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[bWM#012] Passing script/html-filter with special chars (multibrowser)

From: <ben.moeckel () online de>
Date: Sun, 3 Aug 2003 13:58:01 +0200


badWebMasters security advisory #012 

Passing script/html-filter with special chars (multibrowser) 

Discovery date: 2003-07-16 

Update: 2003-07-31 
  
Original advisory:
http://badwebmasters.net/advisory/012/ (text/html) 
 
  
Legal Notice:
Copyright 2003 by Benjamin Klimmek (ben moeckel - badWebMasters)!
You may distribute it unmodified.
You may not modify it and distribute it or distribute parts of it
without giving credits and the URL where the original advisory can be
found!
This document may change without notice. 
 
  
Author:
ben moeckel (http://distressed.de)
mailto: badwebmasters () online de 
 
  
Description:
When webbrowsers parse html they remove special chars, this behavior may
be used by an malicious user to fool script/html-filters in
webapplications.
 
  
Detail: 
badWebMasters showed in their advisory #011 how to pass the "Snitz
Forums"-scriptfilter with the Tab-Char (09). After "Opera" and
"Mozilla"-users noticed that the provided exploit didn't work on their
system I decided to start some new testings, with an amazing result!

To detect what kind of special chars can be used in html- parameters I
set up the following asp-page:

----------------------------------------test2.asp---------
<%@LANGUAGE=JScript%><%

Response.BinaryWrite('<html><script>function a(o){alert(o)}</script>');
Response.BinaryWrite('<img src="javascript:a(\'test\')" />');
for(i=0;i<256;++i){
        uc = "%"+chk(i.toString(16));
        Response.BinaryWrite('<img src="ja'+unescape(uc)+'vascript:a('+i+')"
/>\r\n');
}
function chk(sInp){if(sInp.length<2){
        return String("0"+sInp)
}else{return sInp}}

%>
---------------------------------------------------------

 
  
Result:

The page has been viewed with Mozilla, Opera and Internet- Explorer, the
alert-box poped up in this order:

   Mozilla 1.3.1 (Win32):       - (with restricions)

   Opera 7.11 (Win32):          0, 9, 10, 13, 173

   Internet Explorer 5.0:       13, 10, 9, 0
        
Webmasters may be carefull with char 173 (ADh) that can be used in Opera
only.

Silly Internet Explorer: reversed order!?
 
  
Test:
http://badwebmasters.net/advisory/012/test.asp 
 
  
Workaround:
Make sure control-chars are removed before badwords are! Even better:
Only allow trusted protocols in user-images!
 
  
References:
[bWM #011]: Cross-Site-Scripting @ Snitz Forums
- http://cert.uni-stuttgart.de/archive/bugtraq/2003/04/msg00247.html 
 
  
Thanks:
Thanks to "jelmer " for correcting my demo page! 
 
  
Feedback:
Comments, suggestions, updates, anything else?
   -> mailto:badwebmasters () online de 
 

__________________________________________

badWebMasters - ben moeckel security research
http://badwebmasters.de http://badwebmasters.net
copyright 2k1-3 by Benjamin Klimmek / Germany
mailto:badwebmasters () online de
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Current thread:

[bWM#012] Passing script/html-filter with special chars (multibrowser) ben.moeckel (Aug 03)
- <Possible follow-ups>
- [bWM#012] Passing script/html-filter with special chars (multibrowser) ben.moeckel (Aug 03)