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 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';
}
?>