Re: [scip_Advisory 1746] Microsoft Internet Explorer 6.0 embedded content cross site scripting

[tim tompkins <timt () spiderlinks org>]

I discovered something similar recently, though, where a *valid* jpg 
containing an XML header was issued to IE (via a direct link to the 
image) with a content-type of application/octet-stream and IE attempted 
to render the XML rather than rendering the image (or prompting for an 
action by the user, which is what I was going for).  IE subsequently 
issued an error indicating that it was unable to render the XML page 
(although it was a jpeg image with a .jpg extension).

While this may not be a direct threat if the server is sending the 
correct content-type, it could be a threat otherwise.  This, being 
untested speculation, would require testing to see exactly what you can 
get IE to do with embedded code.


Regards,
Tim Tompkins



Brion Vibber wrote:

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> Marc Ruef wrote:
> | III. EXPLOITATION
> |
> | The following proof-of-concept has been published in the articles "Wie
> | mit GIF-Bildern Cross Site Scripting-Angriffe im Internet Explorer
> | umgesetzt werden können" in scip monthly Security Summary Issue 19.
> | September 2005 (pp. 12-14)[1] and "GIF-Bug im Internet Explorer 6 -
> | Proof of Concept" at computec.ch[2]:
> |
> |     01 <GIF89aŸ 8 ÷™fÿ™™>
>
> The reason that this works in this case is that this is *not* a GIF
> header; GIF headers do not begin with "<". It is well known that IE will
> interpret files as HTML that contain certain HTML tags if a another type
> detection doesn't override it.
>
> For Microsoft's vague documentation on this process, see:
> http://msdn.microsoft.com/workshop/networking/moniker/overview/appendix_a.asp 
>
>
> If you remove the "<" at the beginning, then IE will detect the GIF
> signature, overriding its HTML detection, and show a 'broken image' icon
> with no interpretation of JavaScript.
>
> (Tested MSIE 6.0.2900.2180.xpsp_sp2)
>
> However the advice is good; as a general rule sites accepting uploads
> should validate them as carefully as possible, as IE may not recognize
> all file types properly. Invalid image file headers and HTML-like tags
> near the start of a file should be considered suspicious.
>
> - -- brion vibber (brion @ pobox.com)
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