Re: Bypassing of web filters by using ASCII

[Fixer <fixer () gci net>]

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This also affects IE 7 Beta 2.

Did you shoot this over to Microsoft?

k.huwig () iku-ag de wrote:
> _______________________________________________________________________
>
>
>                            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
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>   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
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> 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
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> set the bit on arbitraty characters without changing the look of the
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> page. But virus scanners and content filters see completely different
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> characters, so that there programs cannot detect viruses or spam.
>
>
> This offers spammers and virus writers the possibility to bypass
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> 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
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> 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
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> 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 

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