Security hole in rexec?

Troels Therkelsen t_therkelsen at
Sat Aug 24 18:42:09 CEST 2002

Hello everybody,

I have managed to stumble onto something with the rexec module that I
do not quite understand.  As I understand it, the rexec framework is
meant to create a sandbox area within the Python interpreter,
technically with an instance of the rexec.RExec class.  It is supposed
to be impossible to break out of this sandbox unless you do something
careless like inserting non-rexec objects into the rexec namespace.

Let me demonstrate with some code:

  Python 2.2.1 (#1, Jun 27 2002, 10:29:04) 
  [GCC 2.95.3 20010315 (release)] on linux2
  Type "help", "copyright", "credits" or "license" for more
  >>> import rexec
  >>> r = rexec.RExec()
  >>> r.r_exec("import sys; print sys.stdout")
  Traceback (most recent call last):
    File "<stdin>", line 1, in ?
    File "/usr/local/lib/python2.2/", line 254, in r_exec
      exec code in m.__dict__
    File "<string>", line 1, in ?
  AttributeError: 'module' object has no attribute 'stdout'

This is as you'd expect, 'stdout' is not in the default ok_sys_names
attribute of the rexec.RExec class, so you are not supposed to be able
to see it from within the 'sandbox'.  But observe:

  >>> r.r_exec("del __builtins__")
  >>> r.r_exec("import sys; print sys.stdout")
  <open file '<stdout>', mode 'w' at 0x80fe2a0>

If __builtins__ is so critical to the operation of the 'sandbox' how
is it possible to break it from within the 'sandbox'?  Have I stumbled
across a bug in rexec?  Have I misunderstood something important?

I've used the id() function to get the 'address' of the __builtins__
object and I have verified that the new __builtins__ which gets
re-added has a different id so it is definitely a different
__builtins__ than the one I used del on.  It would appear that exec
and family adds __builtins__ to the namespace it runs in if it doesn't
exist.  But where does it get it from?  Why doesn't rexec deal with
this quirk of exec?  Maybe it's a new feature/bug of exec?

I'll stop with the questions now.  Suffice to say, I really need rexec

Best regards,

Troels Therkelsen

More information about the Python-list mailing list