Upgrade Security

Documentations 

Note

If you use the Form Helper and enable the CSRF filter globally, then form_open() will automatically insert a hidden CSRF field in your forms. So you do not have to upgrade this by yourself.

What has been changed 

The method to implement CSRF tokens to HTML forms has been changed.

Upgrade Guide 

To enable CSRF protection in CI4 you have to enable it in app/Config/Filters.php:

<?php

namespace Config;

use CodeIgniter\Config\BaseConfig;

class Filters extends BaseConfig
{
    // ...

    public $globals = [
        'before' => [
            // 'honeypot',
            'csrf',
        ],
    ];

    // ...
}

Within your HTML forms you have to remove the CSRF input field which looks similar to <input type="hidden" name="<?= $csrf['name'] ?>" value="<?= $csrf['hash'] ?>" />.
Now, within your HTML forms you have to add <?= csrf_field() ?> somewhere in the form body, unless you are using form_open().

Code Example
CodeIgniter Version 3.x<?php

$csrf = array(
    'name' => $this->security->get_csrf_token_name(),
    'hash' => $this->security->get_csrf_hash()
);

?>

<form>
    <input name="name" type="text">
    <input name="email" type="text">
    <input name="password" type="password">

    <input type="hidden" name="<?= $csrf['name'] ?>" value="<?= $csrf['hash'] ?>">
    <input type="submit" value="Save">
</form>

CodeIgniter Version 4.x<form>
    <input name="name" type="text">
    <input name="email" type="text">
    <input name="password" type="password">

    <?= csrf_field() ?>
    <input type="submit" value="Save">
</form>