From:	 Markus Arndt <markus-arndt@web.de>
To:	 bugtraq@securityfocus.com
Subject: Philip Chinery's Guestbook 1.1 fails to filter out js/html
Date:	 Sun, 21 Apr 2002 11:03:07 +0200

Target:
 Philip Chinery's Guestbook 1.1 (maybee older versions?)

Vendor:
 http://www.sector7g.de.vu

Notified Vendor:
 Sure

Affected Systems:
 Webservers that run "Philip Chinery's Guestbook 1.1"

Found by:
 Markus Arndt<markus-arndt@web.de>

Short Description:
 Philip Chinery's Guestbook 1.1 fails to filter out JScript/HTML (CrossSiteScripting)

This nice lil' guestbook let's the owner choose to filter out Jscript- and/or HTML-entrys..
Let's see the start of it's sub where it saves an entry:

---code starts---

sub SaveData
{
    if($kill_html == 1) {
    $Text =~ s/<([^>]|\n)*>//g;
	}
   
    if($kill_html == 2) {
    $Text =~ s/</&lt;/g;
    $Text =~ s/>/&gt;/g;
	}
	
    if ($kill_java) {
	$Text =~ s/<!--(.|\n)*-->//g;
	}
    $Text       =~ s/\n/&nbsp;<br>/g;
    $Text       =~ tr/|/ /;
    $Text       =~ s/\t/&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/g;
    $Text		=~ s/\cM//g;

---code ends---

That's all it filters out.. As we can see it does only filter the comment itself a user wrote!
For example the fields "Name", "EMail" or "Homepage" are NOT checked!


So let's build an url to exploit this..

http://[target]/cgi-bin/guestbook.pl?action=sign&cwrite=none&Name=<script>alert("gotcha!");</script>&EMail=example@example.com&Text=css%20example

This would post a message that would display an alertbox on a visiotrs screen
when accessing the gb..

As I noticed the guestbook logs ipadresses but doesn't prevent spam.
It also automaticly redirects posters back to the mainguestbook-page.
That makes it very easy to post entrys that e.g. force visitors to spam the guestbook (really anoying).


Sorry for bad english, hope you can understand what i'm talkin' about. ;)


Markus Arndt<markus-arndt@web.de>
http://skka.de
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