CVE-2026-6621: 1024bit extend-deep RCE Vulnerability

CVE-2026-6621 Overview

A prototype pollution vulnerability has been identified in the 1024bit extend-deep npm package up to version 0.1.6. The vulnerability exists in an unknown function within the index.js file, where improper handling of the __proto__ argument allows attackers to modify object prototype attributes. This flaw enables remote exploitation through network-accessible attack vectors.

The exploit has been publicly disclosed and may be actively utilized by threat actors. Of particular concern is that the project's code repository has not been active for many years, meaning no security patches are expected from the maintainer.

Critical Impact

Remote attackers can exploit this prototype pollution vulnerability to modify JavaScript object prototypes, potentially leading to denial of service, property injection, or in certain application contexts, remote code execution.

Affected Products

• 1024bit extend-deep version 0.1.6 and earlier
• Applications and packages that depend on vulnerable versions of extend-deep
• Node.js projects utilizing extend-deep for deep object merging operations

Discovery Timeline

• April 20, 2026 - CVE-2026-6621 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6621

Vulnerability Analysis

This vulnerability falls under CWE-94 (Improper Control of Generation of Code), specifically manifesting as a prototype pollution attack. Prototype pollution occurs when an attacker can inject properties into existing JavaScript object prototypes, affecting all objects that inherit from the polluted prototype.

In the case of extend-deep, the library is designed to perform deep object merging operations. When processing user-controlled input containing the __proto__ property, the library fails to properly sanitize or reject these dangerous property keys. This allows an attacker to inject arbitrary properties into the Object.prototype, which then propagates to all JavaScript objects in the application.

The vulnerability is exploitable remotely through network vectors, requiring no authentication or user interaction to trigger. The impact includes potential confidentiality, integrity, and availability compromises at a limited level, as attackers can manipulate application behavior by injecting unexpected properties into object prototypes.

Root Cause

The root cause of this vulnerability lies in the inadequate input validation within the index.js file of the extend-deep package. The deep merge functionality does not implement safeguards against prototype pollution attacks, specifically failing to:

1. Block or sanitize dangerous property names such as __proto__, constructor, and prototype
2. Use Object.hasOwnProperty() checks to prevent prototype chain traversal
3. Implement a safe object creation pattern using Object.create(null) for property storage

Since the project repository has been inactive for many years, these security best practices were likely not implemented when the package was originally developed.

Attack Vector

The attack is network-based and can be executed remotely without requiring authentication or user interaction. An attacker can craft malicious JSON payloads containing __proto__ properties and send them to applications that process this input using the vulnerable extend-deep library.

When the malicious payload is processed by the deep merge function, the __proto__ property values are written to Object.prototype, effectively polluting all objects in the JavaScript runtime. This can lead to various exploitation scenarios depending on how the application uses object properties, including denial of service, security bypasses, or in certain contexts, remote code execution when combined with other application-specific vulnerabilities.

Technical details and proof-of-concept documentation are available at the GitHub PoC Repository. Additional vulnerability information can be found at VulDB #358256.

Detection Methods for CVE-2026-6621

Indicators of Compromise

• Presence of extend-deep package version 0.1.6 or earlier in package.json or package-lock.json files
• Incoming HTTP requests or API payloads containing __proto__ or constructor.prototype property names
• Unexpected properties appearing on JavaScript objects that were not explicitly defined
• Application crashes or unexpected behavior related to object property access

Detection Strategies

• Implement static code analysis tools to identify usage of the vulnerable extend-deep package in your codebase
• Configure web application firewalls (WAF) to detect and block payloads containing prototype pollution patterns such as __proto__ and constructor
• Use software composition analysis (SCA) tools to identify vulnerable dependencies in your Node.js projects
• Deploy runtime application self-protection (RASP) solutions to monitor for prototype pollution attempts

Monitoring Recommendations

• Monitor application logs for unusual object property access patterns or undefined property errors
• Implement input validation logging to track payloads that contain potentially malicious property names
• Set up dependency vulnerability scanning in CI/CD pipelines to catch vulnerable packages before deployment
• Review network traffic for JSON payloads containing prototype pollution attack patterns

How to Mitigate CVE-2026-6621

Immediate Actions Required

• Audit your Node.js projects for dependencies on extend-deep version 0.1.6 or earlier
• Replace extend-deep with actively maintained alternatives such as lodash.merge (with appropriate configuration) or deepmerge
• Implement input validation to reject or sanitize objects containing __proto__, constructor, or prototype properties
• Consider freezing Object.prototype in security-critical applications to prevent runtime pollution

Patch Information

No official patch is available for this vulnerability. The project repository has not been active for many years, indicating that the package is abandoned and no security updates are expected. Organizations using this package should migrate to an alternative library immediately.

Alternative packages that provide similar functionality with better security maintenance include:

• deepmerge - Actively maintained deep merge library
• lodash.merge - Part of the well-maintained lodash ecosystem (note: configure to prevent prototype pollution)
• @fastify/deepmerge - Security-focused implementation

Workarounds

• Implement a sanitization layer that removes __proto__, constructor, and prototype keys from all user-controlled input before passing to extend-deep
• Wrap the extend-deep function with a proxy that validates input objects for dangerous property names
• Use Object.freeze(Object.prototype) at application startup to prevent prototype modifications (note: may break compatibility with some libraries)
• Apply strict schema validation on all API inputs to reject unexpected or dangerous property names

# Check for vulnerable extend-deep in your project
npm ls extend-deep

# Replace with a safer alternative
npm uninstall extend-deep
npm install deepmerge

# Audit all dependencies for known vulnerabilities
npm audit