How can I sandbox Python in pure Python?

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I’m developing a web game in pure Python, and want some simple scripting available to allow for more dynamic game content. Game content can be added live by privileged users.

It would be nice if the scripting language could be Python. However, it can’t run with access to the environment the game runs on since a malicious user could wreak havoc which would be bad. Is it possible to run sandboxed Python in pure Python?

Update: In fact, since true Python support would be way overkill, a simple scripting language with Pythonic syntax would be perfect.

If there aren’t any Pythonic script interpreters, are there any other open source script interpreters written in pure Python that I could use? The requirements are support for variables, basic conditionals and function calls (not definitions).

This is really non-trivial.

There are two ways to sandbox Python. One is to create a restricted environment (i.e., very few globals etc.) and exec your code inside this environment. This is what Messa is suggesting. It’s nice but there are lots of ways to break out of the sandbox and create trouble. There was a thread about this on Python-dev a year ago or so in which people did things from catching exceptions and poking at internal state to break out to byte code manipulation. This is the way to go if you want a complete language.

The other way is to parse the code and then use the ast module to kick out constructs you don’t want (e.g. import statements, function calls etc.) and then to compile the rest. This is the way to go if you want to use Python as a config language etc.

Another way (which might not work for you since you’re using GAE), is the PyPy sandbox. While I haven’t used it myself, word on the intertubes is that it’s the only real sandboxed Python out there.

Based on your description of the requirements (The requirements are support for variables, basic conditionals and function calls (not definitions)) , you might want to evaluate approach 2 and kick out everything else from the code. It’s a little tricky but doable.

AFAIK it is possible to run a code in a completely isolated environment:

exec somePythonCode in {'__builtins__': {}}, {}

But in such environment you can do almost nothing 🙂 (you can not even import a module; but still a malicious user can run an infinite recursion or cause running out of memory.) Probably you would want to add some modules that will be the interface to you game engine.

Roughly ten years after the original question, Python 3.8.0 comes with auditing. Can it help? Let’s limit the discussion to hard-drive writing for simplicity – and see:

from sys import addaudithook
def block_mischief(event,arg):
    if 'WRITE_LOCK' in globals() and ((event=='open' and arg[1]!='r') 
            or event.split('.')[0] in ['subprocess', 'os', 'shutil', 'winreg']): raise IOError('file write forbidden')

addaudithook(block_mischief)

So far exec could easily write to disk:

exec("open('/tmp/FILE','w').write('pwned by l33t h4xx0rz')", dict(locals()))

But we can forbid it at will, so that no wicked user can access the disk from the code supplied to exec(). Pythonic modules like numpy or pickle eventually use the Python’s file access, so they are banned from disk write, too. External program calls have been explicitly disabled, too.

WRITE_LOCK = True
exec("open('/tmp/FILE','w').write('pwned by l33t h4xx0rz')", dict(locals()))
exec("open('/tmp/FILE','a').write('pwned by l33t h4xx0rz')", dict(locals()))
exec("numpy.savetxt('/tmp/FILE', numpy.eye(3))", dict(locals()))
exec("import subprocess; subprocess.call('echo PWNED >> /tmp/FILE', shell=True)",     dict(locals()))

An attempt of removing the lock from within exec() seems to be futile, since the auditing hook uses a different copy of locals that is not accessible for the code ran by exec. Please prove me wrong.

exec("print('muhehehe'); del WRITE_LOCK; open('/tmp/FILE','w')", dict(locals()))
...
OSError: file write forbidden

Of course, the top-level code can enable file I/O again.

del WRITE_LOCK
exec("open('/tmp/FILE','w')", dict(locals()))

Sandboxing within Cpython has proven extremely hard and many previous attempts have failed. This approach is also not entirely secure e.g. for public web access:

  1. perhaps hypothetical compiled modules that use direct OS calls cannot be audited by Cpython – whitelisting the safe pure pythonic modules is recommended.

