The challenge is simple:

• Open a fresh Python interpreter and do:

  >>> from safelite import FileReader

• You can use FileReader to read files on your filesystem
• Now find a way to write to the filesystem from your interpreter

This has been discussed extensively for the last few days on python-dev. It’s funny how code seems to be pretty secure at first glance, then someone comes up with another loophole.

It especially piqued my interest since I am working on yet another searchable card database, this time using Google App Engine. Kind of like Gatherer, but for a different CCG than Magic, and (hopefully) more flexible. What does this have to do with security? Simple: the most flexible way to search cards is to store them as Python objects, then search them using a Python expression, e.g.

card.red and card.black and (card.creature or card.instant) and card.cost > 2

…or, a more convoluted query:

(card.red or card.black) and not card.multicolor \
and card.type == 'Dragon' and card.set.year > 2006

Now, executing an arbitary Python expression entered on a web page, is of course very unsafe. So I need to find ways to make it more secure, while still preserving the flexibility of a Python-based search. Although I’m not sure how much it matters in this particular case, because:

• According to the App Engine docs, an application cannot write to the system, so no overwriting of files (?).

• I’m not using the data store at all, and there is no user registration, so there’s no sensitive data to be accessed or manipulated.

• Projects like Try Python and Try Ruby seem to fare pretty well without imposing many restrictions on the user.

That said, there might be other ways to mess with the site. Personally I don’t care if a user manages to screw up their own session due to malicious hackery, as long as it doesn’t affect other users. :-)

Anyway, the site isn’t ready yet, I still need to add more cards and flesh out the API. If you want to try it (locally), drop me a note, and I’ll send you the code.