CSRF attacks and token protection in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a session in PHP.

PHP

<?php
[1];
?>

Drag options to blanks, or click blank then click option'

Asession_start()

Bstart_session()

Csession()

Dbegin_session()

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that does not exist like start_session()

Forgetting the parentheses () after the function name

In PHP, session_start() is used to start a session or resume the current one.

2fill in blank

medium

Complete the code to generate a CSRF token using PHP's random_bytes function.

PHP

<?php
$token = bin2hex([1](32));
?>

Drag options to blanks, or click blank then click option'

Arandom_bytes

Bmt_rand

Crandom_int

Dopenssl_random_pseudo_bytes

Attempts:

3 left

💡 Hint

Common Mistakes

Using random_int which returns integers, not bytes

Using mt_rand which is not cryptographically secure

random_bytes(32) generates 32 random bytes suitable for cryptographic use.

3fill in blank

hard

Fix the error in the code that checks if the CSRF token from POST matches the session token.

PHP

<?php
if (isset($_POST['csrf_token']) && hash_equals($_SESSION['csrf_token'], [1])) {
    // Valid token
}
?>

Drag options to blanks, or click blank then click option'

A$_COOKIE['csrf_token']

B$_GET['csrf_token']

C$_SESSION['csrf_token']

D$_POST['csrf_token']

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing the session token to itself

Using $_GET or $_COOKIE instead of $_POST

The CSRF token sent by the form is in $_POST['csrf_token'], so it must be compared to the session token.

4fill in blank

hard

Fill both blanks to create a hidden input field in an HTML form that sends the CSRF token stored in PHP session.

PHP

<input type="hidden" name="csrf_token" value="<?php echo [1]; ?>">
<?php echo [2]; ?>

Drag options to blanks, or click blank then click option'

A$_SESSION['csrf_token']

B$token

Chtmlspecialchars($token)

Dhtmlspecialchars($_SESSION['csrf_token'])

Attempts:

3 left

💡 Hint

Common Mistakes

Not escaping the token before output

Using a variable that is not defined

Use htmlspecialchars($_SESSION['csrf_token']) to safely output the token in HTML.

5fill in blank

hard

Fill all three blanks to store a new CSRF token in session, generate it securely, and include it in a form.

PHP

<?php
session_start();
$_SESSION['csrf_token'] = [1];
?>
<form method="post">
  <input type="hidden" name="csrf_token" value="<?php echo [2]; ?>">
  <button type="submit">Submit</button>
</form>
<?php
echo [3];
?>

Drag options to blanks, or click blank then click option'

Abin2hex(random_bytes(16))

Bhtmlspecialchars($_SESSION['csrf_token'])

C$_SESSION['csrf_token']

Drandom_bytes(16)

Attempts:

3 left

💡 Hint

Common Mistakes

Not generating the token securely

Not escaping output when echoing

Using the wrong variable in the form value

First, generate a secure token with bin2hex(random_bytes(16)) and store it in session. Then output the raw session token in the hidden input's value. Finally, echo the escaped token for safe HTML display.

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