**This post is based on the one from** [**https://itnext.io/prototype-pollution-attack-on-nodejs-applications-94a8582373e7**](https://itnext.io/prototype-pollution-attack-on-nodejs-applications-94a8582373e7)\*\*\*\*
First of all, we need to understand `Object`in JavaScript. An object is simply a collection of key and value pairs, often called properties of that object. For example:
In Javascript, `Object`is a basic object, the template for all newly created objects. It is possible to create an empty object by passing `null`to `Object.create`. However, the newly created object will also have a type that corresponds to the passed parameter and inherits all the basic properties.
```javascript
console.log(Object.create(null)); // prints an empty object
Previously we learned that an Object in javascript is collection of keys and values, so it makes sense that a `null` object is just an empty dictionary: `{}`
## Functions / Classes in Javascript <a id="55dd"></a>
In Javascript, the concepts of the class and the function are quite interrelated \(the function itself acts as the constructor for the class and the actual nature has no concept of “class” in javascript\). Let’s see the following example:
One thing to note is that the prototype attribute can be changed/modified/deleted when executing the code. For example functions to the class can be dynamically added:
In a prototype-based program, objects inherit properties/methods from classes. The classes are derived by adding properties/methods to an instance of another class or by adding them to an empty object.
Note that, if you add a property to an object that is used as the prototype for a set of objects \(like the myPersonObj\), the objects for which it is the prototype also get the new property, but that property is not printed unless specifically called on.
You should have already learned that **every object in JavaScript is simply a collection of key and value** pairs and that **every object inherits from the Object type in JavaScript**. This means that if you are able to pollute the Object type **each JavaScript object of the environment is going to be polluted!**
This is fairly simple, you just need to be able to modify some properties \(key-value pairs\) from and arbitrary JavaScript object, because as each object inherits from Object, each object can access Object scheme.
```javascript
function person(fullName) {
this.fullName = fullName;
}
var person1 = new person("Satoshi");
```
From the previous example it's possible to access the structure of Object using the following ways:
```javascript
person1.__proto__.__proto__
person.__proto__.__proto__
```
So, as it was mentioned before, if now a property is added to the Object scheme, every JavaScript object will have access to the new property:
person1.printHello() //This now works and prints hello
//Add constant as new property
person1.__proto__.__proto__.globalconstant = true
person1.globalconstant //This now works and is "true"
```
So now each JS object will contain the new properties: the function `printHello` and the new constant `globalconstant`
## prototype pollution
This technique isn't as effective as the previous one as you cannot pollute the scheme of JS Object. But in cases where the **keyword `__proto__`is forbidden this technique can be useful**.
If you are able to modify the properties of a function, you can modify the `prototype` property of the function and **each new property that you adds here will be inherit by each object created from that function:**
**In this case only the objects created from the `person` class will be affected, but each of them will now inherit the properties `sayHello` and `newConstant`.**
If you manage to do that, each JS object will be able to execute the function `sayBye`.
## Examples
### Basic Example
So where’s the prototype pollution? It happens when there’s a bug in the application that makes it possible to overwrite properties of `Object.prototype`. Since every typical object inherits its properties from `Object.prototype`, we can change application behaviour. The most commonly shown example is the following:
Imagine that we have a prototype pollution that makes it possible to set `Object.prototype.isAdmin = true`. Then, unless the application explicitly assigned any value, `user.isAdmin` is always true!
For example, `obj[a][b] = value`. If the attacker can control the value of `a` and `value`, then he only needs to adjust the value of `a`to `__proto__`\(in javascript, `obj["__proto__"]` and `obj.__proto__`are completely equivalent\) then property `b` of all existing objects in the application will be assigned to `value`.
However, the attack is not as simple as the one above, according to [paper](https://github.com/HoLyVieR/prototype-pollution-nsec18/blob/master/paper/JavaScript_prototype_pollution_attack_in_NodeJS.pdf), we can only attack when one of the following three conditions is met:
if (isObject(target[key]) && isObject(source[key])) {
merge(target[key], source[key]);
} else {
target[key] = source[key];
}
}
return target;
}
function clone(target) {
return merge({}, target);
}
clone(USERINPUT);
let proc = fork('VersionCheck.js', [], {
stdio: ['ignore', 'pipe', 'pipe', 'ipc']
});
```
You can observe that the merge function is coping one by one all the key-value pairs from a dictionary into another one. This may seem secure, but it isn't as the copy of the `__proto__` or `prototype` properties from a dictionary into an object may modify completely the structure of the rest of the JS objects \(as it was previously explained\).
In [https://research.securitum.com/prototype-pollution-rce-kibana-cve-2019-7609/](https://research.securitum.com/prototype-pollution-rce-kibana-cve-2019-7609/) you can see a way to exploit this vulnerability and obtain a RCE abusing environmental variables:
$ .extend, if handled incorrectly, can change the properties of the object `prototype`\(the template of the objects in the app\). This attribute will then appear on all objects. Note that only the “deep” version \(ie g\) of $ .extened is affected.
Programmers often use this function to duplicate an object or fill in new properties from a default object. For example:
We can imagine `myObject`is an input field from the user and is serialized into the DB\)
In this code, we often think, when running will assign the attribute `isAdmin`into the newly created object. But essentially, it is assigned directly to `{}` and then `{}.isAdmin` will be `true`. If after this code, we perform the following check:
If the user has not yet existed \( `undefined`\), the property`isAdmin`will be searched in its parent object, which is the Object added `isAdmin` with the value `true` above.
These errors can affect a lot of Javascript projects, especially NodeJS projects, the most practical example is the error in Mongoose, the JS library that helps manipulate MongoDB, in December 2018.
[Lodash](https://www.npmjs.com/package/lodash) is also a well-known library that provides a lot of different functions, helping us to write code more conveniently and more neatly with over 19 million weekly downloads. And It got the same problem as JQuery.
**CVE-2018–3721**
**CVE-2019–10744**
This bug affects all versions of Lodash, already fixed in version 4.17.11.