Why the Latest Rails Exploit Is Indicative of a Bigger Problem
blog.sdelements.com
blog.sdelements.com
-- by Rohit Sethi on February 13, 2013 -- http://blog.sdelements.com/author/rohit/
The latest Rails security flaw [1] is example of a common anti-pattern. Ned Batchelder wrote an awesome post [2] explaining how a similar issue may also exist in Python’s YAML parser [3]. Looking at these vulnerabilities, I am reminded of similar flaws in other frameworks and libraries.
The issue in each case is an abuse of extensibility. At first glance the idea is clever: allow for run-time execution of new code or binding of server-side variables without changing your compiled code, thereby greatly enhancing extensibility. For example, provide extensions to your Python YAML parser that allow you to create arbitrary objects and execute Python code; provide extensions to XML Template parsing that allows for arbitrary command execution [4]; or dynamically assign user-supplied parameters to server-side variables [5] (aka mass assignment) based on the parameter name. This kind of vulnerability is by design in contrast to many other by accident vulnerabilities. We called the mass assignment anti-pattern out [6] several years ago when doing a security analysis of the Core Java EE Patterns for OWASP.
I have a strong feeling we’ll see more vulnerabilities of this type, particularly with the rising popularity of standards like SAML that are built upon several layers of libraries implementing and extending complex specifications. These kind of issues can sometimes be hard to catch with an automated scanning technology, which means most organizations adopting the status quo of application security due diligence [7] will undoubtedly miss detecting some instances of extensibility abuse.
Security-minded developers can protect themselves by taking the following steps:
Turn off unnecessary extensibility in third party libraries and frameworks Do not use untrusted input in libraries that provide broad extensibility, such as Apache’s Xalan [8] with extensions enabled.
Be vigilant about monitoring for and patching newly discovered vulnerabilities in frameworks and third party libraries. Wherever possible, sign up for security mailing lists or groups like Ruby on Rails Security [9]
[1] http://www.zweitag.de/en/blog/ruby-on-rails-vulnerable-to-ma... [2] http://nedbatchelder.com/blog/201302/war_is_peace.html [3] http://pypi.python.org/pypi/PyYAML [4] http://labs.securitycompass.com/tutorials/xslt-command-execu... [5] http://www.codeproject.com/Articles/471784/Exploiting-Micros... [6] https://www.owasp.org/index.php/Category:OWASP_Security_Anal... [7] http://blog.sdelements.com/raising-the-bar-on-application-se... [8] http://xml.apache.org/xalan-j/extensions.html [9] https://groups.google.com/forum/?hl=en&fromgroups#!forum...
Hypothesis: developers don't like code generation. Partly because build systems are unreasonably difficult to tweak, partly because debugging tools are too crude to handle code generation graciously.
Maybe I'm just too jaded...
Many of these systems are written with the use case in the developer's mind where the data can be trusted to some degree, because perhaps they assume it's going to be read only by the program that generated it and only viewed by the user who already owns the computer, etc. In some sense the error only occurs when one takes this sort of software and then exposes it to the network, but it can be hard to realize when you... or somebody buried three layers deep in $YOUR_FAVORITE_FRAMEWORK... has done that.
Although both are convenience niceties, the latter is based on user input while the former is not.
The current state of having a half dozen of each of these is complete chaos. Presumably nobody thinks they're accountable because everyone has the option of using another package instead if they're not happy, basically passing the hot-potato constantly.
Is there a non-Ruby project that has a good implementation of these worth studying?
A ton of packages have default choices that are inherently bad/insecure (mail servers listening on all interfaces by default, SSH servers accepting root login by default, and so on). Packaging is just as important as development.
Security problems often happen when two components that are perfectly safe in their original contexts are tied together in a way that their designers didn't intend. These parser problems are examples of that.
So it's actually a good thing that frameworks like Rails try to tie together many components for you "magically". The integration of all those little simple pieces is always a chance to screw up security. It's better to do that hard work once, collaboratively as a community, than to have every developer make those risky choices separately every single time.
It's the same cost/benefit analysis that explains why you should never roll your own crypto. Anybody can make a system that's "secure" enough to keep themselves from breaking it, but that's not nearly good enough. You increase security by increasing scrutiny.
I remember being pleasantly surprised when I wrote my first website with a framework after a couple of years building sites from practically nothing.
Compared to frameworkless PHP, Rails is magic but then so is Django or ASP.NET MVC.
if zero-framework development is a hammer, then development with a framework is a power-tool combination drill/hammer/driver. the power-tool does so much more then the basic tool that it feels "magical". its not a meaningless term then. its a description of the power of the tool.
Rails is more oriented in a "convention over configuration" mindset, in this sense it might be more "magic" than others.
In the context of dynamic languages, "magic" does not mean someone casting spells. Of course it is just ruby code. It is magic ruby code. Code which is executing a whole bunch of stuff behind the scenes without the user of that code (the web developer) being aware.
Pointing out that doing really bad things like this is bad is no more transparently self-serving that you claiming "oh rails is totally fine and doing stupid shit is cool because its not really magic". Yes, it is a systemic fundamental flaw in the framework. The entire framework is built from the ground up on the idea that doing this kind of nonsense is good.
But isn't this the whole point of a framework? That stuff gets done for you so you don't have to write everything from the ground up?
