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
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
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
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
This issue also affects RHEL2.1
Affects RHEL4 as well.
This issue doesn't affect RHEL 2.1 as the vsprintf in preproc.c/error() is
missing completely in nasm-0.98-8.
Fix for this is now applied in RHEL3 and RHEL4.
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.