# Misc JS Tricks & Relevant Info
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## Javascript Fuzzing
### Valid JS Comment Chars
```javascript
//This is a 1 line comment
/* This is a multiline comment*/
#!This is a 1 line comment, but "#!" must to be at the beggining of the line
-->This is a 1 line comment, but "-->" must to be at the beggining of the line
for (let j = 0; j < 128; j++) {
for (let k = 0; k < 128; k++) {
for (let l = 0; l < 128; l++) {
if (j == 34 || k ==34 || l ==34)
continue;
if (j == 0x0a || k ==0x0a || l ==0x0a)
continue;
if (j == 0x0d || k ==0x0d || l ==0x0d)
continue;
if (j == 0x3c || k ==0x3c || l ==0x3c)
continue;
if (
(j == 47 && k == 47)
||(k == 47 && l == 47)
)
continue;
try {
var cmd = String.fromCharCode(j) + String.fromCharCode(k) + String.fromCharCode(l) + 'a.orange.ctf"';
eval(cmd);
} catch(e) {
var err = e.toString().split('\n')[0].split(':')[0];
if (err === 'SyntaxError' || err === "ReferenceError")
continue
err = e.toString().split('\n')[0]
}
console.log(err,cmd);
}
}
}
//From: https://balsn.tw/ctf_writeup/20191012-hitconctfquals/#bounty-pl33z
// From: Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker (p. 43). Kindle Edition.
log=[];
for(let i=0;i<=0xff;i++){
for(let j=0;j<=0xfff;j++){
try {
eval(`${String.fromCodePoint(i,j)}%$£234$`)
log.push([i,j])
}catch(e){}
}
}
console.log(log)//[35,33],[47,47]
```
### Valid JS New Lines Chars
```javascript
//Javascript interpret as new line these chars:
String.fromCharCode(10) //0x0a
String.fromCharCode(13) //0x0d
String.fromCharCode(8232) //0xe2 0x80 0xa8
String.fromCharCode(8233) //0xe2 0x80 0xa8
for (let j = 0; j < 65536; j++) {
try {
var cmd = '"aaaaa";'+String.fromCharCode(j) + '-->a.orange.ctf"';
eval(cmd);
} catch(e) {
var err = e.toString().split('\n')[0].split(':')[0];
if (err === 'SyntaxError' || err === "ReferenceError")
continue;
err = e.toString().split('\n')[0]
}
console.log(`[${err}]`,j,cmd);
}
//From: https://balsn.tw/ctf_writeup/20191012-hitconctfquals/#bounty-pl33z
```
### Valid JS Spaces in function call
```javascript
// Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker (pp. 40-41). Kindle Edition.
// Check chars that can be put in between in func name and the ()
function x(){}
log=[];
for(let i=0;i<=0x10ffff;i++){
try {
eval(`x${String.fromCodePoint(i)}()`)
log.push(i)
}catch(e){}
}
console.log(log)v//9,10,11,12,13,32,160,5760,8192,8193,8194,8195,8196,8197,8198,8199,8200,8201,8202,813 232,8233,8239,8287,12288,65279
```
### **Valid chars to Generate Strings**
```javascript
// Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker (pp. 41-42). Kindle Edition.
// Check which pairs of chars can make something be a valid string
log=[];
for(let i=0;i<=0x10ffff;i++){
try {
eval(`${String.fromCodePoint(i)}%$£234${String.fromCodePoint(i)}`)
log.push(i)
}catch(e){}
}
console.log(log) //34,39,47,96
//single quote, quotes, backticks & // (regex)
```
### **Surrogate Pairs BF**
This technique won't be very useful for XSS but it could be useful to bypass WAF protections. This python code receive as input 2bytes and it search a surrogate pairs that have the first byte as the the last bytes of the High surrogate pair and the the last byte as the last byte of the low surrogate pair.
