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@ziffdavis.com -----Original Message----- From: Eli K. [mailto:elik@beyondsecurity.com] Sent: Tuesday, February 24, 2004 5:36 AM To: sunglasses@bay-watch.com; bugtraq@securityfocus.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@bay-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."
