From djb@cr.yp.to Wed Dec 15 14:20:08 2004
Date: 15 Dec 2004 08:09:36 -0000
From: D. J. Bernstein <djb@cr.yp.to>
To: securesoftware@list.cr.yp.to, asher@wildspark.com
Subject: [remote] [control] dxfscope 0.2 overflows ent_name buffer

Ariel Berkman, a student in my Fall 2004 UNIX Security Holes course, has
discovered a remotely exploitable security hole in dxfscope, a viewer
for DXF drawings. I'm publishing this notice, but all the discovery
credits should be assigned to Berkman.

You are at risk if you take a DXF document from email (or a web page or
any other source that could be controlled by an attacker) and feed that
document through dxfscope. Whoever provides that document then has
complete control over your account: he can read and modify your files,
watch the programs you're running, etc.

The dxfscope documentation does not tell users to avoid taking input
from the network. One can easily find DXF files placed on the web for
public viewing; see, e.g., http://www.acipco.com/afc/dxf.cfm.

Proof of concept: On an x86 computer running FreeBSD 4.10, type

   wget http://wildspark.com/dxfscope/dxfscope-current.tar.gz
   gunzip < dxfscope-current.tar.gz | tar -xf -
   cd dxfscope-0.2
   make CC='gcc -DM_PIl=M_PI -I/usr/X11R6/include' SHAREDIR=`pwd`

to download and compile the dxfscope program, version 0.2 (current).
Then save the DXF document attached to this message as 2.dxf, and run

   ./dxfscope 2.dxf

with the unauthorized result that a file named x is created (and its
previous contents destroyed) in the current directory. (I tested this
with a 452-byte environment, as reported by printenv | wc -c; this
particular file 2.dxf is fairly fragile, allowing only a 60-byte range
of environment sizes.)

Here's the bug: In d.c, dxfin() uses fscanf(...,"%s",...) to read any
number of bytes into a 255-byte ent_name array.

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago

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