Re: Bypassing of web filters by using ASCII
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This also affects IE 7 Beta 2.
Did you shoot this over to Microsoft?
k.huwig@xxxxxxxxx wrote:
> _______________________________________________________________________
>
>
> iKu Advisory
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> _______________________________________________________________________
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>
> Product : Microsoft InternetExplorer 6
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> : various filter applications
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> Date : June 20th 2006
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> Affected versions : all
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> Vulnerability Type : bypassing security filters
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> Severity (1-10) : 10
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> Remote : yes
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> _______________________________________________________________________
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> 0. contents
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>
> 1. problem description
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> 2. affected software
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> 3. bug description/possible fix
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> 4. sample code
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> 5. workaround
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>
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> 1. problem description
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> The character set ASCII encodes every character with 7 bits. Internet
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> 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.
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> Of the tested browsers Firefox 1.5, Opera 8.5 and InternetExplorer 6,
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> only the InternetExplorer does this correctly, the others evaluate the
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> bit and display the characters as if they were from the character set
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> ISO-8859-1. Although the behaviour of the InternetExplorer is the
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> 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.
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> This offers spammers and virus writers the possibility to bypass
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> installed spam and virus filters.
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>
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> 2. affected software
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> Only the InternetExplorer displays ASCII encoded web pages as 7 bit. We
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> checked several hardware router and antivirus solutions, all of which
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> failed to detect malicious JavaScript in manipulated web pages.
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>
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> 3. bug description/possible fix
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> It should be quite easy to close this hole within filter/scan
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> applications by clearing the most significant bit on ASCII encoded web
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> pages before analysing them.
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> 4. sample page
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> At
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> http://www.iku-ag.de/ASCII
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> you can find a test page that displays a secret message. IE6 displays
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> the text correctly, Firefox 1.5 and Opera 8.5 display glibberish text.
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> 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.
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> Updated information can be found at
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> http://www.iku-ag.de/sicherheit/ascii-eng.jsp
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>
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> 5. workaround
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> 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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