Cross-Site Scripting (XSS) Tutorial: Learn About XSS Vulnerabilities, XSS Injections and How to Prevent Cross Site Scripting Attacks
XSS - What is Cross-Site Scripting?
XSS is the most common security vulnerability in software today. This should not be the case as XSS is easy to find and easy to fix. XSS vulnerabilities can have consequences such as tampering and sensitive data theft.
Key Concepts of XSS
- XSS is a Web-based attack performed on vulnerable Web applications.
- In XSS attacks, the victim is the user and not the application.
Explaining Cross-Site Scripting
An XSS vulnerability arises when Web applications take data from users and dynamically include it in Web pages without first properly validating the data. XSS vulnerabilities allow an attacker to execute arbitrary commands and display arbitrary content in a victim user's browser. A successful XSS attack leads to an attacker controlling the victim’s browser or account on the vulnerable Web application. Although XSS is enabled by vulnerable pages in a Web application, the victims of an XSS attack are the application's users, not the application itself. The potency of an XSS vulnerability lies in the fact that the malicious code executes in the context of the victim's session, allowing the attacker to bypass normal security restrictions.
Cross-Site Scripting Video
XSS Attack Examples
Impact of Cross-Site Scripting
When attackers succeed in exploiting XSS vulnerabilities, they can gain access to account credentials. They can also spread Web worms or access the user’s computer and view the user’s browser history or control the browser remotely. After gaining control to the victim’s system, attackers can also analyze and use other intranet applications.
By exploiting XSS vulnerabilities, an attacker can perform malicious actions, such as:
- Hijack an account.
- Spread Web worms.
- Access browser history and clipboard contents.
- Control the browser remotely.
- Scan and exploit intranet appliances and applications.
Identifying Cross-Site Scripting Vulnerabilities
XSS vulnerabilities may occur if:
- Input coming into Web applications is not validated
- Output to the browser is not HTML encoded
For example, the HTML snippet:
<title>Example document: %(title)</title>
is intended to illustrate a template snippet that, if the variable title has value Cross-Site Scripting, results in the following HTML to be emitted to the browser:
<title>Example document: XSS Doc</title>
A site containing a search field does not have the proper input sanitizing. By crafting a search query looking something like this:
Sitting on the other end, at the Webserver, you will be receiving hits where after a double space is the users cookie. You might strike lucky if an administrator clicks the link, allowing you to steal their sessionID and hijack the session.
Suppose there's a URL on Google's site, http://www.google.com/search?q=flowers, which returns HTML documents containing the fragment
<p>Your search for 'flowers' returned the following results:</p>
i.e., the value of the query parameter q is inserted into the page returned by Google. Suppose further that the data is not validated, filtered or escaped.
Evil.org could put up a page that causes the following URL to be loaded in the browser (e.g., in an invisible<iframe>):
When a victim loads this page from www.evil.org, the browser will load the iframe from the URL above. The document loaded into the iframe will now contain the fragment
<p>Your search for 'flowers <script>evil_script()</script>'
returned the following results:</p>
Loading this page will cause the browser to execute evil_script(). Furthermore, this script will execute in the context of a page loaded from www.google.com!
XSS Cheat Sheet: Prevent a Cross-Site Scripting Attack
The Cross-Site Scripting Cheat Sheet provides a summary of what you need to know about Cross-Site Scripting. Our XSS cheat sheet details the different types of Cross-site Scripting and shows you how to protect against Cross-site Scripting vulnerabilities.
Written by: Fergal Glynn