From djb@cr.yp.to Wed Dec 15 14:21:57 2004
Date: 15 Dec 2004 08:23:11 -0000
From: D. J. Bernstein <djb@cr.yp.to>
To: securesoftware@list.cr.yp.to, moinejf@free.fr
Subject: [remote] [control] abcm2ps 3.7.20 put_words overflows str buffer

Limin Wang, a student in my Fall 2004 UNIX Security Holes course, has
discovered a remotely exploitable security hole in abcm2ps. I'm
publishing this notice, but all the discovery credits should be assigned
to Wang.

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 document through abcm2ps. Whoever provides the ABC file then
has complete control over your account: she can read and modify your
files, watch the programs you're running, etc.

The abcm2ps 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, as root, type

   cd /usr/ports/print/abcm2ps
   make install

to download and compile the abcm2ps program, version 3.7.20 (current).
Then, as any user, save the file 35.abc attached to this message, and
type

   abcm2ps 35.abc > 35.ps

with the unauthorized result that a file named x is removed from the
current directory. (I tested this with a 470-byte environment, as
reported by printenv | wc -c; beware that 35.abc is sensitive to the
environment size.)

Here's the bug: In subs.c, put_words() copies any amount of data into
an 81-byte str[] array. Wang also comments that t->text is not
0-terminated.

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

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