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Date: Wed, 25 Feb 2004 10:48:31 -0500
From: "Larry Seltzer" <larry@...ryseltzer.com>
To: "'Eli K.'" <elik@...ondsecurity.com>, <sunglasses@...-watch.com>,
	<bugtraq@...urityfocus.com>
Subject: RE: Windows XP explorer.exe heap overflow.


The sample someone sent around that caused the 100% CPU hogging had the Size field set
to 0000h. Try that. Perhaps it's not just a matter of the value being lower, but below
some small threshold.

Larry Seltzer
eWEEK.com Security Center Editor
http://security.eweek.com/
larryseltzer@...fdavis.com 

-----Original Message-----
From: Eli K. [mailto:elik@...ondsecurity.com] 
Sent: Tuesday, February 24, 2004 5:36 AM
To: sunglasses@...-watch.com; bugtraq@...urityfocus.com
Subject: Re: Windows XP explorer.exe heap overflow.


I have tried to verify this issue with a malformed EMF file. Either I didn't 
understand something or the vulnerability doesn't exist.

Here's what I did:

I took a sample EMF file and modified it's Size field (offset 0030h) to some smaller
value such as 0020h. The reported header size (offset 04h) 
was 0000006Ch. I've experimented with the values a little but to no avail.

Windows XP seems to be immune to this. I didn't see any point on trying a different OS
(such as Win2K).

Maybe the poster to the list can provide some details or a malformed EMF file so we
could verify the issue ?

Eli

On Friday 20 February 2004 20:45, sunglasses@...-watch.com wrote:
> Vulnerability in XP explorer.exe image loading
> ----------------------------------------------
>
> Systems affected:
>   Current XP - others not tested.
>
> Degree:
>   Arbitrary code execution.
>
> Summary
> -------
> A malformed .emf (Enhanced Metafile, a graphics format) file can cause 
> an exploitable heap overflow in (or near) shimgvw.dll.
>
> Details
> -------
> The image preview code that explorer uses has an exploitable buffer 
> overflow.
>
> An .emf file with a "total size" field set to less than the header 
> size will causes explorer.exe to crash in the heap routines - in 
> classic heap overflow style that should be exploitable a la the RPC 
> exploits.
>
> There are two overflows here:
>
> 1. A buffer is allocated with the size indicated in the header (no 
> validity checks), then the header is copied into it - if the size is 
> less than the header size, that's one overflow.
>
> 2. They then proceed to read the rest of the file to a length of 
> (size-headersize), which allows for an integer overflow causing the 
> rest of the file to be appended to the already blown buffer.
>
> Exploit
> -------
> To exploit this flaw (in explorer), simply place a malformed (invalid 
> "size" field) .emf file in any directory, open explorer to that path, 
> and view as Thumbnails. Bang. In it's simplest form it's a DOS - it 
> affects all explorer windows, including File Open dialogs for many 
> programs.
>
> Alternatively, without viewing as a Thumbnail, open the picture 
> preview window for the .emf file. (It's the default double-click 
> action). Using this trigger causes a different crash point, which may 
> not be exploitable, but I wouldn't rule it out.
>
> Additional notes
> ----------------
> It may be worth checking out similar issues in .wmf files, as they are 
> similar.
>
>
> - Jellytop, 2004
>
> "If a man will begin with certainties, he shall end in doubts; but if 
> he will be content to begin with doubts he shall end in certainties."





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