Advisory Title: CGI.pm vulnerable to Cross-site Scripting. 
Release Date: July 19 2003

Application: CGI.pm - which is by default included in many common Perl
distributions. 


Platform: Most platforms. Tested on Apache and IIS. 

Version: CGI.pm 

Severity: Effects scripts which make use of start_form()

Author: 
Obscure^ 
[ obscure@eyeonsecurity.org ]

Vendor Status: 
first informed on 30th April 2003
Although the author told EoS that he will be releasing a fix within a
week from his last correspondence (May15), no fix is out yet on his
website.


Web: 

http://stein.cshl.org/WWW/software/CGI/
http://eyeonsecurity.org/advisories/


Background.

(extracted from 
http://stein.cshl.org/WWW/software/CGI/)

This perl 5 library uses objects to create Web fill-out forms on the fly
and to parse their contents. It provides a simple interface for parsing
and interpreting query strings passed to CGI scripts. However, it also
offers a rich set of functions for creating fill-out forms. Instead of
remembering the syntax for HTML form elements, you just make a series of
perl function calls. An important fringe benefit of this is that the
value of the previous query is used to initialize the form, so that the
state of the form is preserved from invocation to invocation. .


Problem

CGI.pm has the ability to create forms by making use of the start_form()
function. The developer/perl scripter can also makes use of
start_multipart_form() which relies on start_form() and is therefore
vulnerable to the same issue. When the action for the form is not
specified, it is given the value of $self->url(-absolute=>1,-path=>1) -
which means that when the url is something like the following :

http://host/script.pl?">some%20text<!--%20

.. the form becomes <form action="http://host/script.pl">some text<!-- "
>

In such case, it is possible to exploit this issue to launch a Cross
Site Scripting attack.  

Exploit Examples.

--
#!/usr/bin/perl
# example of exploitable script
#

use CGI;

$q = new CGI;
print $q->header;
print $q->start_html('CGI.pm XSS');
print $q->start_form();
print $q->end_form();
print $q->end_html;

--

Fix.

I fixed my CGI.pm by adding the following code at line 1537

$action =~ s/\"/\%22/g; 


Disclaimer.

The information within this document may change without notice. Use of
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condition. There are NO warranties with regard to this information.
In no event shall the author be liable for any consequences whatsoever
arising out of or in connection with the use or spread of this
information. Any use of this information lays within the user's
responsibility.


Feedback.

Please send suggestions, updates, and comments to:

Eye on Security
mail : obscure@eyeonsecurity.org
web : http://www.eyeonsecurity.org