PHP: Null bytes related issues

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';
}

?>

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User Contributed Notes 2 notes

down

Anonymous ¶

11 years ago

Looks like this issue was fixed in PHP 5.3 https://bugs.php.net/bug.php?id=39863

down

cornernote [at] gmail.com ¶

10 years ago

clean input of null bytes:

<?php
$clean = str_replace(chr(0), '', $input);
?>

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