(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.");
}
Segurança do Sistema de Arquivos
Índice
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 moldado para permitir acesso em nível de usuário ao sistema de arquivos, é possível escrever um script PHP que permitirá ler arquivos do sistema como o /etc/passwd, modificar suas conexões de rede, enviar inúmeros trabalhos de impressão, 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 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!";
?>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!";
?>- Dar permissões limitadas ao usuário web binário do PHP.
- Checar todas as variáveis que são enviadas.
Exemplo #3 Checagem com 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);
?>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', $userfile)) {
die("Bad username/filename");
}
//etc...
?>Dependendo do sistema operacional, existe uma variedade enorme de arquivos com os quais se preocupar, incluindo dispositivos (/dev/ or COM1), arquivos de configuração (arquivos /etc/ e .ini), áres de armazenamento conhecidas (/home/, My Documents), etc. Por essa razão, normalmente é mais fácil criar uma política onde se proibe tudo exceto aquilo que for explicitamente permitido.
add a note
User Contributed Notes 7 notes
anonymous ¶
18 years ago
devik at cdi dot cz ¶
23 years ago
Well, the fact that all users run under the same UID is a big problem. Userspace security hacks (ala safe_mode) should not be substitution for proper kernel level security checks/accounting.
Good news: Apache 2 allows you to assign UIDs for different vhosts.
devik
fmrose at ncsu dot edu ¶
19 years ago
All of the fixes here assume that it is necessary to allow the user to enter system sensitive information to begin with. The proper way to handle this would be to provide something like a numbered list of files to perform an unlink action on and then the chooses the matching number. There is no way for the user to specify a clever attack circumventing whatever pattern matching filename exclusion syntax that you may have.
Anytime you have a security issue, the proper behaviour is to deny all then allow specific instances, not allow all and restrict. For the simple reason that you may not think of every possible restriction.
Latchezar Tzvetkoff ¶
15 years ago
A basic filename/directory/symlink checking may be done (and I personally do) via realpath() ...
<?php
if (isset($_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($file);
} else {
die('blah!');
}
}
?>
cronos586(AT)caramail(DOT)com ¶
22 years ago
when using Apache you might consider a apache_lookup_uri on the path, to discover the real path, regardless of any directory trickery.
then, look at the prefix, and compare with a list of allowed prefixes.
for example, my source.php for my website includes:
if(isset($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
1 at 234 dot cx ¶
19 years ago
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.
eLuddite at not dot a dot real dot addressnsky dot ru ¶
23 years ago
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.
:-)
