PHP 8.1.15 Released!

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';
}
?>
add a note

User Contributed Notes 2 notes

up
6
cornernote [at] gmail.com
7 years ago
clean input of null bytes:

<?php
$clean
= str_replace(chr(0), '', $input);
?>
up
6
Anonymous
8 years ago
Looks like this issue was fixed in PHP 5.3 https://bugs.php.net/bug.php?id=39863
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