Dailydave: Re: Google Apps Engine

["Aidan Thornton" <makosoft () googlemail com>]

On 4/11/08, Lutz Böhne <lboehne () damogran de> wrote:
> > Even those could easily be sanitized by just some fun with function
> > pointers.
> >
> >     >>> open=lambda *x: "no"
> >     >>> open('/etc/passwd')
> >     'no'
> Unless there are other ways to find these functions:
>
>     >>> __builtins__.__dict__["open"]( '/etc/passwd')
>     <open file '/etc/passwd', mode 'r' at 0xb7dac7b8>
>
> or even:
>
>     >>> open=lambda *x: "no"
>     >>> open('/etc/passwd')
>     'no'
>     >>> del open
>     >>> open('/etc/passwd')
>     <open file '/etc/passwd', mode 'r' at 0xb7db44a0>
>
> Python is fun, there are so many ways to have it do what you want ;)
>
> It might be possible to remove these functions like this:
>
>     >>> del __builtins__.__dict__["open"]
>     >>> open('/etc/passwd')
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     NameError: name 'open' is not defined
>     [...]
>
> But i don't know whether that'd get rid of all problems.
>
> Best regards,
>
> Lutz
Hi,

The quick answer is no, it wouldn't be enough. For example, try
type(sys.stdin)('/etc/passwd') or the equivalent
sys.stdin.__class__('/etc/passwd'). Also, as
http://mail.python.org/pipermail/python-dev/2006-July/067291.html
points out, file can be obtained from object.__subclasses__(). (object itself can be found by working up the 
inheritance tree from any new-style class - say, a string - using __bases__)

Python's powerful introspection support and lack of data hiding make
doing any sort of meaningful sandboxing within the language itself very difficult. There used to be a bundled module 
called rexec to do this (via a combination of hooks into the interpreter and built-in support), but it was depreciated 
due to security issues. They might be doing something similar - it seems to strip what functions from native-code 
modules can be imported to some safe whitelist (and load all modules written in Python within the sandbox).

Aidan
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