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Javascript Characters Brute-Force

JavaScript Comments BF

//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

Javascript New Lines BF

//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

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:

.map js files

"--" 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

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')

{% 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

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 */ })()

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();
})();
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