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.