Client-side validation can provide faster response and better user experience, and reduce server load. But relying client-side validation alone is a terrible idea.  

As user can easily bypass the validation, change value of javascript object using tools like Chrome DevTools or Firebug. We must always appropriate server-side validation as well.
Using Chrome DevTools to Hack it
One website limits the max value of one field, and I am wondering whether I can bypass the verification.
1. Found the pattern
I noticed that when I input invalid value, and go to next field, it will change the color of the field to red.
2. Beak on DOM Attributes Modifications 
So when the color of the field changes, I want Chrome to stop at that moment, so I can figure out how this website do the verification.

So I right click the input textbox, click “Inspect Element”, this will open Chrome DevTools, and select the checked element in “Elements” tab. 
<input title=”” class=”input_field_” type=”text” name=”Memory” id=”memory”>
Right click it, and select “Break on…” -> “Attributes modifications”.

document.write("(adsbygoogle = window.adsbygoogle || []).push({});”)
} else {
if (window.CHITIKA === undefined) {
window.CHITIKA = { ‘units’ : [] };
var unit = {
‘publisher’ : “jefferyyuan”,
‘width’ : 728,
‘height’ : 90,
‘type’ : “mpu”,
‘sid’ : “Chitika Default”,
‘color_site_link’ : “FFFFFF”,
‘color_title’ : “FFFFFF”,
‘color_border’ : “FFFFFF”,
‘color_text’ : “4E2800”,
‘color_bg’ : “F7873D”
var placement_id = window.CHITIKA.units.length;

var s = document.createElement(‘script’);
s.type = ‘text/javascript’;
s.src = ‘http://scripts.chitika.net/getads.js&#8217;;
try {
} catch(e) {

This will cause Chrome pauses when attribute of this input textbox changes.
It would be better if Chrome DevTools allows us to stop when a particular attribute changes, in this case: class field. or even better to stop when a particular attribute changes to a particular value. But anyhow this works.
3. Reproduce It
Now input the invalid value, then trigger the event that will cause the client side validation. Usually this will happen when we focus leaves current field or we click next or submit.

This will cause Chrome stops execution, and switch to “Sources” tab and stops at the code that change 
4. Check the Call Stack
Now we can check the call stack. 
To make  javascript much easier to read, we can press the “Pretty print” button (marked with curly braces{}) from the bottom of the “Sources” tab.

Follow the call stack, I found it do verification like below:

function validateSteps(step) 
    var isStepValid = true;
  // omitted
    if (step == step_config) {
        return validateField('#memory', selectedPolicy.minMemory, selectedPolicy.maxMemory) &&...);
    return isStepValid; // we can set a breakpoint value here to change isStepValid to true. 

The code compare the value from memory textbox with selectedPolicy.minMemory, selectedPolicy.maxMemory.
5. Hack it
Now there are several ways to hack it: we can step into validateField method and change return value.
The simpler way is to directly modify the object selectedPolicy, change maxMemory to a much bigger value. We go to the “Console” tab, and type selectedPolicy.maxMemory, this will print current value, it’s 4. We can type selectedPolicy.maxMemory=100.

Now unclick all breakpoints, and press resume, and submit the form. It works, this site doesn’t check the value in server side at all.

Later if I want to so the same trick: bypass the verification, I can go to “Console” tab directly, and type selectedPolicy.maxMemory=100.

In Console tab, we can do many things, run any valid javascript code in current scope.
We can check windows object, and detect any custom properties and functions.

Happy Hacking!!!
Tips And Tricks

via Blogger http://lifelongprogrammer.blogspot.com/2013/12/using-chrome-devtools-to-hack-client-only-verification.html