CVE-2021-41165: CKEditor4 XSS Vulnerability

CVE-2021-41165 Overview

CVE-2021-41165 is a Cross-Site Scripting (XSS) vulnerability discovered in CKEditor 4, an open source WYSIWYG HTML editor. The vulnerability exists in the core HTML processing module and may affect all plugins used by CKEditor 4. Attackers can exploit this flaw by injecting malformed HTML comments that bypass content sanitization, ultimately allowing the execution of arbitrary JavaScript code in the context of a user's browser session.

This vulnerability poses significant risk to web applications utilizing CKEditor 4 for content editing, as it could enable attackers to steal session cookies, perform actions on behalf of authenticated users, or deliver malicious payloads to website visitors.

Critical Impact

Malformed HTML comment injection bypasses content sanitization, enabling JavaScript execution that could lead to session hijacking, data theft, or phishing attacks on users of affected web applications.

Affected Products

• CKEditor CKEditor (versions prior to 4.17.0)
• Drupal Drupal (multiple versions)
• Oracle Agile Product Lifecycle Management 9.3.6
• Oracle Application Express
• Oracle Banking APIs (versions 19.1, 19.2, 20.1, 21.1)
• Oracle Banking Digital Experience (versions 19.1, 19.2, 20.1, 21.1)
• Oracle Commerce Guided Search 11.3.2
• Oracle PeopleSoft Enterprise PeopleTools (versions 8.58, 8.59)
• Oracle WebCenter Portal (versions 12.2.1.3.0, 12.2.1.4.0)

Discovery Timeline

• November 17, 2021 - CVE-2021-41165 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-41165

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Cross-Site Scripting). The flaw resides in how CKEditor 4's core HTML processing module handles and sanitizes HTML comments. When processing user-supplied content, the sanitization logic fails to properly validate malformed HTML comment structures, creating an opportunity for attackers to embed executable JavaScript code that evades security filters.

The impact of successful exploitation includes unauthorized access to sensitive user data, session hijacking through cookie theft, defacement of web content, and the potential to redirect users to malicious websites. Since CKEditor 4 is widely deployed across enterprise applications including Oracle products and Drupal-based websites, the attack surface is substantial.

Root Cause

The root cause lies in insufficient input validation within CKEditor 4's HTML parsing and sanitization routines. The HTML processing module fails to account for edge cases involving malformed comment syntax, allowing specially crafted comment strings to pass through sanitization checks while still being interpreted as executable content by web browsers.

Attack Vector

This vulnerability is exploitable over the network and requires low-privilege access with user interaction. An attacker would typically craft malicious content containing specially formatted HTML comments designed to bypass sanitization. When a victim views or interacts with content containing the malicious payload through a vulnerable CKEditor instance, the injected JavaScript executes in their browser context.

The attack scenario involves injecting malformed HTML comments that appear benign to the sanitizer but are interpreted as valid script content by browsers. This could occur through content submission forms, comment sections, or any feature that allows user input through CKEditor.

The malicious payload exploits inconsistencies between how the CKEditor sanitizer parses HTML comments versus how browsers interpret them. By crafting specific comment sequences with embedded script tags or event handlers, attackers can achieve JavaScript execution. For detailed technical information, refer to the GitHub Security Advisory GHSA-7h26-63m7-qhf2.

Detection Methods for CVE-2021-41165

Indicators of Compromise

• Unusual HTML comment patterns in stored content, particularly those containing script tags or JavaScript event handlers
• Web application logs showing attempts to submit content with malformed HTML comment sequences
• User reports of unexpected browser behavior when viewing content created through CKEditor interfaces
• Network traffic containing suspicious payloads targeting CKEditor endpoints

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block malformed HTML comment injection attempts
• Deploy Content Security Policy (CSP) headers to limit the impact of any successful XSS exploitation
• Monitor application logs for patterns indicative of XSS payload attempts, including encoded script tags and event handler attributes
• Conduct regular security scanning of web applications using CKEditor to identify unpatched instances

Monitoring Recommendations

• Enable verbose logging for content submission endpoints that utilize CKEditor
• Set up alerts for suspicious content patterns matching known XSS payloads
• Monitor browser console errors from client-side applications that may indicate script injection attempts
• Track CKEditor version deployments across your infrastructure to ensure timely patch application

How to Mitigate CVE-2021-41165

Immediate Actions Required

• Upgrade CKEditor 4 to version 4.17.0 or later immediately across all deployments
• Apply vendor-specific patches for Oracle products as outlined in Oracle Security Alerts
• Update Drupal installations to patched versions as described in Drupal Security Advisory SA-CORE-2021-011
• Implement Content Security Policy headers as a defense-in-depth measure while patching is in progress

Patch Information

The vulnerability has been patched in CKEditor 4 version 4.17.0. Organizations should prioritize upgrading to this version or later. For detailed changelog information, refer to the GitHub CKEditor Change Log.

Oracle customers should apply patches from the Oracle Security Alert January 2022 and Oracle Security Alert April 2022. Drupal users should consult the Drupal Security Advisory SA-CORE-2021-011 for platform-specific guidance.

Workarounds

• Implement strict Content Security Policy (CSP) headers to prevent inline script execution
• Deploy server-side content filtering to sanitize HTML comments before storage
• Consider temporarily disabling CKEditor features that accept HTML input until patches can be applied
• Use input validation at the application layer as an additional defense mechanism

# Example CSP header configuration to mitigate XSS impact
# Add to web server configuration (Apache/Nginx)
Content-Security-Policy: default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'

# Verify CKEditor version in your deployment
grep -r "version" ckeditor/ckeditor.js | head -1