How to Securely Implement CSRF Token Protection in PHP Forms?

Managing CSRF Defense Techniques: A Walkthrough with PHP Forms

Introduction

Adding proper security fortifications to your website involves addressing potential vulnerabilities, including Cross-site Request Forgery (CSRF). This article delves into effectively adding a CSRF token using PHP.

Understanding the Issue

You've implemented CSRF token protection on your website, but the token's value is inconsistently present in the HTML forms. Specifically, the "contact us" and AJAX forms are exhibiting this issue.

Reviewing the Code

Let's scrutinize the code used for injecting the token:

PHP:

if (!isset($_SESSION)) {
    session_start();
    $_SESSION['formStarted'] = true;
}

if (!isset($_SESSION['token'])) {
    $token = md5(uniqid(rand(), TRUE));
    $_SESSION['token'] = $token;
}

HTML:

<input type="hidden" name="token" value="<?php echo $token; ?>" />

Addressing the Weakness

Unfortunately, the current token generation method employs flawed techniques:

• rand() is not a reliable source of randomness.
• uniqid() produces a limited number of entropy.
• md5() does not contribute to entropy, but rather permutes it.

Implementing a Robust Token Generator

To safeguard your forms, utilize the following PHP code:

PHP 7:

session_start();
if (empty($_SESSION['token'])) {
    $_SESSION['token'] = bin2hex(random_bytes(32));
}
$token = $_SESSION['token'];

PHP 5.3 (or with ext-mcrypt):

session_start();
if (empty($_SESSION['token'])) {
    if (function_exists('mcrypt_create_iv')) {
        $_SESSION['token'] = bin2hex(mcrypt_create_iv(32, MCRYPT_DEV_URANDOM));
    } else {
        $_SESSION['token'] = bin2hex(openssl_random_pseudo_bytes(32));
    }
}
$token = $_SESSION['token'];

Verifying the Token

Next, properly validate the CSRF token to prevent attempts at exploitation:

if (!empty($_POST['token'])) {
    if (hash_equals($_SESSION['token'], $_POST['token'])) {
         // Proceed to process the form data
    } else {
         // Log this as a warning and monitor such attempts
    }
}

Additional Considerations

• Consider making the CSRF tokens specific to each form for enhanced security.
• For further protection, utilize the Twig templating engine to simplify token handling and integrate it into your templates.
• Explore the use of specialized libraries like Paragon Initiative Enterprises' Anti-CSRF library for unique, one-time use CSRF tokens.

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