Re: IE8 img tag HiJacking

[Dan Kaminsky <dan () doxpara com>]

Interesting use, using filesize to back into the actual CAPTCHA used for a
given query.  Sneaky!

So it's possible to read not only filesize, but image dimensions
cross-domain.  I actually found a use for this -- it's a good way to
exchange a small amount of data between sites that mutually distrust one
another.  The reason for this is that images are pretty much the only
resources that can be loaded cross-domain that won't have embedded script
executed by a browser.

(Side note:  At this point, you're probably thinking:  Vladimir just said
that some browsers allow SVG to load via <img> -- and SVG can embed script
with nothing but a script tag  and a smile!  Doesn't this mean a bunch of
sites are in trouble?

Turns out, no, not as far as I can tell anyway.  IE and Firefox both block
<img> to SVG entirely, while Chrome, Safari, and Opera allow it.  But there
appears to be a script firewall (or more accurately, a missing connection)
between <img>-loaded SVG and the script engine.  Static SVG renders just
fine, but don't expect it to do anything unless you top-level nav, inline,
or use something like embed.)

Back to image dimensions, it turns out that this information channel cannot
be closed; even if the dimensions of the object itself couldn't be queried,
the XY positioning of the objects *around* the imported images must be both
queryable and dependent on image properties.

I was curious however if img.fileSize would leak filesizes of non-image
content.  Doesn't look like it does -- undefined in everything but IE, -1 in
IE.


2010/4/22 T Biehn <tbiehn () gmail com>

> It could be used as a technique for defeating the login images used as
> "two-factor-authentication" by some online services.
> The application of using filesize to fingerprint an image is somewhat
> novel. This is a decidedly 'old' vector, though.
>
> -Travis
>
> 2010/4/21 Владимир Воронцов <vladimir.vorontsov () onsec ru>
>
> Hello Full disclosure!
> > Once again, unwinding theme HiJacking found a fun way to get the very
> > least information about the target resource when the user is located at
> > the
> > attacker.
> >
> > Already crocked <img> tag opens new opportunities using the method
> > fileSize, described here:
> > http://msdn.microsoft.com/en-us/library/ms533752
> > (v = VS.85). Aspx
> >
> > Consider a simple example - a Web application after authentication
> > provides some sort of picture for the user, for example:
> >
> > http://example.com/getImage.php?image=myAvatar
> >
> > The attacker, knowing this can create a page to read:
> >
> > <img id="onsec" src="http://example.com/getImage.php?image=myAvatar";>
> >
> > <input type="button" onclick="if (onsec.fileSize> 0) (alert ('authorized
> > on example.com') else (alert ('not authorized on example.com')}">
> >
> > Thus, the attacker learns the simplest case, whether the target user
> > access to example.com.
> >
> > Continuing the theme, I want to note that in some cases, can obtain
> > additional information from the very values of the size of the picture. It
> > can be any logical information Web applications, say, the same script can
> > show administrators a picture of the same size, and users - of another.
> > Thus, we obtain the user rights. And so on.
> >
> > I'd like to return the size of the method is not only "valid" images, but
> > also HTML pages, JSON, etc. But, unfortunately, does not work. Maybe, of
> > course, there are exceptions, call to investigate the matter.
> >
> > I have some thoughts on the study of vector images in XML format, because
> > HTML is often valid XML, and then ...
> >
> > Check for the test version IE9, but he did not support SVG inside tag
> > <img>, but only as a separate tag.
> >
> > Works in IE8, in Opera 10.52 does not work on check writing, if not
> > difficult.
> >
> > Original at russian language: http://oxod.ru/?p=113
> >
> > --
> > Best regards,
> > Vladimir Vorontsov
> > ONsec security expert
> >
> > _______________________________________________
> > Full-Disclosure - We believe in it.
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>
>
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