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# Using Register Globals

One feature of PHP that can be used to enhance security is configuring PHP with register_globals = off. By turning off the ability for any user-submitted variable to be injected into PHP code, you can reduce the amount of variable poisoning a potential attacker may inflict. They will have to take the additional time to forge submissions, and your internal variables are effectively isolated from user submitted data.

While it does slightly increase the amount of effort required to work with PHP, it has been argued that the benefits far outweigh the effort.

Example 5-14. Working with register_globals=on

    <?php
    if ($username) {  // can be forged by a user in get/post/cookies
        $good_login = 1;
    }

    if ($good_login == 1) { // can be forged by a user in get/post/cookies,
        fpassthru ("/highly/sensitive/data/index.html");
    }
    ?>

Example 5-15. Working with register_globals = off

    <?php
    if($_COOKIE['username']){
        // can only come from a cookie, forged or otherwise
        $good_login = 1;
        fpassthru ("/highly/sensitive/data/index.html");
    }
    ?>

By using this wisely, it's even possible to take preventative measures to warn when forging is being attempted. If you know ahead of time exactly where a variable should be coming from, you can check to see if submitted data is coming from an inappropriate kind of submission. While it doesn't guarantee that data has not been forged, it does require an attacker to guess the right kind of forging.

Example 5-16. Detecting simple variable poisoning

    <?php
    if ($_COOKIE['username'] &&
        !$_POST['username'] &&
        !$_GET['username'] ) {
        // Perform other checks to validate the user name...
        $good_login = 1;
        fpassthru ("/highly/sensitive/data/index.html");
    } else {
       mail("admin@example.com", "Possible breakin attempt", $_SERVER['REMOTE_ADDR']);
       echo "Security violation, admin has been alerted.";
       exit;
    }
    ?>

Of course, simply turning off register_globals does not mean code is secure. For every piece of data that is submitted, it should also be checked in other ways.

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