Misc JS Tricks & Relevant Info

Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE) Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)

Support HackTricks

Check the subscription plans!
Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.
Share hacking tricks by submitting PRs to the HackTricks and HackTricks Cloud github repos.

Javascript Fuzzing

Valid JS Comment Chars

//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 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

// 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

// 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.

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:

`javascript{}:` Protocol Fuzzing

// 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
<a href="&#12;javascript:alert(1337)">Test</a>

URL Fuzzing

// 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

// 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=`<!----${String.fromCodePoint(i)}><span></span>-->`;
    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

.map js files

Trick to download .map js files: 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

"--" 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.

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).

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)

// 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:

// 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.

//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')

Note that using bind you can manipulate the this object that is going to be used when calling the function.

Function code leak

If you can access the object of a function you can get the code of that function

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:

(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:

(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.

// 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
<img src onerror=event.path.pop().alert(1337)>
// In other browsers the method is
<img src onerror=event.composedPath().pop().alert(1337)>
// In case of svg, the "event" object is called "evt"
<svg><image href=1 onerror=evt.composedPath().pop().alert(1337)>

// Abusing Error.prepareStackTrace to get Window back
Error.prepareStackTrace=function(error, callSites){
2   callSites.shift().getThis().alert(1337);
3 };
4 new Error().stack

// From an HTML event
// Events from HTML are executed in this context
with(document) {
    with(element) {
        //executed event
    }
}
// Because of that with(document) it's possible to access properties of document like:
<img src onerror=defaultView.alert(1337)>
<img src onerror=s=createElement('script');s.append('alert(1337)');appendChild(s)>

Breakpoint on access to value

// Stop when a property in sessionStorage or localStorage is set/get
// via getItem or setItem functions
sessionStorage.getItem = localStorage.getItem  = function(prop) {
    debugger;
    return sessionStorage[prop];
}

localStorage.setItem = function(prop, val) {
    debugger;
    localStorage[prop] = val;
}

// Stop when anyone sets or gets the property "ppmap" in any object
// For example sessionStorage.ppmap
// "123".ppmap
// Useful to find where weird properties are being set or accessed
// or to find where prototype pollutions are occurring 

function debugAccess(obj, prop, debugGet=true){

    var origValue = obj[prop];

    Object.defineProperty(obj, prop, {
        get: function () {
            if ( debugGet )
                debugger;
            return origValue;
        },
        set: function(val) {
            debugger;
            origValue = val;
        }
    });
};

debugAccess(Object.prototype, 'ppmap')

Automatic Browser Access to test payloads

//Taken from https://github.com/svennergr/writeups/blob/master/inti/0621/README.md
const puppeteer = require("puppeteer");

const realPasswordLength = 3000;
async function sleep(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  //Loop to iterate through different values
  for (let i = 0; i < 10000; i += 100) {
    console.log(`Run number ${i}`);
    const input = `${"0".repeat(i)}${realPasswordLength}`;
    console.log(`  https://challenge-0621.intigriti.io/passgen.php?passwordLength=${input}&allowNumbers=true&allowSymbols=true&timestamp=1624556811000`);
    //Go to the page
    await page.goto(
      `https://challenge-0621.intigriti.io/passgen.php?passwordLength=${input}&allowNumbers=true&allowSymbols=true&timestamp=1624556811000`
    );
    //Call function "generate()" inside the page
    await page.evaluate("generate()");
    //Get node inner text from an HTML element
    const passwordContent = await page.$$eval(
      ".alert .page-content",
      (node) => node[0].innerText
    );
    //Transform the content and print it in console
    const plainPassword = passwordContent.replace("Your password is: ", "");
    if (plainPassword.length != realPasswordLength) {
      console.log(i, plainPassword.length, plainPassword);
    }

    await sleep(1000);
  }
  await browser.close();
})();

Support HackTricks

Check the subscription plans!
Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.
Share hacking tricks by submitting PRs to the HackTricks and HackTricks Cloud github repos.

PreviousJS Hoisting NextPDF Injection

Last updated 4 months ago

Javascript Fuzzing

Valid JS Comment Chars

Valid JS New Lines Chars

Valid JS Spaces in function call

Valid chars to Generate Strings

Surrogate Pairs BF

javascript{}: Protocol Fuzzing

URL Fuzzing

HTML Fuzzing

Analizing attributtes

.map js files

"--" Assignment

Functions Tricks

.call and .apply

Arrow functions

Bind function

Function code leak

Sandbox Escape - Recovering window object

Breakpoint on access to value

Automatic Browser Access to test payloads

Javascript Fuzzing

Valid JS Comment Chars

Valid JS New Lines Chars

Valid JS Spaces in function call

Valid chars to Generate Strings

Surrogate Pairs BF

javascript{}: Protocol Fuzzing

URL Fuzzing

HTML Fuzzing

Analizing attributtes

.map js files

"--" Assignment

Functions Tricks

.call and .apply

Arrow functions

Bind function

Function code leak

Sandbox Escape - Recovering window object

Breakpoint on access to value

Automatic Browser Access to test payloads

`javascript{}:` Protocol Fuzzing

`javascript{}:` Protocol Fuzzing