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 a vulnerable code that demonstrates this problem:
Example #1 Script vulnerable to null bytes
<?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:
Example #2 Correctly validating the input
<?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';
}
?>