```python
def unicode(findHex):
for i in range(0,0xFFFFF):
H = hex(int(((i - 0x10000) / 0x400) + 0xD800))
h = chr(int(H[-2:],16))
L = hex(int(((i - 0x10000) % 0x400 + 0xDC00)))
l = chr(int(L[-2:],16))
if(h == findHex[0]) and (l == findHex[1]):
print(H.replace("0x","\\u")+L.replace("0x","\\u"))
```
More info:
* [https://github.com/dreadlocked/ctf-writeups/blob/master/nn8ed/README.md](https://github.com/dreadlocked/ctf-writeups/blob/master/nn8ed/README.md)
* [https://mathiasbynens.be/notes/javascript-unicode](https://mathiasbynens.be/notes/javascript-unicode) [https://mathiasbynens.be/notes/javascript-encoding](https://mathiasbynens.be/notes/javascript-encoding)
### `javascript{}:` Protocol Fuzzing
```javascript
// Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker (p. 34). Kindle Edition.
log=[];
let anchor = document.createElement('a');
for(let i=0;i<=0x10ffff;i++){
anchor.href = `javascript${String.fromCodePoint(i)}:`;
if(anchor.protocol === 'javascript:') {
log.push(i);
}
}
console.log(log)//9,10,13,58
// Note that you could BF also other possitions of the use of multiple chars
// Test one option
let anchor = document.createElement('a');
anchor.href = `javascript${String.fromCodePoint(58)}:alert(1337)`;
anchor.append('Click me')
document.body.append(anchor)
// Another way to test
Test
```
### URL Fuzzing
```javascript
// Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker (pp. 36-37). Kindle Edition.
// Before the protocol
a=document.createElement('a');
log=[];
for(let i=0;i<=0x10ffff;i++){
a.href = `${String.fromCodePoint(i)}https://hacktricks.xyz`;
if(a.hostname === 'hacktricks.xyz'){
log.push(i);
}
}
console.log(log) //0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32
// Between the slashes
a=document.createElement('a');
log=[];
for(let i=0;i<=0x10ffff;i++){
a.href = `/${String.fromCodePoint(i)}/hacktricks.xyz`;
if(a.hostname === 'hacktricks.xyz'){
log.push(i);
}
}
console.log(log) //9,10,13,47,92
```
### HTML Fuzzing
```javascript
// Heyes, Gareth. JavaScript for hackers: Learn to think like a hacker (p. 38). Kindle Edition.
// Fuzzing chars that can close an HTML comment
let log=[];
let div = document.createElement('div');
for(let i=0;i<=0x10ffff;i++){
div.innerHTML=``;
if(div.querySelector('span')){
log.push(i);
}
}
console.log(log)//33,45,62
```
## **Analizing attributtes**
The tool **Hackability inspector** from Portswigger helps to **analyze** the **attributtes** of a javascript object. Check: [https://portswigger-labs.net/hackability/inspector/?input=x.contentWindow\&html=%3Ciframe%20src=//subdomain1.portswigger-labs.net%20id=x%3E](https://portswigger-labs.net/hackability/inspector/?input=x.contentWindow\&html=%3Ciframe%20src=//subdomain1.portswigger-labs.net%20id=x%3E)
## **.map js files**
* Trick to download .map js files: [https://medium.com/@bitthebyte/javascript-for-bug-bounty-hunters-part-2-f82164917e7](https://medium.com/@bitthebyte/javascript-for-bug-bounty-hunters-part-2-f82164917e7)
* You can use this tool to analyze these files [https://github.com/paazmaya/shuji](https://github.com/paazmaya/shuji)
## "--" Assignment
The decrement operator `--` is also an asignment. This operator takes a value and then decrements it by one. If that value is not a number, it will be set to `NaN`. This can be used to **remove the content of variables from the environment**.
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## Functions Tricks
### .call and .apply
The **`.call`** method of a function is used to **run the function**.\
The **first argument** it expects by default is the **value of `this`** and if **nothing** is provided, **`window`** will be that value (unless **`strict mode`** is used).
