CVE-2024-38356: TinyMCE XSS Vulnerability

CVE-2024-38356 Overview

A cross-site scripting (XSS) vulnerability was discovered in TinyMCE, a widely-used open source rich text editor. The vulnerability exists in TinyMCE's content extraction code and specifically affects configurations using the noneditable_regexp option. When this option is enabled, specially crafted HTML attributes containing malicious code can be executed when content is extracted from the editor, potentially allowing attackers to execute arbitrary JavaScript in the context of the victim's browser session.

Critical Impact

Attackers can inject and execute malicious JavaScript code through specially crafted HTML attributes, potentially leading to session hijacking, data theft, or further exploitation of users interacting with affected TinyMCE instances.

Affected Products

• TinyMCE versions prior to 7.2.0
• TinyMCE versions prior to 6.8.4
• TinyMCE versions prior to 5.11.0 LTS

Discovery Timeline

• 2024-06-19 - CVE CVE-2024-38356 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-38356

Vulnerability Analysis

This Cross-Site Scripting (XSS) vulnerability (CWE-79) resides in TinyMCE's content parsing and extraction mechanisms, specifically within the DOM parser and serializer components. The flaw occurs when the noneditable_regexp option is configured, which is designed to mark content matching a regular expression as non-editable. Due to improper validation, malicious content embedded within HTML attributes bypasses the configured regular expression verification before being added to the output, enabling script injection.

The vulnerability is particularly concerning for web applications that rely on TinyMCE for user-generated content, such as content management systems, forums, and web-based email clients. A successful exploit could allow attackers to steal session cookies, perform actions on behalf of authenticated users, or redirect victims to malicious sites.

Root Cause

The root cause lies in insufficient validation of content within HTML attributes when using the noneditable_regexp option. The DOM parser and serializer components failed to properly verify that attribute content matches the configured regular expression before including it in the extracted output. Additionally, the special element handling in the DOM parser did not properly account for noscript elements, allowing encoded content to be improperly processed.

Attack Vector

The attack is network-based and requires user interaction. An attacker can craft malicious HTML content containing specially formatted attributes that bypass the noneditable_regexp validation. When a victim loads or interacts with editor content containing the payload, the malicious JavaScript executes in their browser context. The attack does not require authentication to craft the payload, though exploiting it requires a victim to interact with the malicious content.

The following code shows the security patch applied to fix the DOM parser handling:

   const parentName = parent.name;
   // Exclude the special elements where the content is RCDATA as their content needs to be parsed instead of being left as plain text
   // See: https://html.spec.whatwg.org/multipage/parsing.html#parsing-html-fragments
-  const isSpecial = parentName in specialElements && parentName !== 'title' && parentName !== 'textarea';
+  const isSpecial = parentName in specialElements && parentName !== 'title' && parentName !== 'textarea' && parentName !== 'noscript';

   const childNodes = nativeParent.childNodes;
   for (let ni = 0, nl = childNodes.length; ni < nl; ni++) {

Source: GitHub Commit Update

The patch adds noscript to the list of elements that should have their content parsed rather than left as plain text, preventing encoded malicious content from being improperly handled.

Detection Methods for CVE-2024-38356

Indicators of Compromise

• Unusual or obfuscated JavaScript code within HTML attributes in TinyMCE content
• Unexpected script execution events originating from rich text editor components
• Log entries showing suspicious HTML attribute patterns in user-submitted content
• Reports of session hijacking or unauthorized actions following interaction with editor content

Detection Strategies

• Implement Content Security Policy (CSP) headers to detect and block inline script execution attempts
• Monitor web application logs for HTML content containing suspicious attribute patterns or encoded JavaScript
• Deploy web application firewalls (WAF) with XSS detection rules targeting rich text editor content
• Use browser developer tools or security scanning tools to audit TinyMCE content for script injection patterns

Monitoring Recommendations

• Enable detailed logging for TinyMCE content submission and extraction events
• Configure security information and event management (SIEM) systems to alert on XSS indicator patterns
• Implement client-side monitoring for unexpected script execution in editor contexts
• Regularly audit stored content for malicious payloads that may have been injected prior to patching

How to Mitigate CVE-2024-38356

Immediate Actions Required

• Upgrade TinyMCE to version 7.2.0, 6.8.4, or 5.11.0 LTS depending on your major version
• Review existing content stored by TinyMCE for potential malicious payloads
• Implement Content Security Policy headers to mitigate XSS impact if immediate patching is not possible
• Audit usage of the noneditable_regexp option across your applications

Patch Information

TinyMCE has released security patches addressing this vulnerability. The fix ensures that when using the noneditable_regexp option, any content within an attribute is properly verified to match the configured regular expression before being added to the output. Patches are available for all supported version branches:

• TinyMCE 7.x: Upgrade to version 7.2.0 or later - See TinyMCE 7.2 Release Notes
• TinyMCE 6.x: Upgrade to version 6.8.4 or later - See TinyMCE 6.8.4 Release Notes
• TinyMCE 5.x LTS: Upgrade to version 5.11.0 or later

For detailed technical information about the fix, refer to the GitHub Security Advisory GHSA-9hcv-j9pv-qmph.

Workarounds

• There are no known workarounds for this vulnerability; upgrading to a patched version is required
• As a temporary mitigation, implement strict Content Security Policy headers to reduce XSS impact
• Consider disabling the noneditable_regexp option if not critical to your application functionality while awaiting upgrade
• Deploy additional input validation and output encoding on content processed by TinyMCE

# Upgrade TinyMCE via npm
npm update tinymce

# Or install specific patched version
npm install tinymce@7.2.0

# Verify installed version
npm list tinymce