Prototype Pollution Hunter

Prototype pollution is a critical JavaScript vulnerability that allows attackers to modify the prototype of base objects, potentially affecting every object in an application. As modern web applications increasingly rely on complex JavaScript frameworks and Node.js backends, understanding prototype pollution has become essential for security professionals and developers alike.

How JavaScript Prototypes Work

JavaScript uses a prototype-based inheritance model. Every object has an internal link to another object called its prototype, forming a prototype chain that ultimately leads to Object.prototype. When a property is accessed on an object, JavaScript first checks the object itself, then traverses the prototype chain until the property is found or the chain ends. This inheritance mechanism is powerful but creates a significant security risk when attackers can manipulate the prototype chain.

Understanding Prototype Pollution Attacks

Prototype pollution occurs when an attacker can inject properties into Object.prototype through vulnerable code patterns. The most common attack vector involves recursive object merge or deep copy functions that do not properly sanitize property keys. When user-controlled input contains keys like __proto__ or constructor.prototype, the merge operation writes to the prototype instead of the target object. This means every object in the application inherits the polluted properties, potentially overriding security checks, enabling privilege escalation, or triggering denial of service.

Real-World Impact of Prototype Pollution

Prototype pollution vulnerabilities have been discovered in widely used npm packages including lodash, jQuery, and express-fileupload. In real-world attacks, prototype pollution has been chained with other vulnerabilities to achieve remote code execution in Node.js applications. Common exploitation scenarios include bypassing authentication by polluting isAdmin properties, manipulating template engines to achieve server-side code execution, and poisoning application configuration objects. Security researchers and penetration testers must understand how to identify and exploit prototype pollution to effectively assess modern JavaScript applications.