CVE-2021-41164: CKEditor4 XSS Vulnerability

CVE-2021-41164 Overview

CVE-2021-41164 is a Cross-Site Scripting (XSS) vulnerability discovered in CKEditor 4, a widely-used open source WYSIWYG HTML editor. The vulnerability exists within the Advanced Content Filter (ACF) module and may affect all plugins used by CKEditor 4. Attackers can exploit this flaw to inject malformed HTML that bypasses content sanitization mechanisms, ultimately enabling the execution of arbitrary JavaScript code in the context of a victim's browser session.

This vulnerability impacts a broad range of applications and platforms that integrate CKEditor 4 as their rich text editing component, including content management systems like Drupal and numerous Oracle enterprise products.

Critical Impact

Attackers can execute arbitrary JavaScript code in victim browsers by bypassing CKEditor 4's content sanitization, potentially leading to session hijacking, credential theft, and malicious content injection across affected web applications.

Affected Products

• CKEditor 4 versions prior to 4.17.0
• Drupal (multiple versions)
• 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 Agile PLM 9.3.6
• Oracle Application Express
• Oracle Commerce Guided Search 11.3.2
• Oracle PeopleSoft Enterprise PeopleTools (8.58, 8.59)
• Oracle WebCenter Portal (12.2.1.3.0, 12.2.1.4.0)
• Fedora 36 and 37

Discovery Timeline

• 2021-11-17 - CVE-2021-41164 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-41164

Vulnerability Analysis

This vulnerability (CWE-79) resides in CKEditor 4's Advanced Content Filter (ACF) module, which is responsible for sanitizing and filtering HTML content to prevent malicious code injection. The ACF module is designed to strip potentially dangerous elements and attributes from user-supplied content before it is rendered in the editor or stored.

The flaw allows attackers to craft specially malformed HTML payloads that evade the sanitization logic implemented by the ACF. When these payloads are processed by CKEditor, the malicious content is not properly filtered, allowing embedded JavaScript to execute when the content is rendered in a user's browser.

Because the ACF module is a core component used by virtually all CKEditor 4 plugins, this vulnerability has a wide attack surface affecting any application that relies on CKEditor 4 for user content editing.

Root Cause

The root cause of this vulnerability lies in insufficient input validation and sanitization within the Advanced Content Filter module. The ACF failed to properly handle certain malformed HTML structures, allowing specially crafted payloads to bypass the content filtering mechanisms. This gap in the sanitization logic enabled the preservation of executable script content that should have been stripped or neutralized during the filtering process.

Attack Vector

The attack vector for CVE-2021-41164 is network-based, requiring user interaction. An attacker must craft malicious HTML content containing embedded JavaScript and submit it through an application using a vulnerable version of CKEditor 4. When another user views or edits the content, the malformed HTML bypasses the ACF sanitization, causing the malicious JavaScript to execute in their browser context.

Exploitation scenarios include:

The attacker submits malicious content through a comment form, forum post, or any input field powered by CKEditor 4. The crafted payload uses malformed HTML structures specifically designed to evade the ACF's filtering rules. When legitimate users access the page containing this content, the JavaScript executes with their session privileges, potentially allowing session hijacking, credential theft, or further attacks.

Detection Methods for CVE-2021-41164

Indicators of Compromise

• Unusual or malformed HTML patterns in stored user-generated content, particularly structures designed to confuse HTML parsers
• Unexpected JavaScript execution events originating from CKEditor-rendered content areas
• Web application logs showing suspicious HTML submissions with unconventional tag structures or attribute combinations
• User reports of unexpected browser behavior or redirects when viewing content created through CKEditor interfaces

Detection Strategies

• Implement Content Security Policy (CSP) headers to detect and block unauthorized script execution attempts
• Deploy web application firewall (WAF) rules to identify and log malformed HTML injection attempts targeting rich text editors
• Monitor server-side logs for HTML content submissions containing suspicious script tags or event handler attributes
• Utilize browser-based XSS auditing tools and security extensions to detect client-side script injection

Monitoring Recommendations

• Enable detailed logging for all CKEditor content submissions and modifications in affected applications
• Set up alerting for CSP violation reports that may indicate XSS exploitation attempts
• Monitor for anomalous user session activity that could indicate successful session hijacking
• Review stored content databases periodically for signs of injected malicious payloads

How to Mitigate CVE-2021-41164

Immediate Actions Required

• Upgrade CKEditor 4 to version 4.17.0 or later immediately across all affected applications
• Audit existing stored content for potentially malicious HTML payloads that may have been injected before patching
• Implement Content Security Policy headers to provide defense-in-depth against XSS attacks
• Review and apply relevant security updates from Drupal and Oracle for their respective products

Patch Information

CKEditor has released version 4.17.0 which addresses this vulnerability. The fix is documented in the CKEditor 4 Change Log. Additional security guidance is available in the GitHub Security Advisory GHSA-pvmx-g8h5-cprj.

Drupal users should apply the patches detailed in Drupal Security Advisory SA-CORE-2021-011.

Oracle customers should consult the Oracle Critical Patch Update January 2022, Oracle Critical Patch Update April 2022, and Oracle Critical Patch Update July 2022 for patches addressing affected Oracle products.

Workarounds

• If immediate patching is not possible, consider temporarily disabling user-generated content features that use CKEditor
• Implement server-side content sanitization as an additional layer before storing CKEditor output
• Deploy strict Content Security Policy headers with script-src directives to limit JavaScript execution sources
• Consider using an alternative text editor temporarily until the upgrade can be completed

# Example: Deploy Content Security Policy headers as defense-in-depth
# Add to Apache configuration or .htaccess
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none';"

# For Nginx
add_header Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none';";