In the world of web security, prototype pollution is a subtle yet potentially devastating vulnerability that can lead to severe consequences if not addressed properly. In this blog, we will explore what prototype pollution is, how it occurs, and most importantly, how to prevent it. Let's dive in!

What is Prototype Pollution?

Prototype pollution is a type of vulnerability that affects JavaScript applications. It occurs when an attacker is able to inject properties into an object's prototype, which can then propagate to all objects that inherit from this prototype. This can lead to unexpected behavior, including the ability to overwrite existing methods and properties, ultimately compromising the security and functionality of the application.

How Prototype Pollution Happens

To understand how prototype pollution happens, we need to take a closer look at JavaScript objects and prototypes. In JavaScript, every object has a prototype, which is another object from which the first object inherits properties and methods. This prototype chaining allows for efficient property lookup but also opens the door for potential attacks if not handled correctly.

Here’s a simple example of how prototype pollution can occur:

let obj = {};
console.log(obj.constructor); // function Object() { [native code] }

obj.__proto__.polluted = true;
console.log({}.polluted); // true

In this example, by modifying the proto property of obj, we inadvertently affect all objects that share the same prototype, demonstrating how easy it is to pollute the prototype chain.

Real-World Example of Prototype Pollution

Consider a scenario where user input is used to extend or merge objects without proper validation. A common use case is merging query parameters into a configuration object.

const merge = require('lodash/merge');

let config = {};
let query = JSON.parse('{"__proto__":{"admin":true}}');

merge(config, query);

console.log(config.admin); // undefined
console.log({}.admin); // true

In this example, the merge function from the Lodash library is used to combine config with query. However, the attacker-controlled query object includes a proto property that pollutes the global object prototype, setting admin to true for all objects.

Preventing Prototype Pollution

To safeguard your applications from prototype pollution, consider implementing the following measures:

1. Avoid Extending Native Prototypes:
Do not extend native prototypes (e.g., Object.prototype) directly, as it can lead to conflicts and security vulnerabilities.

Example: Avoid Extending Native Prototypes
Avoid doing this:

Object.prototype.polluted = true; // Extending native prototype

let obj = {};
console.log(obj.polluted); // true

Instead, create utility methods within your own namespace:

const myUtils = {
  polluted: function() {
    // Your method implementation
  }
};

let obj = {};
console.log(obj.polluted); // undefined

2. Validate User Input:
Always validate and sanitize user input before using it to construct or modify objects. Use libraries like Joi or Validator to enforce strict input validation rules.

Example: Validate User Input Using Joi

const Joi = require('joi');

const schema = Joi.object({
  admin: Joi.boolean().required()
});

const input = JSON.parse('{"admin":true}');

const { error, value } = schema.validate(input);

if (error) {
  console.error('Invalid input:', error.details);
} else {
  console.log('Valid input:', value);
}

3. Use Safe Object Methods:

Prefer using safe object methods that do not traverse the prototype chain, such as Object.create(null) to create plain objects without a prototype.

Example: Use Safe Object Methods

let safeObj = Object.create(null);
safeObj.admin = false;

console.log(safeObj.constructor); // undefined
console.log(safeObj.admin); // false

4. Freeze the Prototype:

Freeze the Object.prototype to prevent modifications to the prototype chain. This can be done using Object.freeze().

Example: Freezing the Prototype

Object.freeze(Object.prototype);

let obj = {};
try {
  obj.__proto__.polluted = true;
} catch (e) {
  console.error('Attempt to modify prototype failed:', e);
}

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

5. Update Dependencies:

Regularly update your dependencies to ensure you are using the latest versions that include security patches. Vulnerabilities in third-party libraries are often exploited for prototype pollution attacks.

Example: Updating Dependencies Using npm

npm update

Run this command regularly to ensure that all your packages are up to date.

6. Monitor and Test:
Implement monitoring and automated testing to detect and mitigate prototype pollution vulnerabilities. Tools like npm audit can help identify vulnerable packages in your project.

Example: Monitoring and Testing with npm audit

npm audit

Run this command to scan your project for vulnerabilities. It provides a report of found issues and suggests remediation steps.

Conclusion

Prototype pollution is a critical vulnerability that can have far-reaching consequences if left unchecked. By understanding how it occurs and implementing best practices to prevent it, you can significantly enhance the security of your JavaScript applications. Stay vigilant, keep your dependencies up to date, and always validate user input to protect against this insidious attack vector.

If you found this blog helpful, be sure to share it with your fellow developers and security enthusiasts. Staying informed and proactive is key to maintaining robust web security. Happy coding!