CSRF attacks and token protection in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CSRF stand for and what is it?

CSRF stands for Cross-Site Request Forgery. It is a type of attack where a malicious website tricks a user's browser into performing unwanted actions on a trusted site where the user is logged in.

beginner

How does a CSRF token help protect against CSRF attacks?

A CSRF token is a unique, secret value generated by the server and included in forms. When the form is submitted, the server checks the token. If it matches, the request is valid. This prevents attackers from forging requests because they don't know the token.

intermediate

Show a simple PHP code snippet to generate and store a CSRF token in a session.

<?php
session_start();
if (empty($_SESSION['csrf_token'])) {
    $_SESSION['csrf_token'] = bin2hex(random_bytes(32));
}
?>

beginner

How do you include a CSRF token in an HTML form?

<form method="post" action="submit.php">
  <input type="hidden" name="csrf_token" value="<?php echo htmlspecialchars($_SESSION['csrf_token']); ?>">
  <!-- other form fields -->
  <input type="submit" value="Submit">
</form>

beginner

What is the server-side check for a CSRF token in PHP?

On form submission, the server compares the token sent in the form with the token stored in the session. If they match, the request is accepted; otherwise, it is rejected to prevent CSRF attacks.

What is the main purpose of a CSRF token?

A To speed up form submission

B To verify the user is logged in

C To encrypt user data

D To prevent unauthorized form submissions from other sites

CSRF tokens prevent unauthorized form submissions by ensuring the request comes from the legitimate user.

Where is a CSRF token usually stored on the server side?

A In the user's session data

B In a cookie accessible by JavaScript

C In the URL query string

D In a database table

CSRF tokens are stored in the user's session to keep them secret and tied to the user.

Which PHP function is commonly used to generate a secure CSRF token?

A rand()

B md5()

C bin2hex(random_bytes())

D uniqid()

bin2hex(random_bytes()) generates a cryptographically secure random token.

What happens if the CSRF token in the form does not match the session token?

A The request is rejected to prevent CSRF

B The server redirects to the login page

C The token is automatically updated

D The request is accepted anyway

Mismatch means the request might be forged, so it is rejected.

Why can't attackers easily guess the CSRF token?

A Because it is stored in a cookie

B Because it is a long, random string generated securely

C Because it is encrypted with a password

D Because it changes every second

CSRF tokens are long random strings generated securely, making guessing practically impossible.

Explain what a CSRF attack is and how a CSRF token helps prevent it.

Think about how a malicious site tricks a user and how a secret token stops that.

Describe the steps to implement CSRF token protection in a PHP web form.

Consider what happens before showing the form and after the form is submitted.

Practice

1. What is the main purpose of a CSRF token in a PHP web application?

easy

A. To speed up the loading time of the website

B. To prevent unauthorized commands from being transmitted from a user the website trusts

C. To encrypt user passwords before storing them

D. To validate user input format on forms

Solution

1. Step 1: Understand CSRF attack basics

   CSRF attacks trick a user into submitting unwanted actions on a trusted site.

2. Step 2: Role of CSRF token

   A CSRF token is a unique value that verifies the request is from the legitimate user, preventing unauthorized commands.

3. Final Answer:

   To prevent unauthorized commands from being transmitted from a user the website trusts -> Option B

4. Quick Check:

   CSRF token purpose = prevent unauthorized commands [OK]

Hint: CSRF tokens stop fake requests from trusted users [OK]

Common Mistakes:

• Confusing CSRF tokens with password encryption
• Thinking CSRF tokens speed up websites
• Believing CSRF tokens validate input format

2. Which PHP function is commonly used to generate a CSRF token?

easy

A. random_bytes()

B. md5()

C. htmlspecialchars()

D. strtoupper()

Solution

1. Step 1: Identify secure token generation

   CSRF tokens must be random and hard to guess, so secure random functions are used.

2. Step 2: Choose the correct function

   random_bytes() generates cryptographically secure random bytes, suitable for tokens.

3. Final Answer:

   random_bytes() -> Option A

4. Quick Check:

   Secure token generation = random_bytes() [OK]

Hint: Use random_bytes() for secure token generation [OK]

Common Mistakes:

• Using md5() which is not secure for tokens
• Confusing htmlspecialchars() with token generation
• Using strtoupper() which only changes case

3. What will the following PHP code output if the CSRF token in session matches the token sent in POST?

$_SESSION['csrf_token'] = 'abc123';
if (isset($_POST['csrf_token']) && $_POST['csrf_token'] === $_SESSION['csrf_token']) {
    echo 'Valid request';
} else {
    echo 'Invalid request';
}

medium

A. No output

B. Invalid request

C. Valid request

D. Undefined index error

Solution

1. Step 1: Check token comparison logic

   The code compares the POST token with the session token for equality.

2. Step 2: When tokens match

   If tokens match, it echoes 'Valid request'. If not, 'Invalid request'.

3. Final Answer:

   Valid request -> Option C

4. Quick Check:

   Matching tokens = Valid request [OK]

Hint: Matching tokens means valid request output [OK]

Common Mistakes:

• Assuming error if POST token is set correctly
• Ignoring isset() check causing errors
• Thinking output is empty when tokens match

4. Identify the error in this PHP CSRF token validation snippet:

session_start();
if ($_POST['csrf_token'] == $_SESSION['csrf_token']) {
    echo 'Token valid';
} else {
    echo 'Token invalid';
}

medium

A. session_start() should be called after token check

B. Using == instead of === causes token mismatch

C. Tokens should be compared with strtolower()

D. Missing check if $_POST['csrf_token'] is set before comparing

Solution

1. Step 1: Analyze token existence check

   The code compares $_POST['csrf_token'] without verifying if it exists, risking undefined index error.

2. Step 2: Importance of isset()

   Using isset() prevents errors by ensuring the token is present before comparison.

3. Final Answer:

   Missing check if $_POST['csrf_token'] is set before comparing -> Option D

4. Quick Check:

   Check token existence before compare = Missing isset() [OK]

Hint: Always check if POST token exists before comparing [OK]

Common Mistakes:

• Ignoring isset() causing runtime errors
• Thinking == vs === affects token validation here
• Calling session_start() after token check

5. You want to protect a PHP form from CSRF attacks. Which sequence correctly implements token generation, embedding, and validation? A) Generate token on form display, store in session, embed in hidden input, then check token on form submission. B) Generate token on form submission, embed in hidden input, then store in session. C) Store token in session after form submission, then generate token and embed in form. D) Embed a static token in the form and check it on submission without session.

hard

A. Generate token on form display, store in session, embed in hidden input, then check token on form submission.

B. Generate token on form submission, embed in hidden input, then store in session.

C. Store token in session after form submission, then generate token and embed in form.

D. Embed a static token in the form and check it on submission without session.

Solution

1. Step 1: Token generation and storage

   Token must be generated before showing the form and stored in session to verify later.

2. Step 2: Embedding and validation

   Embed the token in a hidden form field and check it against the session token on submission.

3. Final Answer:

   Generate token on form display, store in session, embed in hidden input, then check token on form submission. -> Option A

4. Quick Check:

   Token lifecycle = generate, store, embed, validate [OK]

Hint: Generate and store token before form, validate after submission [OK]

Common Mistakes:

• Generating token after form submission
• Not storing token in session
• Using static tokens without session storage