1. Introduction

In this tutorial, we'll explore session fixation, a vulnerability that could potentially be exploited by attackers to hijack a user's session. We'll understand its mechanism, how it can be used maliciously, and ways to protect against such attacks.

By the end of this tutorial, you'll be able to:

• Understand what session fixation is and how it works
• Identify potential areas in your web application susceptible to session fixation
• Implement measures to protect your web application from session fixation

Prior knowledge of basic web development and programming languages like JavaScript, PHP or Python will be helpful, but not necessary.

2. Step-by-Step Guide

Session fixation occurs when an attacker sets a user's session id to one known to the attacker, forcing the user's browser into using this session id and making it easier for the attacker to hijack the user's session.

Here are some steps to understand this concept:

• Step 1: The attacker generates a valid session ID but does not use it.
• Step 2: The attacker tricks the victim into using this session ID.
• Step 3: The victim logs in with the session ID, effectively giving the attacker access.

To protect against session fixation, you can:

• Step 1: Regenerate a new session ID after a successful login.
• Step 2: Implement session timeout or inactivity timeout.
• Step 3: Secure your application by using secure and HTTPOnly flags.

3. Code Examples

Here's a simple example in PHP that showcases session fixation protection:

<?php
// Starting session
session_start();

// Regenerating session ID
session_regenerate_id();

// Setting secure and HTTPOnly flags
setcookie(session_name(), session_id(), null, '/', null, null, true);
?>

In this code snippet:

• session_start(); starts a new session or resumes the existing one.
• session_regenerate_id(); regenerates the session id.
• The setcookie() function sets a cookie with the name of the session, its id, and secure and HTTPOnly flags.

This script won't produce any visible output but will start a secure session when executed.

4. Summary

In this tutorial, we've covered:

• What session fixation is and how it works
• How session fixation can be used maliciously
• How to protect your application from session fixation

To learn more about session management and security, you could explore resources such as the OWASP Testing Guide.

5. Practice Exercises

Here are some practical exercises to test your understanding:

1. Create a simple web application and identify potential areas susceptible to session fixation.
2. Implement measures discussed in this tutorial to protect your web application from session fixation.
3. Review the changes and test again for session fixation vulnerabilities.

Remember, the best way to learn is by doing. We learn from both our successes and failures. Happy coding!