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title: The Ruby on Rails _json Juggling Attack
description: Rails wraps a non-hash JSON body into params under a reserved key but never blocks a request that supplies that key itself, so one body can carry both the single-item hash form and the multi-item array form at once. Where authorisation code and action code read different forms of the same request, the mismatch gives an authorisation bypass.
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# The Ruby on Rails _json Juggling Attack

**The Ruby on Rails _json Juggling Attack** - Luke Jahnke, nastystereo.com.

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- Original: <https://nastystereo.com/security/rails-_json-juggling-attack.html>
- Preserved from: https://nastystereo.com/security/rails-_json-juggling-attack.html (stored) on 2026-08-14
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## Content

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The Ruby on Rails _json Juggling Attack

Luke Jahnke9 December 2024

Ruby on Rails is a web framework which mostly provides access to user-provided data via the `params` object. The object is an instance of [ActionController::Parameters](https://api.rubyonrails.org/v7.1.3.3/classes/ActionController/Parameters.html), which can be used similar to a [Hash](https://docs.ruby-lang.org/en/3.3/Hash.html) - a collection of key-value pairs in Ruby. The contents include values from the request body, the query string, and for routes such as `get "/users/:user_id"`, the path.

### # Introducing the _json juggling attack

The _json juggling attack is an in-band signaling attack targeting JSON parsing within Ruby on Rails. The vulnerability exists in code that handles the conflict between the following:

- Rails parses JSON request bodies into `params`, which is always a Hash-like object.
- Rails accepts JSON request bodies that are arrays, strings, numbers, etc which are not Hash-like.

The code responsible for JSON parsing and resolving this conflict can be found inside [actionpack/lib/action_dispatch/http/parameters.rb](https://github.com/rails/rails/blob/1201cb1009e783fbeca62addecd12b272f830df3/actionpack/lib/action_dispatch/http/parameters.rb#L13-L16) and looks as follows:

```
DEFAULT_PARSERS = {
  Mime[:json].symbol => -> (raw_post) {
    data = ActiveSupport::JSON.decode(raw_post)
    data.is_a?(Hash) ? data : { _json: data }
  }
}

```

We see that to maintain the Hash-like nature of `params`, a hash is created with a key of `_json` that references our not-a-Hash object. The critical thing to notice about the above code is the not what is there, but what is missing. There is nothing preventing the supply of a JSON object with a `_json` key. Supplying a key of the string `"_json"` will be functionally equivalent to the Ruby symbol `:_json` (from the above code snippet), as parameters are [stored](https://github.com/rails/rails/blob/1201cb1009e783fbeca62addecd12b272f830df3/actionpack/lib/action_controller/metal/strong_parameters.rb#L294) as a [HashWithIndifferentAccess](https://api.rubyonrails.org/v7.1/classes/ActiveSupport/HashWithIndifferentAccess.html).

For a simple example of how this enables a vulnerability, consider a Rails application that has a delete item endpoint. The code handles both deleting a single item and deleting multiple items. The single item JSON is `{"id": 123}` and the multi-item JSON is `[456, 789]`. The _json juggling attack is where an attacker submits JSON structured to be acceptable in both cases at the same time:

```
{
  "id": 123,
  "_json": [456, 789]
}

```

An authorisation bypass vulnerability then results when an application's authorisation code is inconsistent with the action execution code. For example, the authorisation code preferencing the multi-item form and the action execution code preferencing the single-item form.

### # Rails Parameter Testing Setup

The following Dockerfile can be used to create a Rails application that sends the applications view of `params` back in the response.

```
FROM ruby:3.3

RUN gem install rails:7.1.3.3

RUN rails new param_dumper --minimal --api --skip-active-record

WORKDIR param_dumper/

RUN cat <<"EOF" > route_create_template.rb
  route "root 'dump#index', via: :all"
  route "match '/users/:user_id', to: 'dump#index', via: :all"
EOF

RUN ./bin/rails app:template LOCATION=route_create_template.rb

RUN cat <<"EOF" > app/controllers/dump_controller.rb
class DumpController < ApplicationController
  def index
    render plain: params.inspect
  end
end
EOF

ENTRYPOINT ["./bin/rails", "server", "--binding=0.0.0.0"]

```

It can be used as follows after placing the above in a file named `Dockerfile`:

```
$ docker build -t rails-param-dumper .

$ docker run --name rails -p 127.0.0.1:3000:3000 -d --rm rails-param-dumper

$ curl -d 'b=frombody' http://127.0.0.1:3000/users/frompath?q=fromquery
#<ActionController::Parameters {
  "b"=>"frombody",
  "q"=>"fromquery",
  "controller"=>"dump",
  "action"=>"index",
  "user_id"=>"frompath"
} permitted: false>

$ docker stop rails

```

If you wish to send a JSON request with `curl`, do not forget to set an appropriate `Content-Type` header. This can be achieved with a command such as `curl -H 'Content-Type: application/json' -d '{}' http://127.0.0.1:3000/`

### # Bonus questions about `params`

There are a number of other interesting security-related questions that can be asked:

Q: What is the precedence order of the three sources?
 A: Path is highest, then query string, followed by request body.

Q: What is the `permitted` attribute in `ActionController::Parameters`?
 A: This is used to protect against [https://brakemanscanner.org/docs/warning_types/mass_assignment/](https://nastystereo.com/security/mass assignment) [attacks](https://web.archive.org/web/20130818070952/https://github.com/rails/rails/issues/5239).

Q: Can you clobber the values of `controller` or `action` that are included in `params`?
 A: I tried and was unsuccessful :(

Q: Are there any query strings that cannot be parsed into `params`?
 A: Yes, examples can be found in Rack's [test code](https://github.com/rack/rack/blob/c52c1b6428b4841e7a634b80d73f28e1cffd8c12/test/spec_utils.rb#L230-L240), such as `x[y]=1&x[y]z=2` and `x[y]=1&x[y][][w]=2`. These can be useful to fingerprint the usage of Rack. It used to be possible to use `a&a[]`, as covered in [blog post](https://web.archive.org/web/20121031231234/http://bjeanes.com/2010/10/rails-bug-in-versions-1-2-and-3) by bjeanes in 2010.