Why should exec() and eval() be av

Question

I've seen this multiple times in multiple places, but never have found a satisfying explanation as to why this should be the case.

So, hopefully, one will be presented here. Why should we (at least, generally) not use exec() and eval()?

EDIT: I see that people are assuming that this question pertains to web servers – it doesn't. I can see why an unsanitized string being passed to exec could be bad. Is it bad in non-web-applications?

Answer 1

Same reason you shouldn't login as root: it's too easy to shoot yourself in the foot.

Answer 2

Try this in the interactive interpreter and see what happens:

>>> import sys
>>> eval('{"name" : %s}' % ("sys.exit(1)"))

Of course, this is a corner case, but it can be tricky to prevent things like this.

Answer 3

In contrast to what most answers are saying here, exec is actually part of the recipe for building super-complete decorators in Python, as you can duplicate everything about the decorated function exactly, producing the same signature for the purposes of documentation and such. It's key to the functionality of the widely used decorator module (http://pypi.python.org/pypi/decorator/). Other cases where exec/eval are essential is when constructing any kind of "interpreted Python" type of application, such as a Python-parsed template language (like Mako or Jinja).

So it's not like the presence of these functions are an immediate sign of an "insecure" application or library. Using them in the naive javascripty way to evaluate incoming JSON or something, yes that's very insecure. But as always, its all in the way you use it and these are very essential functions.

Answer 4

Allowing these function in a context where they might run user input is a security issue, and sanitizers that actually work are hard to write.

Answer 5

When you need exec and eval, yeah, you really do need them.

But, the majority of the in-the-wild usage of these functions (and the similar constructs in other scripting languages) is totally inappropriate and could be replaced with other simpler constructs that are faster, more secure and have fewer bugs.

You can, with proper escaping and filtering, use exec and eval safely. But the kind of coder who goes straight for exec/eval to solve a problem (because they don't understand the other facilities the language makes available) isn't the kind of coder that's going to be able to get that processing right; it's going to be someone who doesn't understand string processing and just blindly concatenates substrings, resulting in fragile insecure code.

It's the Lure Of Strings. Throwing string segments around looks easy and fools naïve coders into thinking they understand what they're doing. But experience shows the results are almost always wrong in some corner (or not-so-corner) case, often with potential security implications. This is why we say eval is evil. This is why we say regex-for-HTML is evil. This is why we push SQL parameterisation. Yes, you can get all these things right with manual string processing... but unless you already understand why we say those things, chances are you won't.