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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 <GIF89a 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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