Vulnerability in XP explorer.exe image loading

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Systems affected: 

  Current XP - others not tested.



Degree: 

  Arbitrary code execution.



Summary

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A malformed .emf (Enhanced Metafile, a graphics format) file can cause an exploitable heap overflow in (or near) shimgvw.dll.



Details

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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

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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

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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."