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Preventing Python "Sandbox" Escape?

3 pointshigh_byte1y ago13 comments

I'm using python's exec(code, globals, locals)

I disable __builtins__ so no imports, exec, eval, open, etc. inside that context

but it seems you can still always do object.__subclasses__() and find every system method (eg. open())

it can't be overwritten but looking at the interpreter code is seems like it's possible to hack a workaround for this specific case.

are there other known ways to escape exec()?

13 comments

10 comments · 5 top-level

eesmith1y ago· 2 in thread

Don't do it. Really, really don't do it. People have tried for decades to develop such a sandbox, and it does not work.

high_byteOP1y ago

as I mentioned on another reply, all those projects also wanted system apis like filesystem and sockets and such.

for me I just want to hijack the interpreter so I don't have to write my own. no imports, no sockets.

No, I'm not.

I'm talking about the history beyond why rexec and Bastion, and restricted execution, were removed from Python in the 2.x days. See https://python.readthedocs.io/en/v2.7.2/library/restricted.h... , "In Python 2.3 these modules have been disabled due to various known and not readily fixable security holes."

They started because back in the 1.x days there was a Python web browser called Grail, and the hope was to support restricted Python applets in Grail.

Or from 10 years ago, read https://lwn.net/Articles/574215/ about the failure of 'pysandbox' where one of the ways to break out was to "[use] a traceback object to unwind the stack frame to one in the trusted namespace, then use the f_globals attribute to retrieve a global object." ... "Stinner's tale should serve as a cautionary one to anyone considering a CPython-based solution".

You might consider RestrictedPython at https://restrictedpython.readthedocs.io/en/latest/ which supports only a subset of Python, via AST-walking to limit what the code can do. I have no experience with it.

billpg1y ago· 2 in thread

I'm interested in an answer. Is there a way, by design, to run code from an untrusted source in a restricted manner? So the worse the code could do is call me rude names.

Not staying in Python. Python's run-time is not built for sandboxing.

If you set up a new runtime environment, like a FreeBSD jail, with no access to anything and a short CPU limit, then start you could start a Python subprocess in that environment, where the only thing that gets out is data via a pipe to call you names.

An operating system like FreeBSD is built to run code in a restricted manner.

high_byteOP1y ago

I believe that's basically docker which uses linux seccomp, but there are also sandboxes for language specific applications.

ps. browsers basically do that with javascript

Terr_1y ago· 1 in thread

I would infer that it's insecure, since if it were that easy there wouldn't be various abandoned projects trying to sandbox Python.

It's the curse of any sufficiently useful language. Well, maybe not Lua, but that was specifically designed for embedding. Java also began with that intention back when applets were ahead of their time, though IIRC secure sandboxing is no longer really a feature.

high_byteOP1y ago

but all those projects also wanted system apis like filesystem and sockets and such.

for me I just want to hijack the interpreter so I don't have to write my own. no imports, no sockets.

Relevant:

https://discuss.python.org/t/extending-subinterpreters-with-...

https://stackoverflow.com/questions/3068139

https://wiki.python.org/moin/SandboxedPython

https://github.com/jailctf/pyjailbreaker

https://healeycodes.com/running-untrusted-python-code

https://lwn.net/Articles/574215/

PixelNomad_1231y ago

I agree with eesmith. DONT DO IT. I guess you got your answer restrictedPy

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