Tuesday, 27 November 2012

XSS(Cross Site Scripting) Part 1:


Hello Everyone,
Today I am going to share with you some basic methods of XSS attack.


XSS or Cross Site Scripting is a vulnerability found in web application. It is method by which malicious script is injected into the the websites .These type of attacks is made by injecting some code written in client side scripting code like Javascript or VBscript .And once the code is executed in the browser side it can be used to perform attacks like generating popups , cookie stealing and as well as website defacement.

XSS can be divided into 3 different category 

  • Non Persistent 
  • Persistent
  • DOM Based

In this tutorial I am going to discuss mainly on Non Persistent XSS attacks. The other techniques will be presented in separate tutorial.

CAUTION
"This tutorial should be used for educational purpose only. I won't be responsible if you misuse this techniques and get yourself in trouble.Performing such attacks without the permission of the owner can lead to serious trouble.Most of the websites uses IDS to check the input sent by the user.Any malicious script if detected by these IDS coming from your IP then your IP will be logged and henceforth  blocked"



Prerequisites of this Tutorial:
  • Knowledge on PHP .
  • Knowledge on HTML & JavaScript.
  • Knowledge on Form Processing in HTML.

Non Persistent XSS
Non Persistent XSS attacks are caused due to improper filtering / no filtering of the input sent by the user.They generally does not cause much harm but they can be used to extract information stored in cookies and can be also used for many other purposes.


Scenario
This a simple web application that is developed using PHP and HTML. The working mechanism is simple. It asks for a user input and then displays the input  .

Now lets have a look at the HTML code.


Here we are using POST method to send the input information to the target page display.[hp which is written in PHP .Lets analyze the PHP code of display.php



Now we can see that there is no filtering of the input and and input is directly displayed.
Now I are going to inject some malicious code into the input

Here is my malicious code which I am going to inject in the input field.
                 <script language="javascript">alert("You are hacked");</script>

And here is what I get in the browser.



Now we see a popup. 
The reason why this popup executes is simple . When the injected code is passed ,the browser have no idea if its malicious or not,it just executes it just like a normal javascript code. Now when this code is executed we see this popup. Now you might have a question in your mind how this can be harmful . Well I will explain you it in later posts after I cover the remaining 2 types of XSS.
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