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ECHU.ORG Alert #4: GuppY makes XSS attacks easy
From: "David Suzanne" <das () echu org>
Date: Mon, 29 Sep 2003 5:23:25 +0100

--------------------------------
| GuppY makes XSS attacks easy |
--------------------------------


PROGRAM: GuppY
VULNERABLE VERSIONS: All versions under 2.4p1
IMMUNE VERSIONS: 2.4p1
SEVERITY: high


Tested version
==============
GuppY 2.4 on the official GuppY website.


Description
============ 
"GuppY is a small web portal, meant to be easy to use for the end user that you are, and which does not require a 
database to run.This architecture allows then to install it on almost all Internet web sites hosters (or almost all of 
them), the only preriquisite being that your hoster has to provide the PHP scripting language facility." - GuppY website

GuppY use the postguest.php file for its forum and guestbook modules. This file contain XSS holes, an attacker can 
execute code since [c=codecolor]text[/c] has no filter.


POSTGUEST.PHP
=============
Problems come from this part:

Line 196: $ptxt = preg_replace("/(\[c=(.*?)\])(.*?)(\[\/c\])/i", "<font style=\"color:\\2\">\\3</font>",$ptxt);
Line 197: $ptxt = preg_replace("/(\[c=(.*?)\])(.*?)(\[\/C\])/i", "<font style=\"color:\\2\">\\3</font>",$ptxt);
Line 198: $ptxt = preg_replace("/(\[C=(.*?)\])(.*?)(\[\/c\])/i", "<font style=\"color:\\2\">\\3</font>",$ptxt);
Line 199: $ptxt = preg_replace("/(\[C=(.*?)\])(.*?)(\[\/C\])/i", "<font style=\"color:\\2\">\\3</font>",$ptxt);

A badly disposed member can post a message containing code really easily in forum and guestbook modules. Example:
(This proof-of-concept will show a window that can freeze your computer if using Internet Explorer so take care)

[c=expression(alert('unsecure'))]texte[/c]


Solution
========
In order to avoid the vulnerability, add a filter that will block ;<>*()"'] caracters to lines 196 to 199:

Line 196: $ptxt = preg_replace("/(\[c=([#a-zA-Z0-9]*)\])(.*?)(\[\/c\])/i", "<font 
style=\"color:\\2\">\\3</font>",$ptxt);
Line 197: $ptxt = preg_replace("/(\[c=([#a-zA-Z0-9]*)\])(.*?)(\[\/C\])/i", "<font 
style=\"color:\\2\">\\3</font>",$ptxt);
Line 198: $ptxt = preg_replace("/(\[C=([#a-zA-Z0-9]*)\])(.*?)(\[\/c\])/i", "<font 
style=\"color:\\2\">\\3</font>",$ptxt);
Line 199: $ptxt = preg_replace("/(\[C=([#a-zA-Z0-9]*)\])(.*?)(\[\/C\])/i", "<font 
style=\"color:\\2\">\\3</font>",$ptxt);


Vendor status
==============
GuppY developper's team has been contacted, a patch has been released tonight that fix the problem. Thanks to aldweb 
(freeguppy.org webmaster) for his quick answer.


Links
=====
GuppY: http://www.freeguppy.org
Patch: http://www.freeguppy.org/file/guppy_patch2.zip
Version 2.4p1: http://www.freeguppy.org/file/guppy.zip


This vulnerability's orginal paper can be found here: http://www.echu.org/articles/alertes/echu-alert4.txt


David Suzanne (dAs)
das[a]echu.org
http://www.echu.org 


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