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."
