From djb@cr.yp.to Wed Dec 15 14:23:48 2004
Date: 15 Dec 2004 08:36:25 -0000
From: D. J. Bernstein <djb@cr.yp.to>
To: securesoftware@list.cr.yp.to, guido.gonzato@poste.it
Subject: [remote] [control] abcpp 1.3.0 process_directive overflows token
    buffer

Yosef Klein, a student in my Fall 2004 UNIX Security Holes course, has
discovered a remotely exploitable security hole in abcpp. I'm publishing
this notice, but all the discovery credits should be assigned to Klein.

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

The abcpp documentation does not tell users to avoid taking input from
the network. Many web pages offer ABC files for public consumption.

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

   wget http://abcplus.sourceforge.net/abcpp-1.3.0.tar.gz
   gunzip < abcpp-1.3.0.tar.gz | tar -xf -
   cd abcpp-1.3.0
   gmake

to download and compile the abcpp program, version 1.3.0 (current). Then
save the file 80.abc attached to this message, and type

   ./abcpp 80.abc

with the unauthorized result that a file named EXPLOITED is created in
the current directory. (I tested this with a 613-byte environment, as
reported by printenv | wc -c; beware that 80.abc is sensitive to the
environment size.)

Here's the bug: In abcpp.c lines 557 and 571, handle_directive() copies
a line to various small local arrays. The line can be as large as 1024
bytes.

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

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