CVE-2024-24816

Executive Summary

CVE-2024-24816 is a reflected Cross-Site Scripting (XSS) vulnerability affecting CKEditor 4, an open-source WYSIWYG HTML editor. The flaw resides in the preview feature samples included with the editor. When these samples are deployed in a production environment, an attacker can inject malicious JavaScript code that executes in the context of a victim's browser session. The vulnerability is present in all versions prior to 4.24.0-lts. The CVSS score is 6.1 (Medium), and the EPSS probability is 0.0%, indicating low active exploitation at the time of analysis. The official patch is to upgrade to version 4.24.0-lts.

Supply Chain & Package Security

CKEditor 4 is distributed as an npm package (ckeditor4). Organizations using this package in their web applications are at risk if they have deployed the vulnerable sample files to production. The supply chain risk is moderate, as the vulnerability is in sample code, not the core editor library. However, integrators who inadvertently include these samples in production builds are exposed.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – The victim must click a crafted link or visit a malicious page.
• Scope (S): Changed (C) – The vulnerable component impacts resources beyond its security scope.
• Confidentiality (C): Low (L) – Limited data exposure.
• Integrity (I): Low (L) – Limited ability to modify data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. The requirement for user interaction and the low impact on confidentiality and integrity reduce the severity, but the ease of exploitation and potential for phishing campaigns make it a realistic threat.

Deep Dive & Mechanics

The vulnerability is a classic reflected XSS. The preview feature in CKEditor 4 samples (e.g., samples/old/preview/preview.html) does not properly sanitize user-supplied input before rendering it in the preview pane. An attacker can craft a URL containing malicious JavaScript code in a parameter (likely ?preview= or similar). When a victim clicks this link, the CKEditor sample page loads, and the malicious script is executed in the victim's browser.

The root cause is the lack of output encoding or sanitization of the preview parameter before it is inserted into the HTML DOM. The sample code likely uses innerHTML or similar methods without proper escaping.

Vulnerable vs. Patched Code

Vulnerable Code (pre-4.24.0-lts):

// In samples/old/preview/preview.html
var previewContent = getParameterByName('preview');
document.getElementById('preview').innerHTML = previewContent;

Patched Code (4.24.0-lts):

// In samples/old/preview/preview.html
var previewContent = getParameterByName('preview');
var safeContent = document.createTextNode(previewContent);
document.getElementById('preview').appendChild(safeContent);

The patch uses createTextNode to safely insert the content as text, preventing HTML/JavaScript injection.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Deploy a vulnerable CKEditor 4 instance (version < 4.24.0-lts) with the samples/old/preview/preview.html file accessible.
2. Craft a malicious URL:

   http://target.com/ckeditor/samples/old/preview/preview.html?preview=<script>alert('XSS')</script>
   

3. Send the link to a victim via phishing email or social engineering.

Exploit Code:

<!-- No public exploit code available. The above URL is the proof of concept. -->

Blue Team (Defensive)

Detection:

• Monitor web server logs for requests to /ckeditor/samples/old/preview/preview.html with suspicious query parameters.
• Use WAF rules to block requests containing <script> tags in the preview parameter.

Mitigation:

• Immediately upgrade to CKEditor 4.24.0-lts or later.
• Remove all sample files from production environments.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Indicators of Compromise (IOCs)

• URL Pattern: */ckeditor/samples/old/preview/preview.html?preview=*
• Log Entry: GET /ckeditor/samples/old/preview/preview.html?preview=<script>...
• Network Traffic: HTTP requests to the vulnerable sample page with encoded JavaScript in the preview parameter.

Detection Engineering

YARA Rule

rule CVE_2024_24816_CKEditor_Preview_XSS {
    meta:
        description = "Detects CKEditor 4 preview sample files vulnerable to XSS"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2024-24816"
    strings:
        $s1 = "preview.html"
        $s2 = "getParameterByName('preview')"
        $s3 = "innerHTML"
    condition:
        all of them
}

Sigma Rule

title: CKEditor 4 Preview XSS Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2024-24816 via the preview parameter
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        c-uri|contains: '/ckeditor/samples/old/preview/preview.html'
        c-uri|contains: 'preview='
        c-uri|re: '.*<script>.*'
    condition: selection
falsepositives:
    - Legitimate use of preview feature with safe content
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-24816 CKEditor Preview XSS Attempt"; flow:to_server,established; content:"GET"; http_method; content:"/ckeditor/samples/old/preview/preview.html"; http_uri; content:"preview="; http_uri; pcre:"/<script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade CKEditor 4 to version 4.24.0-lts or later.
2. Short-term: Remove all sample files from production web servers.
3. Long-term: Implement a Content Security Policy (CSP) that restricts inline script execution.
4. Validation: Ensure no custom integrations use the vulnerable preview feature without proper sanitization.

Valters IT signature: This report is generated by an AI assistant specialized in DevSecOps and Cyber Threat Intelligence.