CSRF session token - php

Question

I'm trying to implement a simple CSRF protection on a form, but I can't get it right. If someone can point out what I'm doing wrong, I would appreciate it.
The error: Every time I submit the form, I get "Invalid Submission2" because the token changes (after the form is submitted), since is being generated on the same page?
Edit -- I forgot to mention, another file (config.php) already has session_start().
<?php
class Module_Post extends Base_Module {
public function start()
{
requireLogin();
if (isset($_GET['act'])) {
switch($_GET['act']) {
case 'text':
$this->text();
break;
case 'image':
$this->image();
break;
default:
$this->text();
break;
}
} else {
$this->text();
}
}
private function text()
{
// Generate random unique token to prevent cross-site request forgery (CSRF).
if(empty($_SESSION['form_token']))
{
$form_token = md5(uniqid(rand(), TRUE));
$_SESSION['form_token'] = $form_token;
}
if(isset($_POST['submit']))
{
// Clean the content from cross-site scripting (xss)
$content = trim($_POST['content']);
$content = Xss::clean($content);
// Validate that the form token
if(!isset($_POST['form_token'], $_SESSION['form_token'])) {
$err = 'Invalid Submission';
} elseif ($_POST['form_token'] != $_SESSION['form_token']) {
$err = 'Invalid Submission2';
} elseif (strlen($content) < 10) {
$err = 'Your content contains too few characters.';
}
if(isset($err)) {
unset( $_SESSION['form_token']);
$this->setMessage($err, 'FAIL');
header('Location: index.php?mod=post');
exit;
}
// Insert database data here, then redirect
$this->setMessage('Your post was published successfully.', 'GOOD');
header('Location: index.php');
exit;
}
$this->tpl->assign('form_token', $form_token);
$this->tpl->display('new/text.tpl');
}
}
?>
The HTML (text.tpl file)
<form method='post' enctype='multipart/form-data' action='#'>
<fieldset>
<textarea rows="8" id="new_post" name="content" class="input-block-level"></textarea>
<input type="hidden" name="form_token" value="{$form_token}" />
<button type="submit" name="submit" class="btn btn-info pull-left">Create Post</button>
</fieldset>
</form>

Answer 1

You need to change this line
$this->tpl->assign('form_token', $form_token);
into:
$this->tpl->assign('form_token', $_SESSION['form_token']);
That's because you generate your token only with this condition:
if(empty($_SESSION['form_token']))
{
$form_token = md5(uniqid(rand(), TRUE));
$_SESSION['form_token'] = $form_token;
}
and unset it using this condition:
if(isset($err)) {
unset( $_SESSION['form_token']);
}
So if you send your form once and then reload page (without submitting form just url), $form_token variable is unknown because there is $_SESSION['form_token'] is not empty and then in your form you have empty token.
If you had displaying errors turned on in PHP you would see in this case in PHP:
Undefined variable: form_token in