Yet another attempt at a safe eval() call

Grant Edwards invalid at invalid.invalid
Fri Jan 4 16:53:39 CET 2013

On 2013-01-04, Steven D'Aprano <steve+comp.lang.python at> wrote:
> On Thu, 03 Jan 2013 23:25:51 +0000, Grant Edwards 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?
> Pretty badly, sorry.

I suspected that was the case.

> See trivial *cough* exploit below.
>> 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.
> Here's one exploit. I make no promises that it is the simplest such one.
> # get access to __import__
> s = ("[x for x in (1).__class__.__base__.__subclasses__() "
>      "if x.__name__ == 'catch_warnings'][0]()._module"
>      ".__builtins__['__imp' + 'ort__']")
> # use it to get access to any module we like
> t = s + "('os')"
> # and then do bad things
> urscrewed = t + ".system('echo u r pwned!')"
> lessDangerousEval(urscrewed)
> At a minimum, I would recommend:
> * Do not allow any underscores in the expression being evaluated.
>   Unless you absolutely need to support them for names, they can only
>   lead to trouble.

I can disallow underscores in names.

> [...]

> * Since you're evaluating mathematical expressions, there's probably
>   no need to allow quotation marks either. They too can only lead to
>   trouble.
> * Likewise for dots, since this is *integer* maths.

OK, quotes and dots are out as well.

> * Set as short as possible limit on the length of the string as you
>   can bare; the shorter the limit, the shorter any exploit must be,
>   and it is harder to write a short exploit than a long exploit.
> * But frankly, you should avoid eval, and write your own mini-integer
>   arithmetic evaluator which avoids even the most remote possibility
>   of exploit.

That's obviously the "right" thing to do.  I suppose I should figure
out how to use the ast module.  

> So, here's my probably-not-safe-either "safe eval":
> def probably_not_safe_eval(expr):
>     if 'import' in expr.lower():
>         raise ParseError("'import' prohibited")
>     for c in '_"\'.':
>         if c in expr:
>             raise ParseError('prohibited char %r' % c)
>     if len(expr) > 120:
>         raise ParseError('expression too long')
>     globals = {'__builtins__': None}
>     locals  = symbolTable
>     return eval(expr, globals, locals)  # fingers crossed!
> I can't think of any way to break out of these restrictions, but that may 
> just mean I'm not smart enough.

Thanks!  It's definitely an improvement.

Grant Edwards               grant.b.edwards        Yow! -- I have seen the
                                  at               FUN --

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