[Python-Dev] Some news from my sandbox project (original) (raw)

Victor Stinner victor.stinner at haypocalc.com
Sat Sep 18 11:58:55 CEST 2010

Le samedi 18 septembre 2010 10:24:49, Greg Ewing a écrit :

Victor Stinner wrote: > I'm still developing irregulary my sandbox project since last june. > > Today, the biggest problem is the creation of a read only view of the > builtins dictionary.

Why do you think you need to do this? What form of attack would a writable builtins expose you to that would be prevented by making it read-only?

Replace builtin functions make it possible to modify (indirectly) functions outside the sandbox. Eg. one of the most important function of pysandbox is proxy() (a function to create a read only view of a object outside the sandbox, especially on an import), if you replace isinstance() by a function which always return True: you can get unmodified objects (read+write access). So it's possible to get the real file type from sys.stdout and so open a new file.

The example on a modified version of pysandbox (to get the vulnerability):

sandbox>>> builtins['isinstance']=lambda obj, cls: True sandbox>>> import sys
sandbox>>> type(sys.stdout)('/etc/issue').read() 'Debian GNU/Linux squeeze/sid \n \l\n\n'

It would be possible to create a local copy of each builtin in all pysandbox functions, but it doesn't protect functions outside pysandbox.

-- Victor Stinner http://www.haypocalc.com/

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