From djb@cr.yp.to Wed Dec 15 14:21:37 2004
Date: 15 Dec 2004 08:20:49 -0000
From: D. J. Bernstein <djb@cr.yp.to>
To: securesoftware@list.cr.yp.to, nasm-devel@lists.sourceforge.net
Subject: [remote] [control] NASM 0.98.38 error() overflows buff[]

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

You are at risk if you receive an asm 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 NASM. Whoever provides that asm file then has
complete control over your account: he can read and modify your files,
watch the programs you're running, etc.

Of course, if you _run_ a program, you're authorizing the programmer to
take control of your account; but the NASM documentation does not say
that merely _assembling_ a program can have this effect. It's easy to
imagine situations in which a program is run inside a jail but assembled
outside the jail; this NASM bug means that the jail is ineffective.

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

   cd /usr/ports/devel/nasm
   make install

to download and compile the NASM program, version 0.98.38 (current).
Then, as any user, save the file 22.S attached to this message, and type

   nasm 22.S

with the unauthorized result that a file named EXPLOITED is created in
the current directory. (I tested this with a 525-byte environment, as
reported by printenv | wc -c.)

Here's the bug: In preproc.c, error() uses an unprotected vsprintf() to
copy data into a 1024-byte buff[] array.

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

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