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Date: Fri, 19 Oct 2012 01:29:35 +1100
Subject: Re: use of exec()
From: Chris Angelico <rosuav@gmail.com>
To: python-list@python.org
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On Fri, Oct 19, 2012 at 1:07 AM, lars van gemerden <lars@rational-it.com> wrote:
> Thanks, Chris,
>
> That works like a charm (after replacig "return ns.function" with "return ns['function']" ;-) ).

Err, yes, I forget sometimes that Python doesn't do that. JavaScript
and Pike both let you (though Pike uses -> instead of . for that
operator). Yes, Python has real methods on dictionary objects :)

> About the security, i noticed you can still import and use modules within the exec'ed code. Is there a way to prevent this or otherwise make this approach more secure.

Basically no, there's no real way to make it secure. Without
eliminating exec/eval, destroying insecurity is the hopeless work of a
wasted life, as the oracle said to Alice.

> I should say that the users that will be able to make custom functions, are not end-users, but authenticated designers, however i would like to close a backdoor to the whole framework.

You have to decide one thing: Will you permit them to execute
untrusted code on your system? If so, go ahead (and just warn them
that things like import shouldn't be done, as they can cause other
messes). I run a server that I build with the help of another guy (I
do the code, he does the bulk of the content - descriptions and
stuff), and I'm happy to trust him to not be malicious, so the purpose
of "embedded code in loci" is to make it easier to write tiny bits of
code, without any security requirement. But if you need security,
don't use eval. AT ALL.

There may be a brand new service coming along, though. The ast module
I think is getting a new evaluator that allows a little more
functionality than literal_eval, while still not permitting most
things. But you then have the question of performance, since you
effectively interpret the code at a high level.

ChrisA