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Let's exploit this
From: Tim <tim-pentest () sentinelchicken org>
Date: Wed, 2 Aug 2006 08:57:17 -0400

Hello list,

After a discussion with our moderator about list content, I decided I'll
try to encourage some more in-depth technical discussion about the
exploitation certain quirky bugs.  So, here's one that I came across a
while back.  

It is an exploitable XSS bug, but it isn't immediately obvious (at least
it wasn't to me) how it can be exploited.  Suppose we have a dynamically
generated web page which contains the following line of JavaScript:

  myFunc('FOO', 'BAR');

In this page, the user can control both the values of FOO and BAR.  In
other words, this JavaScript is dynamically generated.  Here's the
catch:  The values FOO and BAR are both checked for apostrophe (single
quote) characters.  Any instances of these are escaped with backslashes.
For instance, if a user supplied values "Nancy's" and "hamster" for FOO
and BAR, respectively, then the following line would be generated:

  myFunc('Nancy\'s', 'hamster');

Apostrophe is the only character checked for, in all it's variants
(unicode?).  Let us suppose an attacker can inject arbitrary binary data
in those strings.

Now, like I said, I know of one way to exploit this behavior to inject
almost arbitrary JavaScript.  I'm hoping this will prompt an interesting
discussion on the topic of browser behavior and perhaps learn of other
ways it can be exploited.


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