Understanding and Preventing Prototype Pollution in JavaScript

Introduction

In the world of JavaScript security, Prototype Pollution is a critical vulnerability that every developer should be aware of. This blog post will dive deep into what Prototype Pollution is, how it can be exploited, and most importantly, how to prevent it in your JavaScript applications.

What is Prototype Pollution?
How Does Prototype Pollution Work?
The Dangers of Prototype Pollution
Real-World Implications
How to Prevent Prototype Pollution
Detecting Prototype Pollution
Conclusion

What is Prototype Pollution?

Prototype Pollution is a type of vulnerability that allows an attacker to modify the prototype of base objects in JavaScript. This can lead to unexpected behavior, application crashes, and in severe cases, remote code execution.

In JavaScript, almost everything is an object, and objects inherit properties and methods from their prototype. The prototype chain ends with Object.prototype. If an attacker can pollute Object.prototype, they can potentially affect every object in the application.

How Does Prototype Pollution Work?

Let's look at a simple example:

function merge(target, source) {
  for (let key in source) {
    if (typeof source[key] === 'object') {
      target[key] = merge(target[key] || {}, source[key])
    } else {
      target[key] = source[key]
    }
  }
  return target
}

const obj = {}
const maliciousPayload = JSON.parse('{"__proto__": {"polluted": true}}')

merge(obj, maliciousPayload)

console.log(obj.polluted) // true
console.log({}.polluted) // true - All objects are now polluted!

In this example, the merge function is vulnerable to prototype pollution. The malicious payload adds a polluted property to Object.prototype, affecting all objects.

The Dangers of Prototype Pollution

Application-wide Impact: Since Object.prototype is at the root of the prototype chain, polluting it affects all objects.
Hard to Detect: The effects of prototype pollution might not be immediately visible and can cause hard-to-debug issues.
Remote Code Execution: In severe cases, attackers can inject malicious functions into the prototype, leading to remote code execution.
Bypass Security Checks: Polluted prototypes can potentially bypass certain types of security checks in applications.

Real-World Implications

Prototype pollution has been found in popular libraries and frameworks. For example:

In 2018, a prototype pollution vulnerability was discovered in the lodash library (CVE-2018-3721 and CVE-2018-16487).
jQuery before 3.4.0 was vulnerable to prototype pollution (CVE-2019-11358).

These vulnerabilities could potentially affect millions of websites and applications.

How to Prevent Prototype Pollution

Use Object.create(null): Create objects without a prototype to store user-controlled data:
```
const safeObject = Object.create(null)
```
Freeze the Prototype: Prevent modifications to Object.prototype:
```
Object.freeze(Object.prototype)
```

Avoid Recursive Merging and Use Safe Merge: Be cautious with functions that recursively merge objects.

function safeMerge(target, source) {
  for (let key in source) {
    if (key === '__proto__' || key === 'constructor') {
      continue
    }
    if (typeof source[key] === 'object') {
      target[key] = safeMerge(target[key] || {}, source[key])
    } else {
      target[key] = source[key]
    }
  }
  return target
}

Sanitize and Validate Input: Always sanitize and validate user input before using it in object operations.
Use Maps Instead of Plain Objects: For storing dynamic keys, consider using Map instead of plain objects:
```
const safeMap = new Map()
safeMap.set('userInput', userValue)
```

Use JSON.parse with a Reviver Function: When parsing JSON, use a reviver function to filter out __proto__ and constructor:

const safeParse = (json) => {
  return JSON.parse(json, (key, value) => {
    if (key === '__proto__' || key === 'constructor') {
      return undefined
    }
    return value
  })
}

Use Object.hasOwn() or Object.hasOwnProperty(): When checking for properties, use these methods to ensure you're not accessing prototype properties:

if (Object.hasOwn(obj, 'prop')) {
  // Safe to use obj.prop
}

for (let key in obj) {
  if (Object.prototype.hasOwnProperty.call(obj, key)) {
    // Safe to use obj[key]
  }
}

Use Modern JavaScript Features: Methods like Object.assign() and the spread operator {...obj} don't merge prototypes.
Keep Dependencies Updated: Always use the latest versions of your dependencies, as they often include security patches.

Detecting Prototype Pollution

To detect potential prototype pollution in your codebase:

Use static analysis tools like ESLint with security plugins.
Implement runtime checks to detect attempts to modify __proto__.
Use security scanning tools that specifically look for prototype pollution vulnerabilities.

Conclusion

Prototype Pollution is a serious security vulnerability in JavaScript that can have far-reaching consequences. As developers, it's crucial to understand how it works and implement proper safeguards in our code.

By following best practices such as using Object.create(null), freezing prototypes, and properly validating input, we can significantly reduce the risk of prototype pollution in our applications.

Remember, security is an ongoing process. Stay informed about the latest security vulnerabilities and regularly audit your code and dependencies for potential issues.

Stay safe, and happy coding!