  2. Definitely there is still the possibility of crashing or overloading the Cpython interpreter.

  3. Maybe there remain even some loopholes to write the files on the harddrive, too. But I could not use any of the usual sandbox-evasion tricks to write a single byte. We can say the “attack surface” of Python ecosystem reduces to rather a narrow list of events to be (dis)allowed: https://docs.python.org/3/library/audit_events.html

I would be thankful to anybody pointing me to the flaws of this approach.


EDIT: So this is not safe either! I am very thankful to @Emu for his clever hack using exception catching and introspection:

#!/usr/bin/python3.8
from sys import addaudithook
def block_mischief(event,arg):
    if 'WRITE_LOCK' in globals() and ((event=='open' and arg[1]!='r') or event.split('.')[0] in ['subprocess', 'os', 'shutil', 'winreg']):
        raise IOError('file write forbidden')

addaudithook(block_mischief)
WRITE_LOCK = True
exec("""
import sys
def r(a, b):
    try:
        raise Exception()
    except:
        del sys.exc_info()[2].tb_frame.f_back.f_globals['WRITE_LOCK']
import sys
w = type('evil',(object,),{'__ne__':r})()
sys.audit('open', None, w)
open('/tmp/FILE','w').write('pwned by l33t h4xx0rz')""", dict(locals()))

I guess that auditing+subprocessing is the way to go, but do not use it on production machines:

https://bitbucket.org/fdominec/experimental_sandbox_in_cpython38/src/master/sandbox_experiment.py

I’m not sure why nobody mentions this, but Zope 2 has a thing called Python Script, which is exactly that – restricted Python executed in a sandbox, without any access to filesystem, with access to other Zope objects controlled by Zope security machinery, with imports limited to a safe subset.

Zope in general is pretty safe, so I would imagine there are no known or obvious ways to break out of the sandbox.

I’m not sure how exactly Python Scripts are implemented, but the feature was around since like year 2000.

And here’s the magic behind PythonScripts, with detailed documentation: http://pypi.python.org/pypi/RestrictedPython – it even looks like it doesn’t have any dependencies on Zope, so can be used standalone.

Note that this is not for safely running arbitrary python code (most of the random scripts will fail on first import or file access), but rather for using Python for limited scripting within a Python application.

This answer is from my comment to a question closed as a duplicate of this one: Python from Python: restricting functionality?

I would look into a two server approach. The first server is the privileged web server where your code lives. The second server is a very tightly controlled server that only provides a web service or RPC service and runs the untrusted code. You provide your content creator with your custom interface. For example you if you allowed the end user to create items, you would have a look up that called the server with the code to execute and the set of parameters.

Here’s and abstract example for a healing potion.

{function_id='healing potion', action='use', target="self", inventory_id='1234'}

The response might be something like

{hp='+5' action={destroy_inventory_item, inventory_id='1234'}}

You’ll probably be interested in the Python language services section of the libref for writing your own parser.

Hmm. This is a thought experiment, I don’t know of it being done:

You could use the compiler package to parse the script. You can then walk this tree, prefixing all identifiers – variables, method names e.t.c. (also has|get|setattr invocations and so on) – with a unique preamble so that they cannot possibly refer to your variables. You could also ensure that the compiler package itself was not invoked, and perhaps other blacklisted things such as opening files. You then emit the python code for this, and compiler.compile it.

The docs note that the compiler package is not in Python 3.0, but does not mention what the 3.0 alternative is.

In general, this is parallel to how forum software and such try to whitelist ‘safe’ Javascript or HTML e.t.c. And they historically have a bad record of stomping all the escapes. But you might have more luck with Python 🙂

I think your best bet is going to be a combination of the replies thus far.

You’ll want to parse and sanitise the input – removing any import statements for example.

You can then use Messa’s exec sample (or something similar) to allow the code execution against only the builtin variables of your choosing – most likely some sort of API defined by yourself that provides the programmer access to the functionality you deem relevant.

You’ll find some ideas in this wiki page, but it does not look like it can be done easily.


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