Prototype pollution

Prototype Pollution: A Deep Dive into a JavaScript Vulnerability

Prototype pollution is a critical vulnerability affecting JavaScript applications, stemming from the flexible nature of JavaScript's prototypal inheritance model. This vulnerability allows attackers to inject properties into an object's prototype, effectively poisoning the base template for all instances of that object. The consequences can range from denial of service to remote code execution, making it a significant security concern for web developers.

Understanding JavaScript Prototypes

JavaScript uses prototypes to achieve inheritance. Every object in JavaScript has a prototype, which is another object it inherits properties from. When accessing a property on an object, if the property doesn't exist directly on the object, JavaScript will search the object's prototype, and then the prototype's prototype, and so on, until the property is found or the prototype chain ends at null.

How Prototype Pollution Works

Prototype pollution exploits the mutability of JavaScript prototypes. Malicious actors can inject properties into the base prototype object (Object.prototype), affecting all objects inheriting from it. This is typically achieved by exploiting functions that recursively merge objects without proper validation of keys. For instance, a common scenario involves user-supplied data being used in a merge function that doesn't sanitize keys like __proto__ or constructor.prototype.

Consider the following vulnerable code:

function merge(target, source) {
  for (const key in source) {
    if (typeof source[key] === 'object') {
      if (!target[key]) {
        target[key] = {};
      }
      merge(target[key], source[key]);
    } else {
      target[key] = source[key];
    }
  }
}

let user_data = JSON.parse('{"__proto__": {"polluted": true}}');
let app_data = {};

merge(app_data, user_data);

console.log({}.polluted); // true

In this example, the malicious __proto__ key in user_data pollutes the prototype chain, adding the polluted property to Object.prototype. This affects all objects, making [].polluted and even (function(){}).polluted evaluate to true.

Impact of Prototype Pollution

The impact of prototype pollution can be far-reaching:

• Denial of Service (DoS): Overwriting critical properties can lead to application crashes and unhandled exceptions, effectively disabling the application.
• Property Override: Attackers can modify existing properties, leading to unexpected behavior and potentially bypassing security checks.
• Gadget Chains and Remote Code Execution (RCE): In certain circumstances, prototype pollution can be chained with other vulnerabilities (gadgets) to achieve remote code execution. Libraries and frameworks often use specific properties in their internal logic, making them potential targets for exploitation.
• Bypassing Client-Side Validations: Prototype pollution can be used to tamper with client-side validations by modifying properties used in validation logic.

Preventing Prototype Pollution

Mitigating prototype pollution requires a multi-pronged approach:

• Avoid using __proto__: Refactor code to avoid using __proto__ directly. Modern JavaScript offers safer alternatives like Object.create() and Object.getPrototypeOf().
• Validate User Input: Sanitize all user-supplied data, particularly when used in recursive merging functions. Avoid assigning properties directly from user input without proper validation.
• Use Object.freeze(): Freezing objects, particularly prototypes, prevents further modification. Object.freeze(Object.prototype) prevents pollution of the base prototype.
• Use Map Instead of Plain Objects: For dynamic property assignment, using Map objects provides a safer alternative, as they don't inherit from Object.prototype.
• Secure Libraries and Frameworks: Keep libraries and frameworks updated to patch known vulnerabilities related to prototype pollution.
• Static Analysis Tools: Integrate static analysis tools into the development workflow to detect potential vulnerabilities early in the development cycle. These tools can identify unsafe merging patterns and flag potential pollution risks.
• Regular Security Audits: Perform regular security audits and penetration testing to identify and address vulnerabilities in the application.

Conclusion

Prototype pollution is a serious vulnerability that can have significant consequences for JavaScript applications. Understanding the mechanics of this vulnerability and adopting appropriate mitigation strategies is crucial for building secure and resilient web applications. By implementing best practices, developers can effectively protect their applications from this increasingly common attack vector.