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Why are my CSRF tokens sometimes empty when used in AJAX and standard forms, and how can I fix this in PHP?
Why are my CSRF tokens sometimes empty when used in AJAX and standard forms, and how can I fix this in PHP?
Dec 14, 2024 pm 08:46 PM
Securing Forms with Proper CSRF Token Implementation in PHP
Question:
When attempting to add a CSRF token to two different forms, one using AJAX and the other a basic contact form, it is observed that the token value in the HTML is sporadically empty. How can this issue be resolved?
Answer:
The problem likely stems from the token generation method, as the provided code is vulnerable to prediction and lack of entropy. This method is also insufficient for one-time-use and per-form token validation.
Generating Strong CSRF Tokens:
Replace the token generation with secure methods for PHP 7 or PHP 5.3 :
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 = $_SESSION['token'];
Verifying CSRF Tokens:
Use hash_equals() to verify tokens securely:
if (!empty($_POST['token'])) {
if (hash_equals($_SESSION['token'], $_POST['token'])) {
// Proceed to process the form data
} else {
// Log this as a warning and keep an eye on these attempts
}
}
Per-Form Token Restrictions:
To further restrict tokens to specific forms, use hash_hmac():
echo hash_hmac('sha256', '/my_form.php', $_SESSION['second_token']);
?>" />
Hybrid Approach with Twig Integration:
For those using Twig templates, a simplified dual strategy can be implemented:
$twigEnv->addFunction(
new \Twig_SimpleFunction(
'form_token',
function($lock_to = null) {
if (empty($_SESSION['token'])) {
$_SESSION['token'] = bin2hex(random_bytes(32));
}
if (empty($_SESSION['token2'])) {
$_SESSION['token2'] = random_bytes(32);
}
if (empty($lock_to)) {
return $_SESSION['token'];
}
return hash_hmac('sha256', $lock_to, $_SESSION['token2']);
}
)
);
With this function, secure general tokens can be used as:
While per-form tokens can be generated with:
Single-Use CSRF Tokens:
For one-time-use token requirements, consider using a dedicated library such as the Anti-CSRF library from Paragon Initiative Enterprises.
The above is the detailed content of Why are my CSRF tokens sometimes empty when used in AJAX and standard forms, and how can I fix this in PHP?. For more information, please follow other related articles on the PHP Chinese website!
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