CVE-2024-39008: fast-loops Prototype Pollution Vulnerability

CVE-2024-39008 Overview

A critical prototype pollution vulnerability exists in robinweser fast-loops v1.1.3, specifically within the objectMergeDeep function. Prototype pollution is a JavaScript-specific vulnerability that allows attackers to inject properties into Object prototype, affecting all objects in the application. In this case, the vulnerability enables attackers to execute arbitrary code or cause a Denial of Service (DoS) by injecting arbitrary properties through the vulnerable deep merge functionality.

Critical Impact

This vulnerability allows remote attackers to manipulate JavaScript object prototypes, potentially leading to remote code execution or application crashes without requiring authentication.

Affected Products

• robinweser fast-loops v1.1.3
• Applications using the objectMergeDeep function from fast-loops

Discovery Timeline

• 2024-07-01 - CVE CVE-2024-39008 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-39008

Vulnerability Analysis

The vulnerability resides in the objectMergeDeep function within the fast-loops JavaScript library. This function is designed to recursively merge objects but fails to properly sanitize input properties. When processing user-controlled input, the function does not validate whether the property keys being merged include dangerous prototype-related properties such as __proto__, constructor, or prototype.

This flaw is classified under CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes). When an attacker supplies a maliciously crafted object containing __proto__ as a key with arbitrary properties, the merge operation pollutes the global Object prototype. Since all JavaScript objects inherit from Object.prototype, this pollution affects every object in the application's runtime environment.

Root Cause

The root cause is insufficient input validation in the objectMergeDeep function. The function fails to check whether incoming object keys are prototype-related properties before performing the deep merge operation. Without proper safeguards like checking hasOwnProperty, using Object.keys(), or explicitly blocking dangerous keys (__proto__, constructor.prototype), the function allows arbitrary prototype manipulation.

Attack Vector

The attack is network-accessible and requires no user interaction or privileges. An attacker can exploit this vulnerability by sending a crafted JSON payload to any application endpoint that processes user input through the vulnerable objectMergeDeep function. The payload would contain __proto__ properties that, when merged, pollute the Object prototype.

Once the prototype is polluted, the attacker-controlled properties become available on all objects in the application. This can lead to:

• Remote code execution if the application later evaluates or executes polluted properties
• Denial of Service by overwriting critical properties that crash the application
• Authentication bypass by injecting properties like isAdmin: true

A proof-of-concept demonstrating this vulnerability is available in the GitHub Gist PoC.

Detection Methods for CVE-2024-39008

Indicators of Compromise

• Unusual properties appearing on JavaScript objects that were not explicitly set
• Application crashes or unexpected behavior following JSON parsing operations
• Error logs indicating type errors or undefined property access patterns
• Incoming HTTP requests containing __proto__, constructor, or prototype keys in JSON bodies

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block JSON payloads containing __proto__, constructor.prototype, or similar prototype pollution patterns
• Use static analysis security testing (SAST) tools to identify usage of fast-loops v1.1.3 in your codebase
• Deploy runtime application self-protection (RASP) solutions that monitor for prototype modifications
• Audit application dependencies using npm audit or similar tools to identify vulnerable versions

Monitoring Recommendations

• Monitor application logs for unexpected property access errors or type coercion failures
• Set up alerts for anomalous JSON parsing behavior or object manipulation patterns
• Track incoming HTTP request payloads for prototype pollution keywords in JSON structures
• Implement integrity monitoring on critical application objects to detect unauthorized property additions

How to Mitigate CVE-2024-39008

Immediate Actions Required

• Audit your application dependencies to identify if fast-loops v1.1.3 is in use
• Consider replacing fast-loops with alternative libraries that have prototype pollution protections
• Implement input validation to sanitize or reject objects containing __proto__ or constructor properties
• Use Object.freeze(Object.prototype) as a temporary defensive measure where application compatibility allows

Patch Information

No official patch information is currently available in the CVE data. Monitor the fast-loops repository and npm for security updates. Check the GitHub Gist PoC for additional technical details and potential mitigation approaches.

Workarounds

• Implement a wrapper function around objectMergeDeep that filters out dangerous keys (__proto__, constructor, prototype) before processing
• Use Object.create(null) for objects that will receive user-controlled data to create prototype-less objects
• Apply input sanitization at the API gateway or middleware level to strip prototype-related properties from incoming JSON
• Consider using JSON.parse() with a reviver function that explicitly rejects dangerous keys

# Check if fast-loops is installed in your project
npm list fast-loops

# Audit your project for known vulnerabilities
npm audit

# Search for objectMergeDeep usage in your codebase
grep -r "objectMergeDeep" ./src

# Freeze Object prototype as temporary mitigation (use with caution)
# Add to application entry point:
# Object.freeze(Object.prototype);