Cross-site request forgery (CSRF) in Cybersecurity - Mini Project: Build & Apply

Understanding Cross-site Request Forgery (CSRF)

📖 Scenario: You are learning about web security. One common attack is called Cross-site Request Forgery (CSRF). It tricks a user's browser into sending unwanted requests to a website where the user is logged in.Imagine you have a simple website where users can update their email address. Without protection, an attacker could make a user unknowingly change their email by visiting a malicious page.

🎯 Goal: Build a step-by-step understanding of how CSRF works and how to protect against it by using a secret token in web forms.

📋 What You'll Learn

Create a dictionary representing a user session with a CSRF token

Add a variable for the expected CSRF token value

Write a function that checks if a submitted token matches the expected token

Complete the example by showing how to verify the token before updating user data

💡 Why This Matters

🌍 Real World

CSRF protection is essential for any website that allows users to perform actions while logged in, such as changing settings or making purchases.

💼 Career

Understanding CSRF helps web developers and security professionals protect applications from unauthorized actions caused by malicious websites.

1

Create a user session dictionary with a CSRF token

Create a dictionary called user_session with these exact entries: 'user_id': 101, 'csrf_token': 'abc123xyz'

Cybersecurity

# Create the user_session dictionary with user_id and csrf_token
# Your code here

Hint

Think of user_session as a small box holding user info and a secret token.

2

Add a variable for the expected CSRF token

Create a variable called expected_token and set it to the string 'abc123xyz'

Cybersecurity

user_session = {'user_id': 101, 'csrf_token': 'abc123xyz'}
# Create expected_token variable with the CSRF token string
# Your code here

Hint

This token is what the server expects to receive from a valid form submission.

3

Write a function to check the CSRF token

Define a function called is_valid_csrf that takes one parameter token and returns True if token equals expected_token, otherwise False

Cybersecurity

user_session = {'user_id': 101, 'csrf_token': 'abc123xyz'}
expected_token = 'abc123xyz'
# Define is_valid_csrf(token) function to compare token with expected_token
# Your code here

Hint

This function helps the server decide if the request is safe by checking the token.

4

Complete the example by verifying the CSRF token before updating

Write code that sets a variable submitted_token to 'abc123xyz' and then uses is_valid_csrf(submitted_token) in an if statement to simulate allowing an email update only if the token is valid

Cybersecurity

user_session = {'user_id': 101, 'csrf_token': 'abc123xyz'}
expected_token = 'abc123xyz'

def is_valid_csrf(token):
    return token == expected_token

# Set submitted_token and check it with is_valid_csrf
# Your code here

Hint

This final step shows how the server uses the token check to protect user actions.

Practice

1. What is the main goal of a Cross-site request forgery (CSRF) attack?

easy

A. To encrypt user data for ransom

B. To steal a user's password by phishing

C. To overload a website with traffic causing it to crash

D. To trick a logged-in user into performing unwanted actions on a website

Solution

1. Step 1: Understand CSRF attack purpose

   CSRF attacks aim to make a user unknowingly perform actions they did not intend on a trusted website.

2. Step 2: Compare with other attack types

   Phishing steals passwords, DoS overloads servers, and ransomware encrypts data, which are different from CSRF.

3. Final Answer:

   To trick a logged-in user into performing unwanted actions on a website -> Option D

4. Quick Check:

   CSRF = trick user actions [OK]

Hint: CSRF tricks users into unwanted website actions [OK]

Common Mistakes:

• Confusing CSRF with phishing attacks
• Thinking CSRF steals passwords directly
• Mixing CSRF with denial-of-service attacks

2. Which of the following is a common method to protect a website against CSRF attacks?

easy

A. Blocking all cookies from the browser

B. Encrypting all user passwords with SHA-1

C. Using secret tokens that must be submitted with requests

D. Disabling JavaScript on the website

Solution

1. Step 1: Identify CSRF protection techniques

   Websites use secret tokens (CSRF tokens) that must be included in requests to verify they come from the user intentionally.

2. Step 2: Eliminate unrelated options

   Encrypting passwords, blocking cookies, or disabling JavaScript do not specifically prevent CSRF attacks.

3. Final Answer:

   Using secret tokens that must be submitted with requests -> Option C

4. Quick Check:

   CSRF protection = secret tokens [OK]

Hint: CSRF defense uses secret tokens in requests [OK]

Common Mistakes:

• Thinking password encryption stops CSRF
• Believing blocking cookies prevents CSRF
• Assuming disabling JavaScript is a CSRF fix

3. Consider this scenario: A user is logged into their bank website. An attacker sends a hidden form on another site that submits a money transfer request to the bank without the user's knowledge. Which security measure would stop this CSRF attack?

medium

A. The bank requires a unique CSRF token with each form submission

B. The bank uses HTTPS for all pages

C. The bank requires a strong password

D. The bank limits login attempts

Solution

1. Step 1: Analyze the attack method

   The attacker tricks the user into submitting a form without their consent, typical of CSRF.

2. Step 2: Identify effective defense

   Requiring a unique CSRF token ensures the form submission is legitimate and from the user, blocking forged requests.

3. Final Answer:

   The bank requires a unique CSRF token with each form submission -> Option A

4. Quick Check:

   CSRF blocked by unique tokens [OK]

Hint: Unique CSRF tokens stop forged form submissions [OK]

Common Mistakes:

• Thinking HTTPS alone stops CSRF
• Believing strong passwords prevent CSRF
• Confusing login limits with CSRF defense

4. A developer wrote this code snippet to protect against CSRF:

<form method="POST" action="/transfer">
  <input type="hidden" name="csrf_token" value="12345" />
  <input type="submit" value="Send" />
</form>

Why might this code still be vulnerable to CSRF attacks?

medium

A. The form uses POST method instead of GET

B. The CSRF token is static and not unique per user session

C. The form does not include a CAPTCHA

D. The form action URL is relative

Solution

1. Step 1: Examine the CSRF token usage

   The token value "12345" is hardcoded and does not change per user or session, making it predictable.

2. Step 2: Understand CSRF token requirements

   Effective CSRF tokens must be unique and unpredictable per user session to prevent attackers from guessing them.

3. Final Answer:

   The CSRF token is static and not unique per user session -> Option B

4. Quick Check:

   Static tokens fail CSRF protection [OK]

Hint: CSRF tokens must be unique per session, not static [OK]

Common Mistakes:

• Thinking POST method alone prevents CSRF
• Assuming CAPTCHA is required for CSRF defense
• Believing relative URLs cause CSRF vulnerabilities

5. A website uses cookies for user sessions but does not implement CSRF tokens. An attacker crafts a malicious link that triggers a fund transfer when clicked by a logged-in user. How can the website improve security to prevent this CSRF attack without removing cookies?

hard

A. Implement a unique CSRF token that must be submitted with sensitive requests

B. Switch all requests to use GET method instead of POST

C. Require users to re-enter their password on every page load

D. Disable cookies and use URL parameters for sessions

Solution

1. Step 1: Identify the vulnerability

   Cookies alone do not prevent CSRF because browsers send them automatically with requests, allowing forged actions.

2. Step 2: Choose effective mitigation

   Adding unique CSRF tokens ensures that only legitimate requests from the user are accepted, blocking forged requests.

3. Final Answer:

   Implement a unique CSRF token that must be submitted with sensitive requests -> Option A

4. Quick Check:

   Unique tokens prevent CSRF despite cookies [OK]

Hint: Add unique CSRF tokens to protect cookie-based sessions [OK]

Common Mistakes:

• Thinking GET requests are safer than POST
• Believing password re-entry on every page is practical
• Assuming disabling cookies is the only fix