Re: Bypassing of web filters by using ASCII
On 21 Jun 2006 at 13:11, k.huwig@xxxxxxxxx wrote:
>
> 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.
>
Not quite. The most significant bit must be set to zero when encoding (from RFC
20: "For
concreteness, we suggest the use of standard 7-bit ASCII embedded in an 8 bit
byte whose
high order bit is always 0"). So a byte whose high bit is set is simply illegal
in US-
ASCII. Which leads to the following point:
In case a message contains message description (in our case, charset
specification, i.e.
charset=US-ASCII) which is inconsistent with the message data (in our case,
data out of the
charset specification, i.e. bytes with the high bit set), what is a message
reader to do?
Security-wise, the best would be to reject the message. Yet of course this
leads to less
than ideal user experience. So the obvious solution is to virtually modify one
of the
elements (either the message description, or the message data), so consistency
is
established.
Now, IE changes the data, i.e. sets each msb to zero, and thus establishes
consistency -
the data becomes valid US-ASCII byte stream. Firefox and Opera, I assume, take
the other
path, and modify the message description to read "ISO-8859-1", and thus
establish
consistency, as now the bytestream is valid ISO-8859-1 data.
> 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.
So what I don't understand now is why IE's "solution" is any better than
Opera/Firefox?
Why is modifying the data (msb) any better than modifying the data-description
(charset)?
Please note: the attack you described is interesting and elegant. I'm just
reserved about
the statement that IE's approach is correct (vs. the other browsers). I was
involved in
research around similar situations wherein the strict RFC was violated, and
different
products interpreted data differently. And in such cases, I think we should be
cautious
about which product is "correct" (except that naturally, security-wise, it's
more corrent
to reject the message altogether).
Food for thought,
-Amit