@theecryptochad/merge-guard has Prototype Pollution in its deepMerge() function

Summary

@theecryptochad/merge-guard versions prior to 1.0.1 are vulnerable to Prototype Pollution via the deepMerge() function. An attacker who controls the source object can inject __proto__ keys that mutate Object.prototype, affecting all objects in the Node.js runtime.

Details

The deepMerge() function recursively merges two objects without sanitizing reserved property keys (__proto__, constructor, prototype). When a source object contains a __proto__ key, its value is assigned to target.__proto__, which JavaScript engines interpret as a write to Object.prototype.

Proof of Concept

const { deepMerge } = require('@theecryptochad/merge-guard');
const payload = JSON.parse('{"__proto__":{"isAdmin":true}}');
deepMerge({}, payload);
console.log({}.isAdmin); // true — Object.prototype is polluted

Impact

Any application using deepMerge() with untrusted input (e.g. user-supplied JSON from HTTP requests, WebSocket messages, or config files) is vulnerable. An attacker can inject arbitrary properties onto Object.prototype, enabling privilege escalation, application logic bypass, and property injection.

Remediation

Upgrade to @theecryptochad/merge-guard >= 1.0.1, which adds an explicit blocklist:

const BLOCKED = new Set(['__proto__', 'constructor', 'prototype']);
if (BLOCKED.has(key)) continue;

References

• CWE-1321: Improper Neutralization of Special Elements in Object Keys
• OWASP: Prototype Pollution
• Fix commit