CVE-2024-24815

Executive Summary

CVE-2024-24815 is a cross-site scripting (XSS) vulnerability in CKEditor4, an open-source WYSIWYG HTML editor. The flaw resides in the core HTML parsing module and affects all editor instances that have full-page editing mode enabled or that allow CDATA elements in Advanced Content Filtering (ACF) configuration (defaults to script and style elements). An attacker can inject malformed HTML content that bypasses the ACF mechanism, potentially leading to arbitrary JavaScript execution in the context of the user's browser. The vulnerability is rated with a CVSS score of 6.1 (Medium) and has a low EPSS score of 0.0%, indicating limited active exploitation. A patch is available in version 4.24.0-lts.

Supply Chain & Package Security

CKEditor4 is distributed as an npm package (ckeditor4). Organizations using this package in their supply chain should verify their dependency version and update to 4.24.0-lts or later. The vulnerability can be introduced through direct or transitive dependencies, making it critical to audit package.json and lock files.

CVSS Breakdown & Threat Level

• CVSS Score: 6.1 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Changed
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

The threat level is considered medium due to the requirement for user interaction (e.g., clicking a link) and the limited impact on confidentiality and integrity. However, the vulnerability can be chained with other attacks to escalate privileges or steal sensitive data.

Deep Dive & Mechanics

The vulnerability exists in the core HTML parsing module of CKEditor4. When full-page editing mode is enabled, or when CDATA elements are allowed in ACF configuration, the parser incorrectly handles malformed CDATA sections. An attacker can craft HTML content that includes a malformed CDATA block, which bypasses the ACF filter. This allows the injection of arbitrary HTML and JavaScript, leading to XSS.

The root cause is a flaw in the CDATA detection logic. The parser does not properly validate the structure of CDATA sections, allowing an attacker to inject content that is interpreted as executable code. For example, a payload like <![CDATA[<script>alert(1)</script>]]> might be processed incorrectly, allowing the script to execute.

Vulnerable vs. Patched Code

Vulnerable Code (prior to 4.24.0-lts): The vulnerable code in the HTML parser does not properly validate CDATA sections. It may treat malformed CDATA as valid and pass it through the ACF filter.

// Vulnerable pseudo-code
function parseCDATA(input) {
    if (input.startsWith('<![CDATA[')) {
        // No validation of malformed content
        return input.slice(9, -3);
    }
    return input;
}

Patched Code (4.24.0-lts): The patched code adds validation to ensure CDATA sections are properly formed and do not contain executable content.

// Patched pseudo-code
function parseCDATA(input) {
    if (input.startsWith('<![CDATA[') && input.endsWith(']]>')) {
        // Validate no script injection
        let content = input.slice(9, -3);
        if (content.includes('<script>')) {
            return '';
        }
        return content;
    }
    return input;
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• CKEditor4 version prior to 4.24.0-lts
• Full-page editing mode enabled or CDATA elements allowed in ACF

Exploitation Steps:

1. Identify a target using CKEditor4 with vulnerable configuration.
2. Craft a malicious payload that includes a malformed CDATA section.
3. Inject the payload into the editor content (e.g., via a form field or API).
4. When the content is rendered, the XSS payload executes.

Example Payload:

<![CDATA[<script>alert('XSS')</script>]]>

Lab Setup:

• Use a local instance of CKEditor4 (e.g., version 4.23.0) with full-page mode enabled.
• Create a simple HTML page that loads the editor.
• Inject the payload via the editor's API or by pasting it into the source view.

Blue Team

Detection:

• Monitor for unusual CDATA sections in user-submitted content.
• Look for patterns like <![CDATA[<script> in logs or network traffic.
• Use WAF rules to block malformed CDATA.

Mitigation:

• Update CKEditor4 to version 4.24.0-lts or later.
• Disable full-page editing mode if not required.
• Restrict CDATA elements in ACF configuration.

Indicators of Compromise (IOCs)

• Network: HTTP requests containing <![CDATA[<script> or similar patterns.
• File: Modified CKEditor4 JavaScript files with suspicious CDATA handling.
• Log: Error logs showing malformed CDATA parsing attempts.
• Behavior: Unexpected JavaScript execution in the context of the editor.

Detection Engineering

YARA Rule

rule CVE_2024_24815_CKEditor4_XSS {
    meta:
        description = "Detects malformed CDATA sections used in CVE-2024-24815"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-24815"
    strings:
        $payload1 = "<![CDATA[<script>" ascii wide nocase
        $payload2 = "<![CDATA[<img" ascii wide nocase
        $payload3 = "<![CDATA[<svg" ascii wide nocase
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-24815 CKEditor4 XSS Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects malformed CDATA sections in HTTP requests targeting CKEditor4
author: Valters IT
date: 2024/01/01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-24815
logsource:
    category: webserver
    product: apache
    service: http
detection:
    selection:
        c-uri|contains:
            - "<![CDATA[<script>"
            - "<![CDATA[<img"
            - "<![CDATA[<svg"
    condition: selection
falsepositives:
    - Legitimate use of CDATA sections in XML
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-24815 CKEditor4 XSS Attempt"; flow:to_server,established; content:"<![CDATA[<script>"; nocase; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Upgrade CKEditor4 to version 4.24.0-lts or later.
2. Configuration: Disable full-page editing mode if not required. Restrict CDATA elements in ACF configuration.
3. WAF: Deploy web application firewall rules to block malformed CDATA sections.
4. Monitoring: Monitor logs for suspicious CDATA patterns and investigate any detections.
5. Training: Educate developers on secure coding practices and the risks of XSS.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert