Problemas relacionados a bytes nulos (Null)

Last updated: Fri, 13 Nov 2009

Segurança do Sistema de Arquivos

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Problemas relacionados a bytes nulos (Null)

O PHP está sujeito à segurança encontrada na maioria dos sistemas de servidor com respeito à permissões de arquivos e diretórios. Isso permite que você controle quais arquivos no sistema podem ser lidos e por quem. É preciso ter cuidado com quaisquer arquivos que são lidos por todos para assegurar que eles podem ser lidos por todos os usuários que tem acesso ao sistema de arquivos.

Já que o PHP foi desenhado para permitir acesso em nível de usuário ao sistema de aruqivos, é possível escrever um script PHP que permitirá ler arquivos do sistema com /etc/passwd, modificar suas conexões de ethernet, enviar trabalhos de impressão gigantes, etc. Isso tem algumas implicações óbvias, já que você precisa ter certeza que os arquivos que você lê e escreve são apropriados.

Considere o seguinte script, onde um usuário indica que ele quer apagar um arquivo no seu diretório home. Isso presume uma situação onde uma interface web PHP é usada regularmente para controle de arquivos, então o usuário do Apache tem permissão de apagar arquivos nos diretórios home dos usuários.

Exemplo #1 Checagem fraca de variáveis resulta em....


<?php
// remove a file from the user's home directory
$username = $_POST['user_submitted_name'];
$userfile = $_POST['user_submitted_filename'];
$homedir  = "/home/$username";

unlink("$homedir/$userfile");

echo "The file has been deleted!";
?>

Já que o nome do usuário e do arquivo são enviáveis pelo formulário, um usuário pode enviar um nome de usuário e de arquivo que pertença a outra pessoa, e apagá-lo, mesmo que eles não tenham permissão para fazê-lo. Nesse caso, você desejaria ter alguma outra forma de autenticação. Considere o que poderia acontecer se as variáveis enviadas forem "../etc/" and "passwd". O código então leria efetivamente:

Exemplo #2 ... um ataque ao sistema de arquivos


<?php
// remove um arquivo de qualquer lugar no disco rígido
// que o usuário do PHP tenha acesso. Se o PHP tiver acesso de administrador (root):
$username = $_POST['user_submitted_name']; // "../etc"
$userfile = $_POST['user_submitted_filename']; // "passwd"
$homedir  = "/home/$username"; // "/home/../etc"

unlink("$homedir/$userfile"); // "/home/../etc/passwd"

echo "The file has been deleted!";
?>

Existem duas medidas importantes que você deve tomar para previnir esses problemas.

Só dar permissões limitadas ao usuário executando o binário do PHP.
Checar todas as variáveis que são enviadas.

Aqui temos um script melhorado:

Exemplo #3 Checagem mais segura do nome do arquivo


<?php
// remove um arquivo do disco rígido que o
// usuário do PHP tenha acesso.
$username = $_SERVER['REMOTE_USER']; // usando um mecanismo de autenticação
$userfile = basename($_POST['user_submitted_filename']);
$homedir  = "/home/$username";

$filepath = "$homedir/$userfile";

if (file_exists($filepath) && unlink($filepath)) {
    $logstring = "Deleted $filepath\n";
} else {
    $logstring = "Failed to delete $filepath\n";
}
$fp = fopen("/home/logging/filedelete.log", "a");
fwrite($fp, $logstring);
fclose($fp);

echo htmlentities($logstring, ENT_QUOTES);

?>

No entanto, ele ainda possui falhas. Se seu sistema de autenticação permitir que os usuários criem seu próprios logins, e um usuário escolher o login "../etc/", o sistema está novamente exposto. Por essa razão, você pode preferir escrever uma checagem mais específica:

Exemplo #4 Checagem mais segura do nome do arquivo


<?php
$username     = $_SERVER['REMOTE_USER']; // usando um mecanismo de autenticação
$userfile     = $_POST['user_submitted_filename'];
$homedir      = "/home/$username";

$filepath     = "$homedir/$userfile";

if (!ctype_alnum($username) || !preg_match('/^(?:[a-z0-9_-]|\.(?!\.))+$/iD', $userfile)) {
    die("Bad username/filename");
}

//etc...
?>

Dependendo do seu sistema operacional, existe uma variedade enorme de arquivos que você tem que se preocupar, incluindo dispositivos (/dev/ or COM1), arquivos de configuração (arquivos /etc/ e .ini), áres de armazenamento conhecidas (/home/, My Documents), etc. Por essa razão, normalmente é mais fácil criar uma política onde você proibe tudo exceto aquilo que for explicitamente permitido.

Problemas relacionados a bytes nulos (Null)

Instalado como módulo do Apache

Last updated: Fri, 13 Nov 2009

add a note User Contributed Notes
Segurança do Sistema de Arquivos

Latchezar Tzvetkoff
10-Mar-2009 12:06


A basic filename/directory/symlink checking may be done (and I personally do) via realpath() ...





<?php





if (isset($_GET['file'])) {


    $base = '/home/polizei/public_html/';  // it seems this one is good to be realpath too.. meaning not a symlinked path..


    if (strpos($file = realpath($base.$_GET['file']), $base) === 0 && is_file($file)) {


        unlink($file);


    } else {


        die('blah!');


    }


}


?>

BAT Svensson
27-Mar-2008 11:20


A note on mapping for added security.



The principle with mapping is: WHAT YOU SEE MAY NOT BE.



Map each entity that can be manipulated to a token. Only the token will be displayed to the end user. For added security, map allowed functionality to each token. Changing the token will at most cause a user to manipulate some else already mapped file. This achieve the goal of control and limited access.



There is a variety to implement this but the main point here is that a physical name should never be displayed to the end user, thus restricting an evil user knowledge of the system and it architecture.



This principle is also know under names such as "black box" or "need to know" or "abstraction", etc etc depending on the context.



The point here is that one does not need to know under lying structure in order to do operation one high levels such as "add/move/remove/edit". Present a view - the view will provide the user with what the user need to know.



It is all about creating an illusion of existence at higher level of something that does not exist on a lover level.

steelchords at yahoo dot com
17-Apr-2007 10:12


It seems to me in this particular instance that a simple check to make sure that name or partial pathname doesn't already exist would prevent this attack... if a 'passwd/etc/...' existed as the password directory, you couldn't create a username to exploit the hole in the first place.  But that's only from a 'script user' perspective, it still doesn't protect your server from other sub-admin's badly written code.



Don For

anonymous
17-Nov-2005 11:58


(A) Better not to create files or folders with user-supplied names. If you do not validate enough, you can have trouble. Instead create files and folders with randomly generated names like fg3754jk3h and store the username and this file or folder name in a table named, say, user_objects. This will ensure that whatever the user may type, the command going to the shell will contain values from a specific set only and no mischief can be done.



(B) The same applies to commands executed based on an operation that the user chooses. Better not to allow any part of the user's input to go to the command that you will execute. Instead, keep a fixed set of commands and based on what the user has input, and run those only. 



For example,

(A) Keep a table named, say, user_objects with values like:

username|chosen_name   |actual_name|file_or_dir

--------|--------------|-----------|-----------

jdoe    |trekphotos    |m5fg767h67 |D

jdoe    |notes.txt     |nm4b6jh756 |F

tim1997 |_imp_ folder  |45jkh64j56 |D



and always use the actual_name in the filesystem operations rather than the user supplied names.



(B)

<?php

$op = $_POST['op'];//after a lot of validations 

$dir = $_POST['dirname'];//after a lot of validations or maybe you can use technique (A)

switch($op){

    case "cd":

        chdir($dir);

        break;

    case "rd":

        rmdir($dir);

        break;

    .....

    default:

        mail("webmaster@example.com", "Mischief", $_SERVER['REMOTE_ADDR']." is probably attempting an attack.");

}

fmrose at ncsu dot edu
10-Oct-2005 01:31


All of the fixes here assume that it is necessary to allow the user to enter system sensitive information to begin with. The proper way to handle this would be to provide something like a numbered list of files to perform an unlink action on and then the chooses the matching number. There is no way for the user to specify a clever attack circumventing whatever pattern matching filename exclusion syntax that you may have.



Anytime you have a security issue, the proper behaviour is to deny all then allow specific instances, not allow all and restrict. For the simple reason that you may not think of every possible restriction.

joshudson';DROP TABLE EMAILS;' gmail.com
01-Sep-2005 06:50


I keep application configuration files in the document root. I found the most effective trick to prevent access to them is to

1. Give them no code that actually runs when included (except for variable assignments),

2. Don't use register globals so nobody can do anything weird,

3. Name them *.php so PHP runs them when asked for

4. Don't have anything before <?php

5. Dont have a ?>

1 at 234 dot cx
23-Jun-2005 02:24


I don't think the filename validation solution from Jones at partykel is complete.  It certainly helps, but it doesn't address the case where the user is able to create a symlink pointing from his home directory to the root.  He might then ask to unlink "foo/etc/passwd" which would be in his home directory, except that foo is a symlink pointing to /.



Personally I wouldn't feel confident that any solution to this problem would keep my system secure.  Running PHP as root (or some equivalent which can unlink files in all users' home directories) is asking for trouble.



If you have a multi-user system and you are afraid that users may install scripts like this, try security-enhanced Linux.  It won't give total protection, but it at least makes sure that an insecure user script can only affect files which the web server is meant to have access to.  Whatever script someone installs, outsiders are not going to be able to read your password file---or remove it.

Jones at partykel dot de
10-Dec-2004 11:00


What about:



<?php

  $file_to_delete = '/home/'.$username.'/'.$userfile;

  

  if ((!ereg('\.\.', $file_to_delete)) and (file_exists($file_to_delete))) {

      unlink($file_to_delete);

  }

?>



I think this should prevent every attempt to go outside the user-directory.

Additionally you should check usernames at the registration.



Another way whould be to use the user-ID as home-directory - so, this can't be changed and every registered user as an unique one (if it's a primary key in your database). But then you still have to check the given $userfile.



So the code above could be taken as a "last instance check" directly before finally deleting of the file.

akul at inbox dot ru
06-May-2004 06:59


Common and simple way to avoid path attack is separating objectname and filesystem space when possible. For example if I have users on my site and directory per user solution is not "httpdocs/users/$login", but "httpdocs/users/".md5($login) or "httpdocs/users/".$userId

tim at correctclick dot com
31-Mar-2002 01:48


One more thing -- 



whenever you connect to a database with a password hard coded into your script, make sure you put the script off of your web document tree.  Put the script somewhere where apache won't serve documents, and then include/require this file in your other scripts.  That way, if the server ever gets misconfigured, it won't serve your PHP scripts with passwords, etc. as plain text for all to see. 



-Tim

cronos586(AT)caramail(DOT)com
06-Jan-2002 12:48


when using Apache you might consider a apache_lookup_uri on the path, to discover the real path, regardless of any directory trickery.

then, look at the prefix, and compare with a list of allowed prefixes.

for example, my source.php for my website includes:

if(isset($doc)) {

    $apacheres = apache_lookup_uri($doc);

    $really = realpath($apacheres->filename);

    if(substr($really, 0, strlen($DOCUMENT_ROOT)) == $DOCUMENT_ROOT) {

        if(is_file($really)) {

            show_source($really);

        }

    }

}

hope this helps

regards,

KAT44

devik at cdi dot cz
21-Aug-2001 04:52


Well, the fact that all users run under the same UID is a big problem. Userspace  security hacks (ala safe_mode) should not be substitution for proper kernel level security checks/accounting.


Good news: Apache 2 allows you to assign UIDs for different vhosts.


devik

eLuddite at not dot a dot real dot addressnsky dot ru
11-Nov-2000 06:44


I think the lesson is clear:





(1) Forbit path separators in usernames.


(2) map username to a physical home directory - /home/username is fine


(3) read the home directory


(4) present only results of (3) as an option for deletion.





I have discovered a marvelous method of doing the above in php but this submission box is too small to contain it.





:-)

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Problemas relacionados a bytes nulos (Null)

Instalado como módulo do Apache

Last updated: Fri, 13 Nov 2009