Null bytes related issues

As PHP uses the underlying C functions for filesystem related operations, it may handle null bytes in a quite unexpected way. As null bytes denote the end of a string in C, strings containing them won't be considered entirely but rather only until a null byte occurs. The following example shows the vulnerable code that demonstrates this problem:

<?php

$file = $_GET['file']; // "../../etc/passwd\0"

if (file_exists('/home/wwwrun/' . $file . '.php')) {
    // File_exists will return true as the file /home/wwwrun/../../etc/passwd exists
    include '/home/wwwrun/' . $file . '.php';

    // The file /etc/passwd will be included
}

?>

Therefore, any tainted string that is used in a filesystem operation should always be validated properly. Here is a better version of the previous example:

<?php

$file = $_GET['file'];

// Whitelisting possible values
switch ($file) {
    case 'main':
    case 'foo':
    case 'bar':
        include '/home/wwwrun/include/' . $file . '.php';
        break;
    default:
        include '/home/wwwrun/include/main.php';
}

?>