[40438] in bugtraq
Re: [Full-disclosure] [scip_Advisory 1746] Microsoft Internet Explorer
daemon@ATHENA.MIT.EDU (Brion Vibber)
Thu Sep 22 18:40:56 2005
Message-ID: <43332875.4000309@pobox.com>
Date: Thu, 22 Sep 2005 14:56:05 -0700
From: Brion Vibber <brion@pobox.com>
MIME-Version: 1.0
To: Marc Ruef <maru@scip.ch>
Cc: bugtraq@securityfocus.com, news@securiteam.com,
submissions@packetstormsecurity.org, partners@secunia.com,
full-disclosure@lists.grok.org.uk
In-Reply-To: <4332F84E.8010207@scip.ch>
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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 <GIF89aź 8 ÷™f˙™™>
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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