Lab 5: Conducting a Cross-Site Scripting (XSS) Attack
Objective
Understand the mechanics of a Cross-Site Scripting (XSS) attack, how attackers exploit vulnerable web applications, and the steps to mitigate these vulnerabilities. This lab focuses on executing a stored or reflected XSS attack in a controlled environment.
Prerequisites
- Kali Linux or Any Linux Distro:
- Ensure you have a web browser and testing tools like Burp Suite or OWASP ZAP installed.
- Vulnerable Web Application:
- Use an intentionally vulnerable application like DVWA (Damn Vulnerable Web Application) or OWASP Juice Shop.
- Tip: Install DVWA using XAMPP, Docker, or directly on your system.
- Basic Web Knowledge:
- Familiarity with HTML, JavaScript, and how websites process user input.
- Insight: Understanding web forms, cookies, and scripts is crucial for this lab.
Step 1: Setting Up the Environment
- Launch your vulnerable web application.
- For DVWA, start the web server and navigate to
http://localhost/dvwa
in your browser.
- For DVWA, start the web server and navigate to
- Set the DVWA security level to low:
- Log in (default credentials:
admin
/password
). - Navigate to
DVWA Security
and set the security level toLow
.
- Log in (default credentials:
- Verify the XSS module is available:
- Click on the XSS (Stored) or XSS (Reflected) section in DVWA.
Step 2: Understanding XSS Types
- Reflected XSS:
- The malicious script is embedded in a URL and reflected back to the user.
- Example: A search bar displays the search query directly in the response.
- Stored XSS:
- The malicious script is stored on the server (e.g., in a comment or form) and executed when users view the affected page.
Step 3: Conducting a Reflected XSS Attack
- Open the XSS (Reflected) module in DVWA.
- Enter a test payload into the input field:
<script>alert('XSS');</script>
- Submit the form.
- Observe if an alert box appears:
- Insight: The alert box indicates the input was reflected and executed by the browser.
- Modify the payload to capture sensitive data:
<script>document.write(document.cookie);</script>
- Tip: Cookies can store session tokens or authentication details.
Step 4: Conducting a Stored XSS Attack
- Open the XSS (Stored) module in DVWA.
- Enter a malicious payload into a form field (e.g., a comment box):
<script>alert('Stored XSS');</script>
- Submit the form.
- Reload the page and observe if the script executes when the comment is displayed.
- Insight: Stored XSS is more dangerous as it affects all users who view the page.
- Use an advanced payload to redirect users to a malicious site:
<script>window.location='http://malicious-site.com';</script>
Step 5: Mitigation Techniques
- Input Validation:
- Validate user input on both client and server sides.
- Reject input containing suspicious characters (e.g.,
<
,>
).
- Output Encoding:
- Encode output to ensure special characters are rendered harmless (e.g.,
<
instead of<
).
- Encode output to ensure special characters are rendered harmless (e.g.,
- Content Security Policy (CSP):
- Implement a CSP to restrict the execution of unauthorized scripts.
- Use HTTPOnly Cookies:
- Prevent JavaScript from accessing sensitive cookies.
Step 6: Cleaning Up
- Remove any malicious payloads or reset the DVWA database to its default state.
- Close any vulnerable services or applications to secure your environment.
Additional Tips and Insights
- Test Safely:
- Only test XSS vulnerabilities in a controlled environment where you have explicit permission.
- Educate Users:
- Encourage awareness about the dangers of clicking on untrusted links or entering sensitive data into unknown sites.
- Use XSS Testing Tools:
- Tools like Burp Suite, OWASP ZAP, or XSSer can automate the detection of XSS vulnerabilities.
- Practice Regularly:
- Experiment with different payloads to understand how various sanitization techniques work.
Key Takeaways
- XSS attacks exploit weaknesses in how web applications handle user input and output.
- Reflected XSS typically targets individual users, while Stored XSS affects all users interacting with the compromised data.
- Preventing XSS requires a combination of input validation, output encoding, and modern security practices like CSP.