Hardening enviroment by overloading __import__?

Steven Bethard steven.bethard at gmail.com
Thu Jun 23 23:41:43 CEST 2005

Steve Juranich wrote:
> I have in some code an 'eval', which I hate, but it's the shortest
> path to where I need to get at this point.

What's this code trying to do?  If you care about malicious code at all, 
you'll avoid 'eval' completely.  A couple reasons why:

With only a little trouble, I can get to the file object and write stuff 
to your machine:

py> eval("().__class__.mro()[1].__subclasses__()[16]")
<type 'file'>

Sure, you can avoid this by supplying your own __builtins__ to disable 
the file constructor:

py> eval("().__class__.mro()[1].__subclasses__()[16]('temp.txt')", 
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
   File "<string>", line 0, in ?
IOError: file() constructor not accessible in restricted mode

But even without the file constructor, I can still access pretty much 
any attribute of any class object by looking at object.__subclasses__():

py> class C(object):
...     def __init__(self):
...         self.f = file('temp.txt', 'w')
py> eval("().__class__.mro()[1].__subclasses__()[-1]().f.write('junk')", 
py> file('temp.txt').read()

Moral of the story: don't use eval if you care about security!


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