Friday, June 06, 2014

In the previous post we talked about how to resolve the exercises 1, 2 and 3 of the XSS-game proposed by Google. Now, we are going to resolve the latest ones.

Exercise 4

This exercise is similar to the previous one (Exercise 3). The main difference is that now,  we have an input. 

It is expected that a number will be typed into the box, but... what will happen if we write a name instead of typing a number?

What happens is that our string has been included into the "img" tag...

So, if we use:  3');alert('Behindthefirewalls the result would be...

And the alert appears...

Exercise 5

I don't know what the reason for looking at "next=confirm" was at first because logic would dictate that the first attempt would be to try to exploit a XSS vulnerability in the input field...

But the first thing that I did was to replace "confirm" by "http://www.behindthefirewalls", reload the page, type my mail and click on "Next" and the result was that I was redirected to my blog...

We have discovered another security issue but what we want to do is  locate a XSS vulnerability.

I was trying different options with no success so I decided to read the hints offered by Google. "If you want to make clicking a link execute Javascript (without using the onclick handler), how can you do it?"

So I tried to use:


And the alert appeared.

Exercise 6 

The fourth hit says: "If you can't easily host your own evil JS file, see if will help you here."

If we change "foo" for "alert" will have included in its code:

So, if we use the link bellow, we can exploit the vulnerability.


I spent some time trying to solve this exercise in a different way. I tried a lot of possibilities to exploit a XSS vulnerability...

... until I remembered a post I read some months ago...


Posted on Friday, June 06, 2014 by Javier Nieto


Wednesday, June 04, 2014

As Google say, "Cross-site scripting (XSS) bugs are one of the most common and dangerous types of vulnerabilities in Web applications. These nasty buggers can allow your enemies to steal or modify user data in your apps..."

So they have decided to help us to learn how to exploit these kinds of vulnerabilities by creating a vulnerable web site at:

There are 6 exercises to resolve. Before starting to resolve these issues... Why should I  know how to exploit a XSS vulnerability?

  1. To be more qualified in the security field.
  2. To make money.
Currently, Google is paying up to $7,500 for dangerous XSS bugs discovered in their most sensitive products.

But Google is not the only one who is paying a bounty for disclosing vulnerabilities. Others like Yahoo, Facebook or Paypal have the same  policy of rewards for discovering bugs.

In this post, we are going to resolve 3 issues proposed by Google. In the next post, we will resolve the latest ones.

Exercise 1

That is the easiest exercise. Our input will be directly included in the page without proper escaping.

By inserting the code below, we will be successful.


Exercise 2

This exercise is an example of how to perform a persistent or stored Cross-Site Scripting attack in a simple way.

<img src=x onerror=alert('BehindTheFirewalls')>


This exercise is a little complex because the user doesn't have an input to try to exploit the XSS. 

But what happen if we rewrite the URI? If we change "#1" by "#11111"...

... we will see that "1111" has been added to the source code. 

So, if we add #11111'onerror=alert('BehindTheFirewalls')> at the end of the URL, the code will be:

<img src='/static/level3/cloud#11111'onerror=alert('BehindTheFirewalls')>'.jpg' />

And the alert will appear.

These are the three posible options to exploit this vulnerability.




Posted on Wednesday, June 04, 2014 by Javier Nieto