BugTraq
Bypassing of web filters by using ASCII Jun 21 2006 01:11PM
k huwig iku-ag de (4 replies)
_______________________________________________________________________

iKu Advisory

_______________________________________________________________________

Product : Microsoft InternetExplorer 6

: various filter applications

Date : June 20th 2006

Affected versions : all

Vulnerability Type : bypassing security filters

Severity (1-10) : 10

Remote : yes

_______________________________________________________________________

0. contents

1. problem description

2. affected software

3. bug description/possible fix

4. sample code

5. workaround

1. problem description

The character set ASCII encodes every character with 7 bits. Internet

connections transmit octets with 8 bits. If the content of such a

transmission is encoded in ASCII, the most significant bit must be ignored.

Of the tested browsers Firefox 1.5, Opera 8.5 and InternetExplorer 6,

only the InternetExplorer does this correctly, the others evaluate the

bit and display the characters as if they were from the character set

ISO-8859-1. Although the behaviour of the InternetExplorer is the

correct one, this creates a security risk: the author of a web page can

set the bit on arbitraty characters without changing the look of the

page. But virus scanners and content filters see completely different

characters, so that there programs cannot detect viruses or spam.

This offers spammers and virus writers the possibility to bypass

installed spam and virus filters.

2. affected software

Only the InternetExplorer displays ASCII encoded web pages as 7 bit. We

checked several hardware router and antivirus solutions, all of which

failed to detect malicious JavaScript in manipulated web pages.

3. bug description/possible fix

It should be quite easy to close this hole within filter/scan

applications by clearing the most significant bit on ASCII encoded web

pages before analysing them.

4. sample page

At

http://www.iku-ag.de/ASCII

you can find a test page that displays a secret message. IE6 displays

the text correctly, Firefox 1.5 and Opera 8.5 display glibberish text.

This page only shows that IE6 displays ASCII-text correctly and does not

contain any content that a filter should sort out.

Updated information can be found at

http://www.iku-ag.de/sicherheit/ascii-eng.jsp

5. workaround

There is no workaround know to us.

--

Kurt Huwig iKu Systemhaus AG http://www.iku-ag.de/ Vorstand Am Römerkastell 4 Telefon 0681/96751-0 66121 Saarbrücken Telefax 0681/96751-66 GnuPG 1024D/99DD9468 64B1 0C5B 82BC E16E 8940 EB6D 4C32 F908 99DD 9468

[ reply ]
Re: Bypassing of web filters by using ASCII Jun 22 2006 10:08PM
Amit Klein (AKsecurity) (aksecurity hotpop com)
Re: Bypassing of web filters by using ASCII Jun 22 2006 02:57PM
Hubert Seiwert (hubert westpoint ltd uk)
Re: Bypassing of web filters by using ASCII Jun 21 2006 11:57PM
RSnake (rsnake shocking com) (2 replies)
Re: Bypassing of web filters by using ASCII Jun 25 2006 03:42PM
David Huecking (d huecking gmx net)
Re: Bypassing of web filters by using ASCII Jun 22 2006 12:34AM
Kurt Huwig (k huwig iku-ag de)
Re: Bypassing of web filters by using ASCII Jun 21 2006 08:15PM
Fixer (fixer gci net) (1 replies)
Re: Bypassing of web filters by using ASCII Jun 21 2006 10:24PM
Paul (pvnick gmail com) (3 replies)
Re: Bypassing of web filters by using ASCII Jun 22 2006 07:46AM
Amit Klein (AKsecurity) (aksecurity hotpop com) (1 replies)
RE: Bypassing of web filters by using ASCII Jun 23 2006 11:55AM
James C. Slora Jr. (james slora phra com) (3 replies)
Re: Bypassing of web filters by using ASCII Jun 26 2006 05:56PM
Hubert Seiwert (hubert westpoint ltd uk) (1 replies)
RE: Bypassing of web filters by using ASCII Jun 26 2006 07:31PM
James C. Slora Jr. (james slora phra com)
RE: Bypassing of web filters by using ASCII Jun 23 2006 10:12PM
RSnake (rsnake shocking com)
RE: Bypassing of web filters by using ASCII Jun 23 2006 07:23PM
Amit Klein (AKsecurity) (aksecurity hotpop com)
Re: Bypassing of web filters by using ASCII Jun 22 2006 03:40AM
Thor (Hammer of God) (thor hammerofgod com)
Re: Bypassing of web filters by using ASCII Jun 21 2006 11:23PM
Kurt Huwig (k huwig iku-ag de)


 

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