CVE-2020-7746: Chart.js Prototype Pollution Vulnerability

CVE-2020-7746 Overview

CVE-2020-7746 is a prototype pollution vulnerability affecting the popular Chart.js JavaScript charting library before version 2.9.4. The vulnerability exists in the options processing functionality where user-supplied configuration options are deeply merged with existing or default options without proper key validation. This allows attackers to inject malicious properties into JavaScript object prototypes, potentially leading to remote code execution, denial of service, or property injection attacks.

Critical Impact

Attackers can exploit this prototype pollution vulnerability to manipulate object prototypes in applications using vulnerable Chart.js versions, potentially achieving remote code execution or application-wide denial of service through network-accessible attack vectors without authentication.

Affected Products

• Chart.js versions prior to 2.9.4
• Node.js applications using vulnerable chartjs:chart.js packages
• Web applications integrating Chart.js charting functionality

Discovery Timeline

• October 29, 2020 - CVE-2020-7746 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-7746

Vulnerability Analysis

This prototype pollution vulnerability occurs during the deep merge operation when Chart.js processes configuration options. When a chart is initialized or updated, the library performs a recursive merge of user-provided options with the existing configuration or default values. The merge function fails to validate object keys before assignment, allowing special properties like __proto__, constructor, or prototype to be injected into the object hierarchy.

Prototype pollution is particularly dangerous in JavaScript applications because modifying the prototype of base objects (like Object.prototype) affects all objects in the application. An attacker who can control the options parameter passed to Chart.js can inject arbitrary properties that will be inherited by all JavaScript objects, leading to various exploitation scenarios depending on how the application uses these objects.

Root Cause

The root cause is the lack of proper key sanitization in the deep merge utility function used by Chart.js when processing configuration options. The merge operation recursively copies properties from the source object to the target object without checking whether the property key is a potentially dangerous prototype-related property. This allows an attacker to craft malicious input that pollutes the object prototype chain.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker can exploit this vulnerability by supplying specially crafted options to Chart.js through any input vector that allows modification of chart configuration. In web applications, this could be through user-controllable form inputs, URL parameters, or API endpoints that pass data to Chart.js chart initialization or update methods.

The attack payload typically involves nested objects with __proto__ keys containing properties the attacker wishes to inject into Object.prototype. Once polluted, these properties become accessible on all objects in the application, potentially enabling further exploitation such as bypassing security checks, manipulating application logic, or achieving code execution if gadgets exist in the codebase.

Detection Methods for CVE-2020-7746

Indicators of Compromise

• Unexpected properties appearing on JavaScript objects that were not explicitly defined
• Application behavior anomalies related to object property lookups returning unexpected values
• Web application firewall logs showing requests containing __proto__, constructor.prototype, or similar payload patterns
• Error logs indicating prototype-related issues or unexpected object behavior

Detection Strategies

• Implement software composition analysis (SCA) to identify Chart.js versions prior to 2.9.4 in your dependency tree
• Deploy web application firewalls with rules to detect prototype pollution payloads in request parameters
• Use static analysis tools to scan for vulnerable deep merge patterns in JavaScript codebases
• Monitor npm audit or Snyk for alerts related to CVE-2020-7746 in project dependencies

Monitoring Recommendations

• Enable detailed logging for Chart.js initialization and configuration changes
• Monitor for suspicious patterns in user-supplied JSON data, particularly nested __proto__ or constructor keys
• Implement runtime application self-protection (RASP) to detect prototype pollution attempts
• Set up alerts for dependency vulnerability scanners detecting outdated Chart.js versions

How to Mitigate CVE-2020-7746

Immediate Actions Required

• Upgrade Chart.js to version 2.9.4 or later immediately
• Audit all applications using Chart.js to identify vulnerable installations
• Review application code for user-controlled data flowing into Chart.js configuration
• Implement input validation to filter prototype pollution payloads before they reach Chart.js

Patch Information

The vulnerability was addressed in Chart.js version 2.9.4. The fix involves proper sanitization of object keys during the deep merge operation to prevent prototype pollution. The patch details can be found in the GitHub Pull Request #7920. Additional vulnerability details are available through Snyk's JavaScript Advisory.

Workarounds

• Implement server-side input validation to strip __proto__, constructor, and prototype keys from user input before processing
• Use Object.freeze() on Object.prototype to prevent runtime modifications (may break some applications)
• Wrap user input in a sanitization function that creates clean objects without prototype chain access
• Consider using safer deep merge libraries that include prototype pollution protection

# Update Chart.js to patched version
npm update chart.js@^2.9.4

# Or install specific patched version
npm install chart.js@2.9.4

# Verify installed version
npm list chart.js