Reverse engineering Popunder.js – Chrome

Where it all started?

It all started with a Youtube video by @LiveOverflow, which is awesome by the way. This video talk is on a bit older version of Popunder.js.

Motivated & intrigued to checkout current version of popunder.js and reverse engineer to understand its engine.

To begin with, I cloned the demo site to my local machine, changed few parts to point my localhost (a simple code change). It consists of 5 files, out of which script.js is the heart of the engine payload.

First thing that caught my eye was the defensive code against Debugger tools.

Defensive Method – 1

var element = document.createElement('div');
element.__defineGetter__('id', function() {
  window.devToolsEnabled = _true;


Whenever a created element is printed onto the console, javascript engine reads the id of the element printed in-turn invoking the getter function defined by user. StackOverflow post talks about this behavior used by coders to detect if the debugger is opened or not. Chrome (74.0.3729.131) and Safari(12.1) both execute fake getter functions, interestingly Mozilla Firefox was immune to this.

Defensive Method – 2 (dead code in library)

Check if a function is proxied or not. If you have watched @LiveOverflow video’s on the Youtube , he wrote a proxy function for to catch the execution of the popunder.js which uses this JS API to open popup in the browser.

Code extract – (Read inline comments)

_isFunctionProxied: function(fnName) {
     var regex, fn, name;

     /* Extracts function name */
     fn = Utils._evaluate(fnName);
     name = fnName.replace(/^(?:.*[^\w])*([\w]+)\s*$/i, '$1');
     try {
       /* Creates a regex to check string "{ [native code] }" */
       regex = new RegExp(n7.c9('function ', name, '\\(\\)\\s* .{\\s*\\[\\s*native code\\s*\\]\\s*}'), 'im');

       /* Execute the regex and determine if proxied */
       return !regex.test(fn.toString());
     } catch (err) {
        if (err.toString().indexOf('is not a function') > -1) {
            return _true;
        if (Browser._isFirefox) {
            return _true;
        return _false;

Next, i started reading the code line by line. There were bunch of helper API’s and stuff. Interesting section was the block of code that were written to target specific browser’s & its version, thus exploiting the flaws to open pop-up which is the library’s goal.

Below are options available for popups to be opened (Extract of SwitchCase)

switch (type) {
    case 'popup':
        popOpts = {
            newTab: false,
            under: false

    case 'tabunder':
        popOpts = {
            newTab: true,
            under: true

    case 'tabup':
        popOpts = {
            newTab: true,
            under: false

        popOpts = {};


Thanks to @LiveOverFlow, filing bug report with Chromium which eliminated the popunder option, now this option defaults to tabunder only on Chrome. Watch this video for more details by @LiveOverFlow. You will realize its an art to exploit a flaw for your own gain.

There was this interesting option called tricksChrome which is a boolean flag that ignites a ChromeDance() function on the about:blank window thats opened on main window.onClick() event trigger.

Refactored Code from Popunder library - Jsfiddle link

window.onclick = function () {
  var w ='about:blank', 'sdffdfds', 'top=9999,left=0,width=100,height=100');
  var popUrl = "";
  var left = 0;
  var run = 0;
  var left = 0;
  function dance() {
    if (run > 0){
    run = 1;

    setInterval(function () {
      if (screenY > 100) 
        window.moveTo(0, 0);
        window.moveTo(left, 9999)
    }, 20)

  setTimeout(function () {
    var oldX = screenX, oldY = screenY,
      c = setInterval(function () {
        if (oldX != screenX || oldY != screenY) {
        oldX = screenX; 
        oldY = screenY;
      }, 50);
  }, 50);

  /*This annoys the user to put popup window in the background*/
  document.onmousemove = dance; 

  function finish() {
    window.moveTo(0, 0);
    window.resizeTo(${screen.width}, ${screen.height});
    window.location.href = "${popUrl}";

  /* when put in background load the target site */
  window.onblur = finish;
  setTimeout(finish, 5 * 60e3);


Voilla! Now you have a successful popup tricking the user. Technically it’s not a bug, but its beautifully thought through to trick the user.

Code extract

map.push([this._PostMessage, function() {
   return Browser._isChrome && Browser._version >= 64 && Browser._version < 67;
map.push([this._PDFViewer, function() {
   return Browser._isChrome && Utils._isPDFViewerEnabled() && Browser._isWin && Browser._version >= 43 && Browser._version < 64;
 map.push([this._PDFTabUnder, function() {
   return Browser._isChrome && Utils._isPDFViewerEnabled() && Browser._isMac && Browser._version >= 49 && Browser._version < 64;

Next section of code is a check for the vulnerable browser versions to exploit (influenced by @LiveOverflow’s reverse engineering disclosure’s), like the window.postMessage trick that is available in Chrome ver 68 and below, PDF as an embedded object to gain focus etc.

For all these vulnerabilities & explanation on older version, I would highly recommend to watch @LiveOverflow videos on Youtube.

Let me know what you think. I would be happy to listen to your feedback.

Published on

May 12, 2019


javascript, reverse engineering