious advertisements, or compromised websites.

Once the victim accesses the crafted URL, the malicious JavaScript executes in their browser context, enabling the attacker to:

• Steal session cookies and authentication tokens
• Capture keystrokes and form data
• Redirect users to phishing sites
• Modify the displayed page content
• Perform actions on behalf of the authenticated user

The vulnerability leverages reflected XSS, where the malicious payload is embedded in the URL and reflected back in the server response without proper sanitization.

Detection Methods for CVE-2023-4771

Indicators of Compromise

• Unusual HTTP requests to /ckeditor/samples/old/ajax.html with encoded JavaScript payloads in URL parameters
• Web server logs showing requests containing <script> tags, javascript: protocols, or event handlers like onerror, onload
• Network traffic containing suspicious redirects or data exfiltration attempts following CKEditor sample file access
• User reports of unexpected browser behavior or session hijacking after visiting internal applications

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS payloads targeting CKEditor sample paths
• Configure intrusion detection systems to alert on requests containing common XSS patterns in URLs
• Enable verbose logging for web servers and monitor for suspicious access patterns to sample directories
• Deploy browser-based security controls that detect and report XSS attempts

Monitoring Recommendations

• Monitor web application logs for access attempts to /ckeditor/samples/ directory paths
• Set up alerts for high volumes of requests to CKEditor sample files from external IP addresses
• Implement Content Security Policy (CSP) headers and monitor for policy violations
• Review authentication logs for anomalous session activity following any detected XSS attempts

How to Mitigate CVE-2023-4771

Immediate Actions Required

• Remove or restrict access to CKEditor sample directories in production environments immediately
• Upgrade CKEditor to the latest version that addresses this vulnerability
• Implement Content Security Policy headers to mitigate the impact of XSS attacks
• Review web server configurations to ensure sample files are not publicly accessible

Patch Information

Organizations should upgrade to a patched version of CKEditor that addresses this vulnerability. Review the INCIBE Security Notice for detailed remediation guidance. Contact CKSource for the latest security updates and patch availability information.

Workarounds

• Delete or relocate the /ckeditor/samples/ directory from production deployments
• Configure web server access controls to deny requests to CKEditor sample files
• Implement strict Content Security Policy headers to prevent inline script execution
• Use web application firewall rules to block requests containing XSS payloads targeting CKEditor paths

# Configuration example - Apache .htaccess to block sample directory access
<Directory "/var/www/html/ckeditor/samples">
    Order deny,allow
    Deny from all
</Directory>

# Nginx configuration to block sample directory
location /ckeditor/samples {
    deny all;
    return 404;
}

# Add Content Security Policy header
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'"