oss-security - Re: Python Unsafe Module Loading

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Date: Thu, 5 Jun 2008 12:15:16 +0200
From: Robert Buchholz <rbu@...too.org>
To: oss-security@...ts.openwall.com
Subject: Re: Python Unsafe Module Loading

On Thursday 05 June 2008, Ned Ludd wrote:
> On Thu, 2008-06-05 at 10:10 +0200, Robert Buchholz wrote:
> > On Wednesday 04 June 2008, Ned Ludd wrote:
> > > So for nearly every python based program you can simply dump 
> > > *.so *.py *.pyc files just about anywhere on the file system
> > > where an admin might invoke python.
> >
> > As I also pointed out in our bug [1], this only happens in two
> > cases: (1) The interactive shell is used to run python code.
> > (2) A python script resides inside an untrusted directory.
> >
> > What I expect to be the most common use case, running python code
> > from /usr, or /home, is safe. Since all out-of-the-box software
> > would be installed in directories that are not world-writable, I am
> > tempted call (2) an error on the user side. Changing the behaviour
> > of python in this manner would also break existing programs.
> >
> >
> > Robert
> >
> > [1] https://bugs.gentoo.org/show_bug.cgi?id=224925
>
> Re: (1)
> How this limited to interactive shells? Our portage/emerge being
> directly not vuln is left to near sheer luck that Nick.C opted to
> shove a path into our portage module a-long time ago.. But our tools
> are questionable as it all depends on load order..
>
> More examples:
>
> solar@...ia /tmp $ touch re.so
> solar@...ia /tmp $ cat foo.py
> import string
> print "foo"
>
> solar@...ia /tmp $ python foo.py
> Traceback (most recent call last):
>   File "foo.py", line 1, in ?
>     import string
>   File "/usr/lib/python2.4/string.py", line 83, in ?
>     import re as _re
> ImportError: /tmp/re.so: file too short
> solar@...ia /tmp $ ls -l re.so
> -rw-r--r-- 1 solar solar 0 Jun  5 01:22 re.so
>
> (2) yeah that's pretty much 50% of the problem.

The problem you demonstrated above was caused by (2), running a python 
script that is located in /tmp. Portage, and most other python 
programs, are not installed in /tmp, and therefore they do not import 
files from /tmp, even if they are actually called there:

rbu@...nut /tmp $ touch re.so
rbu@...nut /tmp $ cat /home/rbu/foo.py
import string
print "foo"

import sys
print sys.path
rbu@...nut /tmp $ python /home/rbu/foo.py
foo
['/home/rbu', '/usr/lib64/portage/pym', '/usr/lib64/python25.zip', '/usr/lib64/python2.5', '/usr/lib64/python2.5/plat-linux2', '/usr/lib64/python2.5/lib-tk', '/usr/lib64/python2.5/lib-dynload', '/usr/lib64/python2.5/site-packages', '/usr/lib64/python2.5/site-packages/Numeric', '/usr/lib64/python2.5/site-packages/gtk-2.0']


Robert

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