eval() == evil? --- How to use it safely?

[Guilherme Polo]

On Thu, Aug 28, 2008 at 6:51 PM, Fett <FettManChu at gmail.com> wrote:
> I am creating a program that requires some data that must be kept up
> to date. What I plan is to put this data up on a web-site then have
> the program periodically pull the data off the web-site.
>
> My problem is that when I pull the data (currently stored as a
> dictionary on the site) off the site, it is a string, I can use eval()
> to make that string into a dictionary, and everything is great.
> However, this means that I am using eval() on some string on a web-
> site, which seems pretty un-safe.
>
> I read that by using eval(code,{"__builtins__":None},{}) I can prevent
> them from using pretty much anything, and my nested dictionary of
> strings is still allowable. What I want to know is:
>
> What are the dangers of eval?
> - I originally was using exec() but switched to eval() because I
> didn't want some hacker to be able to delete/steal files off my
> clients computers. I assume this is not an issue with eval(), since
> eval wont execute commands.
> - What exactly can someone do by modifying my code string in a command
> like: thing = eval(code{"__builtins__":None},{}), anything other than
> assign their own values to the object thing?

By "disabling" __builtins__ you indeed cut some obvious tricks, but
someone still could send you a string like "10 ** 10 ** 10".

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-- 
-- Guilherme H. Polo Goncalves