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Cross-site scripting (XSS) Attacks

Cross-site scripting (XSS) Attacks

Cross-site scripting (XSS) Attacks

Cross-site scripting (XSS) is a type of vulnerability commonly found in web applications. This vulnerability makes it possible for attackers to inject malicious code (e.g. JavaScript programs) into victim’s web browser.

Using this malicious code, the attackers can steal the victim’s credentials, such as cookies. The access control policies (i.e., the same origin policy) employed by the browser to protect those credentials can be bypassed by exploiting the XSS vulnerability. Vulnerabilities of this kind can potentially lead to large-scale attacks.

To demonstrate what attackers can do by exploiting XSS vulnerabilities, we have set up a web application named Elgg in our pre-built Ubuntu VM image. Elgg is an open-source web application for social networking, and it has implemented a number of countermeasures to remedy the XSS threat. To demonstrate how XSS attacks work, we have commented out these countermeasures in Elgg in our installation, intentionally making Elgg vulnerable to XSS attacks. Without the countermeasures, users can post any arbitrary message, including JavaScript programs, to the user profiles. In this lab, students need to exploit this vulnerability to launch an XSS attack on the modified Elgg, in a way that is similar to what Samy Kamkar did to MySpace in 2005 through the notorious Samy worm. The ultimate goal of this attack is to spread an XSS worm among the users, such that whoever views an infected user profile will be infected, and whoever is infected will add you (i.e., the attacker) to his/her friend list.

Environment setup for the problem:

For this problem, we will assume that you have set up the Ubuntu virtual machine environment based on the instructions in the Syllabus under “Special Software Installation Requirements”.

We will need the following:

Firefox web browser
Apache web server
Elgg web application
For the Firefox browser, we need to use the LiveHTTPHeaders extension for Firefox to inspect the HTTP requests and responses (available under the “Tools” menu in Firefox). The pre-built Ubuntu VM image provided to you has already installed the Firefox web browser with the required extension.

The Apache web server is also included in the pre-built Ubuntu image. However, the web server is not started by default. You have to first start the web server using one of the following two commands:

% sudo apache2ctl start


% sudo service apache2 start

The Elgg web application is already set up in the pre-built Ubuntu VM image. We have also created several user accounts on the Elgg server and the credentials are given below (username, password):

admin, seedelgg

alice, seedalice

boby, seedboby

charlie, seedcharlie

samy, seedsamy

You can access the Elgg server using the following URL (the Apache server needs to be started first):

(this URL is only accessible from inside of the virtual machine, because we have modified the /etc/hosts file to map the domain name ( to the virtual machine’s local IP address

Once you log in as a user in Elgg, you can access your Profile and list of Friends by clicking on icons in the upper left part of the browser window.

Note: Some of the project tasks require some basic familiarity with JavaScript. Wherever necessary,

we provide a sample JavaScript program to help you get started



Writing an XSS Worm

In this and next task, we will perform an attack similar to what Samy did to MySpace in 2005 (i.e., the Samy

Worm). First, we will write an XSS worm that does not self-propagate; in the next task, we will make it

self-propagating. From the previous task, we have learned how to steal the cookies from the victim.

In this task, we need to write a malicious JavaScript to forge a HTTP request directly from the victim’s browser. This attack does not require the intervention from the attacker. The JavaScript that can achieve this is called a cross-site scripting worm.

This task consists of two independent sub-tasks.


i: XSS Worm that adds a friend

The objective of the attack in this subtask is to modify the victim’s profile and add Samy as a friend of the victim. To add a friend for the victim, we should first find out how a legitimate user adds a friend in Elgg.

More specifically, we need to figure out what is sent to the server when a user adds a friend. Firefox’s

LiveHTTPHeaders extension can help us (available under the “Tools” menu in Firefox); it can display the header and contents of any HTTP request message sent from the browser. From this, we can identify all the parameters in the request.

There are two common types of HTTP requests, one is HTTP GET request, and the other is HTTP POST request. These two types of HTTP requests differ in how they send the contents of the request to the server. We can use the JavaScript XMLHttpRequest object to send HTTP GET and POST requests to web applications. XMLHttpRequest can only send HTTP requests back to the server, instead of other computers, because the same-origin policy is strongly enforced for XMLHttpRequest. This is not an issue for us, because we do want to use XMLHttpRequest to send a forged HTTP request back to the Elgg server.

To learn how to use XMLHttpRequest, you can study these documents:

If you are not familiar with JavaScript programming, we suggest that you study the following documents to learn some basic JavaScript functions. You will have to use some of these functions.

Essential Javascript – A Javascript Tutorial:–_A_Javascript_Tutorial.pdf

You may also need to debug your JavaScript code. Firebug is a Firefox extension that helps you debug JavaScript code. It can point you to the precise places that contain errors. FireBug is already installed in Firefox in our pre-built Ubuntu VM image (available under the “Tools” menu in Firefox).

For this subtask, the worm program should do the following:

1. Create the correct request to add Samy to the friends list of the user who is executing the malicious code

2. Forge a HTTP GET request to add Samy as a friend.


Code Skeleton. We provide a skeleton of the JavaScript code that you need to write. This JavaScript code is inserted into user Samy’s profile, and any user that views Samy’s profile will then automatically add Samy as their friend. You need to fill in all the necessary details. When you include the final JavaScript code in the message posted to Samy’s profile, you need to remove all the comments, extra space, and new-line characters.


<script id=”worm” type=”text/javascript”>

var Ajax=null;

//Construct the HTTP request to add Samy as a friend.

var sendurl=”…”;

//Create and send Ajax request to add friend.

// The format of the request can be learned from LiveHttpHeaders.

Ajax=new XMLHttpRequest();“GET”,sendurl,true);




// (JavaScript code to access session cookie)





Note that in this case the GET method is used to send the HTTP request.

To modify the victim’s profile, the HTTP request sent by the worm should contain the victim’s __elgg_ts and __elgg_token values in the sendurl variable. These details are present in the web page (right-click and “View Page Source”) and the worm needs to find out and use this information using JavaScript code. The sendurl variable should also contain Samy’s id.


What you need to do:

1. Based on the format of the GET request to add a friend, write a JavaScript script that adds Samy to the friends list of any user who views Samy’s profile. Save your JavaScript script in a file task4-1.txt.

2. Login as user Samy and inject in the “About me” field of Samy’s profile the script from file task4-1.txt. (Make sure to select “Remove editor” before editing this field, in order to disable any automatic formatting)

3. Logout and login as user Alice, and then view Samy’s profile by selecting user Samy from “More => Members” in the Elgg menu. At this point, the malicious Javascript script will be executed and Samy will be added to Alice’s friend list.

4. Include in your project document:

a. a screen printout with Alice’s friends list after viewing Samy’s profile.

b. a printout of your JavaScript file task4-1.txt.

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