php secure comment logic? - php

Question

Ok, this might be obvious but its not clicking quite yet. I am creating a forum/blog esque app.
I grab the posts from the database rather securely but commenting is beginning to be a little more difficult. (I could just be paranoid, right?).
How do I add a comment without exposing the id of the parent message? (like in a hidden form field or query string, or something).
I guess I am a bit paranoid that someone might go into the code with firebug or something and change the hidden form field value to something else before submitting. I guess I would have to make sure the user has permission to comment to that particular post/category?
Things to note :
The user is already logged in.
Its not a public post

Answer 1

I would recommend that you setup your database like so:
Comments
---------
id
encodedID
authorID
parentID
message
Then, for the form field have two hidden values, one will be the encodedID, and the second will be a hash that you make. I would recommend the hash to be:
<?php
$hash = sha1(md5($encodedID . $userID . $_SERVER['REMOTE_ADDR'] . "abc1234"));
?>
Then, when the user submits the form, validate that the hash is valid for the specific encodedID and user. Here is a brief code write up:
<?php
if(isset($_POST['submit']))
{
//Get the variables and all and sanitize the input of 'message'
if(sha1(md5($_POST['value1']. $userID . $_SERVER['REMOTE_ADDR'] . "abc1234")) == $_POST['value2'])
{
//User is valid.
}
else
{
//Invalid user.
//Document this.
}
}
$value1 = $encodedID; //Grab this from your database
$value2 = sha1(md5($value1 . $userID . $_SERVER['REMOTE_ADDR'] . "abc1234"));
?>
<form method="post" action="comment.php">
<input type="text" name="message" />
<input type="hidden" name="value1" value="<?php echo $value1; ?>" />
<input type="hidden" name="value2" value="<?php echo $value2; ?>" />
<input type="submit" name="submit" value="Comment" />
</form>
Edit: Just a small tip, but I would recommend that you change value1 and value2 to something abstract, don't call it encodedID or anything like that, just so that it confuses any users that will attempt to try and break it.
And yes md5 and sha1 are not completely secure, but for this case it will work since you want to be able to process the comments fast and efficiently.

Answer 2

That might be an overkill but if you really want to hide the post_id of the current message then you should consider using session. So instead of using something like this on your form:
<form action="/postcomment.php" method="post" >
<input name="post_id" type="hidden" value="123" />
<textarea name="message"></textarea>
</form>
Reduce it to something like this:
<?php $_SESSION['post_id'] = '123'; ?>
<form action="/postcomment.php" method="post" >
<textarea name="message"></textarea>
</form>
Of course this is "yucky" coding but at least you get the idea.
Oh, don't forget to validate EVERYTHING on postcomment.php. Also escape ALL string input values and make sure all numeric inouts are numbers indeed (multiply them by one?).
[EDIT: Due to insistent public demand, may I, if you please, amend the aforementioned:]
Instead of:
<?php $_SESSION['post_id'] = '123'; ?>
Generate a form id:
<?php $_SESSION['form_id'] = $_SESSION['user_id'].'_'.md5(time()); ?>
Then generate the unique post_id:
<?php $_SESSION[$_SESSION['form_id'].'_post_id'] = '123'; ?>
After submitting get the post_id:
<?php $post_id = $_SESSION[$_SESSION['form_id'].'_post_id']; ?>

Answer 3

you could assign the form an "id" as a hidden field and create a database table to track form ids and their associated post ids, that way when the form gets submitted you could check the post id in the db without ever sending it to the client based on the form id that is returned with the post

Answer 4

You're asking the wrong question here: instead of being concerned about the user getting some internal ID that means nothing outside your application, your primary concern should be about keeping them from doing anything unpleasant with it.
Imagine I just started sending POST requests to add a comment for every ID between 1 and 10,000. I'm sure to hit a real post sooner or later.
Rule #1 about writing secure web applications: Don't trust the user.
In other words, yes, you should check to make sure that they have permission to comment when you receive the results back from the from.