```javascript
function test_call(){
console.log(this.value); //baz
}
new_this={value:"hey!"}
test_call.call(new_this);
// To pass more arguments, just pass then inside .call()
function test_call() {
console.log(arguments[0]); //"arg1"
console.log(arguments[1]); //"arg2"
console.log(this); //[object Window]
}
test_call.call(null, "arg1", "arg2")
// If you use the "use strict" directive "this" will be null instead of window:
function test_call() {
"use strict";
console.log(this); //null
}
test_call.call(null)
//The apply function is pretty much exactly the same as the call function with one important difference, you can supply an array of arguments in the second argument:
function test_apply() {
console.log(arguments[0]); //"arg1"
console.log(arguments[1]); //"arg2"
console.log(this); //[object Window]
}
test_apply.apply(null, ["arg1", "arg2"])
```
### Arrow functions
Arrow functions allow you to generate functions in a single line more easily (if you understand them)
```javascript
// Traditional
function (a){ return a + 1; }
// Arrow forms
a => a + 100;
a => {a + 100};
// Traditional
function (a, b){ return a + b + 1; }
// Arrow
(a, b) => a + b + 100;
// Tradictional no args
let a = 4;
let b = 2;
function (){ return a + b + 1; }
// Arrow
let a = 4;
let b = 2;
() => a + b + 1;
```
So, most of the previous functions are actually useless because we aren't saving them anywhere to save and call them. Example creating the `plusone` function:
```javascript
// Traductional
function plusone (a){ return a + 1; }
//Arrow
plusone = a => a + 100;
```
### Bind function
The bind function allow to create a **copy** of a **function modifying** the **`this`** object and the **parameters** given.
```javascript
//This will use the this object and print "Hello World"
var fn = function ( param1, param2 ) {
console.info( this, param1, param2 );
}
fn('Hello', 'World')
//This will still use the this object and print "Hello World"
var copyFn = fn.bind();
copyFn('Hello', 'World')
//This will use the "console" object as "this" object inside the function and print "fixingparam1 Hello"
var bindFn_change = fn.bind(console, "fixingparam1");
bindFn_change('Hello', 'World')
//This will still use the this object and print "fixingparam1 Hello"
var bindFn_thisnull = fn.bind(null, "fixingparam1");
bindFn_change('Hello', 'World')
//This will still use the this object and print "fixingparam1 Hello"
var bindFn_this = fn.bind(this, "fixingparam1");
bindFn_change('Hello', 'World')
```
{% hint style="info" %}
Note that using **`bind`** you can manipulate the **`this`** object that is going to be used when calling the function.
{% endhint %}
### Function code leak
If you can **access the object** of a function you can **get the code** of that function
```javascript
function afunc(){
return 1+1;
}
console.log(afunc.toString()); //This will print the code of the function
console.log(String(afunc)); //This will print the code of the function
console.log(this.afunc.toString()); //This will print the code of the function
console.log(global.afunc.toString()); //This will print the code of the function
```
In cases where the **function doesn't have any name**, you can still print the **function code** from within:
```javascript
(function (){ return arguments.callee.toString(); })()
(function (){ return arguments[0]; })("arg0")
```
Some **random** ways to **extract the code** of a function (even comments) from another function:
```javascript
(function (){ return retFunc => String(arguments[0]) })(a=>{/* Hidden commment */})()
(function (){ return retFunc => Array(arguments[0].toString()) })(a=>{/* Hidden commment */})()
(function (){ return String(this)}).bind(()=>{ /* Hidden commment */ })()
(u=>(String(u)))(_=>{ /* Hidden commment */ })
(u=>_=>(String(u)))(_=>{ /* Hidden commment */ })()
```
## Sandbox Escape - Recovering window object
The Window object allows to reach globally defined functions like alert or eval.
{% code overflow="wrap" %}
```javascript
// Some ways to access window
window.eval("alert(1)")
frames
globalThis
parent
self
top //If inside a frame, this is top most window
// Access window from document
document.defaultView.alert(1)
// Access document from a node object
node = document.createElement('div')
node.ownerDocument.defaultView.alert(1)
// There is a path property on each error event whose last element is the window
// In other browsers the method is
// In case of svg, the "event" object is called "evt"