Challenge: escape from the pysandbox

Victor Stinner victor.stinner at
Fri Feb 26 13:29:33 CET 2010


pysandbox is a new Python sandbox project under development. By default, 
untrusted code executed in the sandbox cannot modify the environment (write a 
file, use print or import a module). But you can configure the sandbox to 
choose exactly which features are allowed or not, eg. import sys module and 
read the file /etc/issue.

I think that the project reached the "testable" stage. I launch a new 
challenge: try to escape from the sandbox. I'm unable to write strict rules. 
The goal is to access objects outside the sandbox. Eg. write into a file, 
import a module which is not in the whitelist, modify an object outside the 
sandbox, etc.

To test the sandbox, you have 3 choices:
 - interactive interpreter executed in the sandbox, use:
    --verbose to display the whole sandbox configuration,
    --features=help to enable help() function,
    --features=regex to enable regex,
    --help to display the help.
 - <>: execute your script in the sandbox. 
   It has also --features option: use --features=stdout to be able 
   to use the print instruction :-)
 - use directly the Sandbox class: use methods call(), execute()
   or createCallback()

Don't use "with sandbox: ..." because there is known but with local frame 
variables. I think that I will later drop this syntax because of this bug. 
Except of debug_sandbox, I consider that all features are safe and so you can 
enable all features :-)

There is no prize, it's just for fun! But I will add the name of hackers 
founding the best exploits.

pysandbox is not ready for production, it's under heavy development. Anyway I 
*hope* that you will quickly find bugs!


Use to found some examples of how you can escape a sandbox. pysandbox 
is protected against all methods described in ;-)

See the README file to get more information about how pysandbox is implemented 
and get a list of other Python sandboxes.

pysandbox is currently specific to CPython, and it uses some ugly hacks to 
patch CPython in memory. In the worst case it will crash the pysandbox Python 
process, that's all. I tested it under Linux with Python 2.5 and 2.6. The 
portage to Python3 is not done yet (is someone motivated to write a 
patch? :-)).

Victor Stinner

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