PHP : Forms - [9.8] Securing PHP's Form Processing

9.8.1 Problem

You want to securely process form input variables and not allow someone to maliciously alter variables in your code.

9.8.2 Solution

Disable the register_globals configuration directive and access variables only from the $_REQUEST array. To be even more secure, use $_GET , $_POST, and $_COOKIE to make sure you know exactly where your variables are coming from.

To do this, make sure this line appears in your php.ini file:

register_globals = Off

As of PHP 4.2, this is the default configuration.

9.8.3 Discussion

When register_globals is set on, external variables, including those from forms and cookies, are imported directly into the global namespace. This is a great convenience, but it can also open up some security holes if you're not very diligent about checking your variables and where they're defined. Why? Because there may be a variable you use internally that isn't supposed to be accessible from the outside but has its value rewritten without your knowledge.

Here is a simple example. You have a page in which a user enters a username and password. If they are validated, you return her user identification number and use that numerical identifier to look up and print out her personal information:

// assume magic_quotes_gpc is set to Off
$username = $dbh->quote($_GET['username']);
$password = $dbh->quote($_GET['password']);

$sth = $dbh->query("SELECT id FROM users WHERE username = $username AND
                    password = $password");

if (1 == $sth->numRows( )) { 
    $row = $sth->fetchRow(DB_FETCHMODE_OBJECT);
    $id = $row->id;
} else {
    "Print bad username and password";
}

if (!empty($id)) {
    $sth = $dbh->query("SELECT * FROM profile WHERE id = $id");
}

Normally, $id is set only by your program and is a result of a verified database lookup. However, if someone alters the GET string, and passes in a value for $id, with register_globals enabled, even after a bad username and password lookup, your script still executes the second database query and returns results. Without register_globals, $id remains unset because only $_REQUEST['id'] (and $_GET['id']) are set.

Of course, there are other ways to solve this problem, even when using register_globals. You can restructure your code not to allow such a loophole.

$sth = $dbh->query("SELECT id FROM users WHERE username = $username AND
                    password = $password");
 
if (1 == $sth->numRows( )) { 
    $row = $sth->fetchRow(DB_FETCHMODE_OBJECT);
    $id = $row->id;
    if (!empty($id)) {
        $sth = $dbh->query("SELECT * FROM profile WHERE id = $id");
    }
} else {
    "Print bad username and password";
}

Now you use $id only when it's been explicitly set from a database call. Sometimes, however, it is difficult to do this because of how your program is laid out. Another solution is to manually unset( ) or initialize all variables at the top of your script:

unset($id);

This removes the bad $id value before it gets a chance to affect your code. However, because PHP doesn't require variable initialization, it's possible to forget to do this in one place; a bug can then slip in without a warning from PHP.