Denial of Service and Unsafe Object Creation Vulnerability in JSON Gem

https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/4_YvCpLzL58

by ontoillogical • 13 years ago

72 18 13 years ago

groups.google.com

Tho853• 13 years ago

Some details on how this can be exploited:

http://www.zweitag.de/en/blog/ruby-on-rails-vulnerable-to-ma...

tenderlove• 13 years ago

Thanks for reporting this issue to us! :-D

<3<3<3<3

Tho853• 13 years ago

Was a pleasure!

With love :-) Thomas

benmmurphy3• 13 years ago

nice find. there is sql injection in 2.3.x and it effects all adapters and not just postgresql.

i just tried quoted_id and it works against mysql on 3.2.x as well. quoted_id is defined in abstract/quoting.rb and any adapter that forwards quotes to the superclass will use it.

Tho853• 13 years ago

You're right! Thanks for the hint, I updated the blog post accordingly.

zyang• 13 years ago

Is it Monday again?

mapgrep• 13 years ago

>`JSON.load` should never be given input from unknown sources. If you are processing JSON from an unknown source, always use `JSON.parse`.

This seems like poor method naming; I would not intuitively understand that "load" is far more dangerous than "parse."

Why not deprecate these and do names like

JSON.load_trusted

JSON.load_untrusted

jdleesmiller• 13 years ago

A few further notes on this, following some reading... The docs aren't very clear about it, either (http://flori.github.com/json/doc/index.html). There is a JSON.parse! method that they explicitly say is to be used only for trusted input, and it looks like JSON.load has some of the same default options.

To further confuse things, JSON.load has the very useful property that you can pass it a String or an IO (e.g. from Rack::Request#body), whereas JSON.parse only accepts a String.

edit: the docs will soon contain a warning about JSON.load; see https://github.com/flori/json/blob/master/lib/json/common.rb

dbaupp• 13 years ago

Better would be load and load_trusted so that the safer function has the short (and expected) name.

MatthewPhillips• 13 years ago

Why have the unsafe function at all?

mikeash• 13 years ago

Better yet would be to remove the trusted functionality entirely and give a good talking-to to whoever wrote it. A generic JSON parser shouldn't have an unsafe parsing method on it period. If you want to implement clever nonsense like this, do it at the next layer up and call it something else.

lkrubner• 13 years ago

I apologize for the ignorant question, but how does Ruby survive this in normal operation?

"Since Ruby symbols are not garbage collected, this can result in a denial of service attack."

If you have a long running Ruby app,and it does not garbage collect symbols, then those symbols are... constants I guess?That survive till the app stops operating? So I guess the assumption is that no app should use too many symbols (and they don't use much memory anyway?)

Confusion• 13 years ago

A long running app will normally reach a steady state in which no new symbols are created anymore. If you for any reason create new symbols (which are basically interned strings) every n seconds or requests, you will eventually run out of memory. However, that is almost never a concern in practice, because you rarely need to do that.

oinksoft2• 13 years ago

This is not an ignorant question. Any half-experienced Erlang developer can tell you that you use `list_to_existing_atom' rather than `list_to_atom' if you are ever doing dynamic things with atoms. So, if you're trying to accomplish dynamic module lookup and foo_baz.beam is in your code path, default startup will create the `foo_baz' atom, and you know that this atom will exist at runtime.

Symbols in Ruby are atoms (the term "atom" spans languages), and GC/space issues plague any persistent term like an atom, in any language.

And thus I arrive at a key question: Does Ruby have something like `list_to_existing_atom', or some mechanism for telling if a symbol exists already? I see no analog to this, only the `ID2SYM' macro in the extensions API, and similar calls like String#to_sym.

Perhaps there is some way to clean up symbols after they are created. This to me would seem like the ideal route. It's good they've got a stop-gap fix by changing defaults, but it feels to me like they're punting here. Perhaps users who do [ab]use this feature also would not like DOS attacks?

I hope others who know more about Ruby extension development, and symbol management capabilities, can chime in on these questions.

benmmurphy3• 13 years ago

there are methods in the ruby reflection api that take strings and silently convert them to symbols so it is really easy to accidentally leak memory.

however, on ruby trunk they added a method: rb_check_id which can be used to check if a string has been already symbolized (https://github.com/ruby/ruby/blob/trunk/parse.y#L10465). this means when these reflection methods get passed a string and it hasn't been symbolized they can bail out and not symbolize the string. (https://github.com/ruby/ruby/blob/trunk/object.c#L2073)

sdevlin• 13 years ago

Symbols in Ruby are basically interned strings.

So if you reference the symbol :foo, it will transparently get pinned in memory somewhere. Referencing :foo again will not allocate an additional object - the interpreter will just give you the existing symbol.

This is usually not an issue, because an application typically operates on a small, fixed set of known symbols. But if an attacker can generate arbitrary symbols (e.g. if you call .to_sym on user input), he can exhaust memory.

benmmurphy3• 13 years ago

also if you have done require 'json/add/rails' you are in for fun (https://github.com/ruby/ruby/blob/v1_9_2_381/ext/json/lib/js...)

    irb(main):001:0> require 'json/add/rails'
    => true
    irb(main):002:0> class Foo
    irb(main):003:1> end
    => nil
    irb(main):004:0> Foo.json_create({"x" => "bar"})
    => #<Foo:0x007fc5f3149540>

https://github.com/search?q=require+%27json%2Fadd%2Frails%27...

Denial of Service and Unsafe Object Creation Vulnerability in JSON Gem

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