Plus if you want to know exactly how everything is done, you can see it for yourself. As we all know Rails is open source and the code is perfectly readable for anyone that knows Ruby.
parse_yaml_from_get_params_with_a_really_long_rails_function_name()
> But isn't this the whole point of a framework? That stuff gets done for you so you don't have to write everything from the ground up?
There is a vast difference between a framework that does stuff for you when you explicitly ask it to, and where what happens is fairly transparent, and a framework that does stuff for you because of code that without knowledge of the framework looks fairly "inert".
E.g. calling methods to define attributes and attribute-accessors on a class that maps to a database is not magic (though depending on how extensive, you might argue parts of what a specific ORM does in that case is): The code makes it plain that something will happen, and unless the naming of the method is atrocious, a developer will have a good shot at guessing roughly what from the code itself. You can make a framework that does a lot for a developer while still being explicit like this.
Automatically defining attributes pulled from the database based just on including a module, or inheritance, on the other hand, is well into "magic" territory: Not only might it not be clear it's going to be pulling stuff from the database and define lots of attributes for you, you don't know what it will define at all without looking at a schema that might not even be available together with the code, and that might change independent of the code.
It's not an either-or, but a sliding scale, and what is magic to some will be plain and obvious to others (the Arthur C. Clarke quote springs to mind). E.g. "everyone" who knows Active Record will know at least the basics of what Active Record will do to your class, and so won't be surprised by most of the method that to another seemingly magically materialize on it.
The trade-off is how far you go before the surprises to your target audience become too many.
I beg to differ on the readability of the Rails source code. Even for someone with a strong command of Ruby, there are a lot of layers of abstraction, functions that call other functions that call other functions, and subclasses of subclasses of subclasses, spread across many different files in different places in the folder/project structure. It's quite difficult to figure out what is actually going on when you look under the hood of Rails, because it's complicated to the point where it's "indistinguishable from magic" until you have spend a lot of time with it.
All software deals in layers of abstraction, which makes it easier to reason about general function, but obscures specific function. All users of all software must make a constant trade-off between assuming abstractions are functioning properly and personally verifying their function, and all software must decide where the line between too little abstraction and too much obscurity should be drawn. When people talk about the "magic" in Rails, they mean that Rails has drawn that line quite far toward the abstraction side, which is true.
There are some abstractions that are a net positive for security, and some that hinder it. While I don't agree with the condescending attitude of your parent and the many similar commenters, who seem to think no software they've ever written or integrated with has ever had a security issue, I think it is fair at this point to say that the level of abstraction in Rails' handling of user data has been a hindrance to its security. I think that is probably also a fair thing to say about most pieces of software that have ever handled potentially-malicious user data.
I like your analogy to toys and children, except that the danger of these toys is not immediately apparent to adults either. But the toy company has been very prompt and transparent in finding ways for people to avoid the danger once it becomes apparent.
Can you give a concrete example of this?
So in ActionController::Metal (a superclass of ActionController::Base) you have:
def params
@_params ||= request.params
end
def params
@params ||= self.GET.merge(self.POST)
rescue EOFError
self.GET
end
A few points:
* It took me about 25 seconds to find this code.
* Making a method available in a subclass doesn't strike me as magic, its just inheritance.
* Aside from the aforementioned inheritance "magic", the rest of the things you described come from Rack, not Rails.
The condescending attitude is not necessary. I never claimed that it is somehow impossible or prohibitively difficult to determine how things in Rails work - it is very possible and I do it myself all the time. What I claimed is that Rails has chosen abstraction over obviousness, and gave an example of a place where I believe that is demonstrated.
People usually inherit from AC::Base, not AC::Metal, which includes a number of modules, any of which may change the behavior of `params`.
You didn't take your expose far enough. As I mentioned in my post, `params` is an already-parsed representation of user data. You traced the call chain a couple of steps, but did not get to the point where parameters are actually parsed.
That's fine, I'm not at all interested in the specifics of how parameter parsing works, and I'm certain that you or I could go track it down when necessary. My point is only that it involves a number of layers of abstraction to get there - cataloging those layers is not a very compelling counterpoint.
1. Provide the action with the raw request body as a string. It is then solely responsible for any and all parsing.
2. Provide the action with the raw request body as a string, but provide standard parsing classes that the action can use depending on what they are specifically doing.
Both of those solutions would make the parsing code less abstract than the Rails solution, with different trade-offs.
I believe this entire thread started because I claimed that Rails has chosen to be further toward the abstraction side of the scale. I can't tell if you really think I am wrong about that, or if you merely think that the choice Rails has made is the right one. If it's the latter, I agree with you, and never claimed otherwise. But it is a choice, and it has trade-offs.
Not even security related. If I decide to move from rails to django, now I have to wonder if somebody out there decided to talk to my app using json instead of regular get/post, and deal with migrating/breaking that client.
If you didn't publicly document the fact that you accept a particular format you have no obligation to continue supporting it.
Your point about the XML stands though, that sucked!
Please see my comment downthread.
ActionDispatch::Routing::RouteSet::NamedRouteCollection.add calls 'eval' on the property name passed to it. This invites exploitation. It also fails 'static_typed's desideratum of each component having a single responsibility. And I don't think calling it "magic" is too strong.
I don't know to what extent the Ruby culture encourages calling 'eval' in random API routines like this, but when I previously argued here on HN that that had to be a bad idea, I got some pushback. I would certainly call this a dangerous anti-pattern.
EDITED to add: reference: http://rubysource.com/anatomy-of-an-exploit-an-in-depth-look...
There are probably ways you can use it safely. But it's not worth the risk.