Python eval: is it still dangerous if I disable builtins and attribute access?

python eval python-internals
asmeurer picture asmeurer · Mar 4, 2016 · Viewed 7.5k times · Source

We all know that eval is dangerous, even if you hide dangerous functions, because you can use Python's introspection features to dig down into things and re-extract them. For example, even if you delete __builtins__, you can retrieve them with 

[c for c in ().__class__.__base__.__subclasses__()  
 if c.__name__ == 'catch_warnings'][0]()._module.__builtins__

However, every example I've seen of this uses attribute access. What if I disable all builtins, and disable attribute access (by tokenizing the input with a Python tokenizer and rejecting it if it has an attribute access token)?

And before you ask, no, for my use-case, I do not need either of these, so it isn't too crippling.

What I'm trying to do is make SymPy's sympify function more safe. Currently it tokenizes the input, does some transformations on it, and evals it in a namespace. But it's unsafe because it allows attribute access (even though it really doesn't need it).

Answer

vaultah picture vaultah · Mar 4, 2016

I'm going to mention one of the new features of Python 3.6 - f-strings.

They can evaluate expressions,

>>> eval('f"{().__class__.__base__}"', {'__builtins__': None}, {})
"<class 'object'>"

but the attribute access won't be detected by Python's tokenizer:

0,0-0,0:            ENCODING       'utf-8'        
1,0-1,1:            ERRORTOKEN     "'"            
1,1-1,27:           STRING         'f"{().__class__.__base__}"'
2,0-2,0:            ENDMARKER      ''

Related questions

How does str(list) work?

Why does str(list) returns how we see list on the console? How does str(list) work? (any reference to the CPython code for str(list))? >>> x = ['abc', 'def', 'ghi'] >>> str(x) "['abc', 'def', …

Read More
What does Python's eval() do?

In the book that I am reading on Python, it keeps using the code eval(input('blah')) I read the documentation, and I understand it, but I still do not see how it changes the input() function. What does it …

Read More
What's the difference between eval, exec, and compile?

I've been looking at dynamic evaluation of Python code, and come across the eval() and compile() functions, and the exec statement. Can someone please explain the difference between eval and exec, and how the different modes of compile() fit in?

Read More