From djb@cr.yp.to Wed Dec 15 14:20:08 2004 Date: 15 Dec 2004 08:09:36 -0000 From: D. J. Bernstein 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 [ Part 2, Text/PLAIN (charset: unknown-8bit) 6 lines. ] [ Unable to print this part. ]