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====== Security - XSS (Cross Site Scripting) - About XSS ======

===== What is XSS? =====

Cross-site scripting (XSS) is a code injection attack that allows an attacker to execute malicious JavaScript in another user's browser.

The attacker does not directly target his victim.  Instead, he exploits a vulnerability in a website that the victim visits, in order to get the website to deliver the malicious JavaScript for him.  To the victim's browser, the malicious JavaScript appears to be a legitimate part of the website, and the website has thus acted as an unintentional accomplice to the attacker.

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===== How the malicious JavaScript is injected =====

The only way for the attacker to run his malicious JavaScript in the victim's browser is to inject it into one of the pages that the victim downloads from the website.  This can happen if the website directly includes user input in its pages, because the attacker can then insert a string that will be treated as code by the victim's browser.

In the example below, a simple server-side script is used to display the latest comment on a website:

<code html>
print "<html>"
print "Latest comment:"
print database.latestComment
print "</html>"
</code>

The script assumes that a comment consists only of text.  However, since the user input is included directly, an attacker could submit this comment: <color red>**"<script>...</script>"**</color>.  Any user visiting the page would now receive the following response:

<code html>
<html>
Latest comment:
<script>...</script>
</html>
</code>

When the user's browser loads the page, it will execute whatever JavaScript code is contained inside the **<script>** tags.  The attacker has now succeeded with his attack.

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===== What is malicious JavaScript? =====

At first, the ability to execute JavaScript in the victim's browser might not seem particularly malicious.  After all, JavaScript runs in a very restricted environment that has extremely limited access to the user's files and operating system.  In fact, you could open your browser's JavaScript console right now and execute any JavaScript you want, and you would be very unlikely to cause any damage to your computer.

However, the possibility of JavaScript being malicious becomes more clear when you consider the following facts:

  * JavaScript has access to some of the user's sensitive information, such as cookies.
  * JavaScript can send HTTP requests with arbitrary content to arbitrary destinations by using XMLHttpRequest and other mechanisms.
  * JavaScript can make arbitrary modifications to the HTML of the current page by using DOM manipulation methods.

These facts combined can cause very serious security breaches, as we will explain next.

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===== The consequences of malicious JavaScript =====

Among many other things, the ability to execute arbitrary JavaScript in another user's browser allows an attacker to perform the following types of attacks:

  * **Cookie theft**\\ The attacker can access the victim's cookies associated with the website using document.cookie, send them to his own server, and use them to extract sensitive information like session IDs.

  * **Keylogging**\\ The attacker can register a keyboard event listener using addEventListener and then send all of the user's keystrokes to his own server, potentially recording sensitive information such as passwords and credit card numbers.

  * **Phishing**\\ The attacker can insert a fake login form into the page using DOM manipulation, set the form's action attribute to target his own server, and then trick the user into submitting sensitive information.

Although these attacks differ significantly, they all have one crucial similarity: because the attacker has injected code into a page served by the website, the malicious JavaScript is executed in the context of that website.  This means that it is treated like any other script from that website: it has access to the victim's data for that website (such as cookies) and the host name shown in the URL bar will be that of the website.  For all intents and purposes, the script is considered a legitimate part of the website, allowing it to do anything that the actual website can.

This fact highlights a key issue:

> If an attacker can use your website to execute arbitrary JavaScript in another user's browser, the security of your website and its users has been compromised.

To emphasize this point, some examples in this tutorial will leave out the details of a malicious script by only showing **<script>...</script>**.  This indicates that the mere presence of a script injected by the attacker is the problem, regardless of which specific code the script actually executes.

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