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OAuth 2.0 Authorization Server as a Rack module
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OAuth 2.0 Authorization Server as a Rack module. Because you don't allow strangers into your app, and OAuth 2.0 is the new awesome.

Step 1: Setup Your Database

The authorization server needs to keep track of clients, authorization requests, access grants and access tokens. That could only mean one thing: a database.

The current release uses MongoDB. You're going to need a running server and open connection in the form of a +Mongo::DB+ object. Because MongoDB is schema-less, there's no need to run migrations.

If MongoDB is not your flavor, you can easily change the models to support a different database engine. All the persistence logic is located in lib/rack/oauth2/models and kept simple by design. And if you did the work to support a different database engine, send us a pull request.

Step 2: Use The Server

For Rails 2.3.x, Rack::OAuth2::Server automatically adds itself as middleware when required, but you do need to configure it from within config/environment.rb (or one of the specific environment files). For example: do |config|
  config.oauth.database =["my_db"]
  config.oauth.scopes = %w{read write}
  config.oauth.authenticator = lambda do |username, password|
    user = User.find(username)
    user if user && user.authenticated?(password)

  . . .

For Sinatra and Padrino, first require rack/oauth2/sinatra and register Rack::OAuth2::Sinatra into your application. For example:

require "rack/oauth2/sinatra"

class MyApp < Sinatra::Base
  register Rack::OAuth2::Sinatra

  oauth.database =["my_db"]
  oauth.scopes = %w{read write}
  oauth.authenticator = lambda do |username, password|
    user = User.find(username)
    user if user && user.authenticated?(password)

  . . .

With any other Rack server, you can +use Rack::OAuth2::Server+ and pass your own {Rack::OAuth2::Server::Options} object.

The configuration options are:

  • :access_token_path – Path for requesting access token. By convention defaults to /oauth/access_token.

  • :authenticator – For username/password authorization. A block that receives the credentials and returns identity string (e.g. user ID) or nil.

  • :authorization_types – Array of supported authorization types. Defaults to [“code”, “token”], and you can change it to just one of these names.

  • :authorize_path – Path for requesting end-user authorization. By convention defaults to /oauth/authorize.

  • :database – +Mongo::DB+ instance.

  • :host – Only check requests sent to this host.

  • :path – Only check requests for resources under this path.

  • :param_authentication – If true, supports authentication using query/form parameters.

  • :realm – Authorization realm that will show up in 401 responses. Defaults to use the request host name.

  • :scopes – Array listing all supported scopes, e.g. [“read”, “write”].

  • :logger – The logger to use. Under Rails, defaults to use the Rails logger. Will use +Rack::Logger+ if available.

If you only intend to use the UI authorization flow, you don't need to worry about the authenticator. If you want to allow clients applications to create access tokens by passing the end-user's username/password, then you need an authenticator. This feature is necessary for some client applications, and quite handy during development/testing.

Step 3: Let Users Authorize

Authorization requests go to /oauth/authorize. Rack::OAuth2::Server intercepts these requests and validates the client ID, secret, redirect URI, authorization type and scope. If the request fails validation, the user is redirected back to the client application with a suitable error code.

If the request passes validation, Rack::OAuth2::Server sets the request header oauth.authorization to the authorization handle, and passes control to your application. Your application will ask the user to grant or deny the authorization request.

Once granted, your application signals the grant by setting the response header oauth.authorization to the authorization handle it got before, and setting the response header oauth.identity to the authorized identity. This is typicaly the user ID or account ID, but can be anything you want, as long as it's a string. Rack::OAuth2::Server intercepts this response and redirects the user back to the client application with an authorization code or access token.

To signal that the user denied the authorization requests your application sets the response header oauth.authorization as before, and returns the status code 401 (Unauthorized). Rack::OAuth2::Server will then redirect the user back to the client application with a suitable error code.

In Rails, the entire flow would look something like this:

class OauthController < ApplicationController
  def authorize
    if current_user
      render :action=>"authorize"
      redirect_to :action=>"login", :authorization=>oauth.authorization

  def grant
    head oauth.grant!(params[:authorization],

  def deny
    head oauth.deny!(params[:authorization])

Rails actions must render something. The oauth method returns a helper object ({Rack::OAuth2::Server::Helper}) that cannot render anything, but can set the right response headers and return a status code, which we then pass on to the head method.

In Sinatra/Padrino, it would look something like this:

get "/oauth/authorize" do
  if current_user
    render "oauth/authorize"
    redirect "/oauth/login?authorization=#{oauth.authorization}"

post "/oauth/grant" do
  oauth.grant! params[:authorization], "Superman"

post "/oauth/deny" do
  oauth.deny! params[:authorization]

The view would look something like this:

<h2>The application <% link_to h(oauth.client.display_name), %>
  is requesting to <%= oauth.scope.to_sentence %> your account.</h2>
<form action="/oauth/grant">
  <input type="hidden" name="authorization" value="<%= oauth.authorization %>">
<form action="/oauth/deny">
  <input type="hidden" name="authorization" value="<%= oauth.authorization %>">

Step 4: Protect Your Path

Rack::OAuth2::Server intercepts all incoming requests and looks for an Authorization header that uses OAuth authentication scheme, like so:

Authorization: OAuth e57807eb99f8c29f60a27a75a80fec6e

It can also support the oauth_token query parameter or form field, if you set param_authentication to true. This option is off by default to prevent conflict with OAuth 1.0 callback.

If Rack::OAuth2::Server finder a valid access token in the request, it sets the request header oauth.identity to the value you supplied during authorization (step 3). You can use oauth.identity to resolve the access token back to user, account or whatever you put there.

If the access token is invalid or revoked, it returns 401 (Unauthorized) to the client. However, if there's no access token, the request goes through. You might want to protect some URLs but not others, or allow authenticated and unauthenticated access, the former returning more data or having higher rate limit, etc.

It is up to you to reject requests that must be authenticated but are not. You can always just return status code 401, but it's better to include a proper WWW-Authenticate header, which you can do by setting the response header oauth.no_access to true, or using oauth_required to setup a filter.

You may also want to reject requests that don't have the proper scope. You can return status code 403, but again it's better to include a proper WWW-Authenticate header with the required scope. You can do that by setting the response header oauth.no_scope to the scope name, or using oauth_required with the scope option.

In Rails, it would look something like this:

class MyController < ApplicationController

  before_filter :set_current_user
  oauth_required :only=>:private
  oauth_required :only=>:calc, :scope=>"math"

  # Authenticated/un-authenticated get different responses.
  def public
    if oauth.authenticated?
      render :action=>"more-details"
      render :action=>"less-details"

  # Must authenticate to retrieve this.
  def private

  # Must authenticate with scope math to do this.
  def calc
    render :text=>"2+2=4"


  def set_current_user
    @current_user = User.find(oauth.identity) if oauth.authenticated?


In Sinatra/Padrino, it would look something like this:

before do
  @current_user = User.find(oauth.identity) if oauth.authenticated?

oauth_required "/private"
oauth_required "/calc", :scope=>"math"

# Authenticated/un-authenticated get different responses.
get "/public" do
  if oauth.authenticated?
    render "more-details"
    render "less-details"

# Must authenticate to retrieve this.
get "/private" do
  render "secrets"

# Must authenticate with scope math to do this.
get "/calc" do
  render "2 + 2 = 4"

Step 5: Register Some Clients

Before a client application can request access, there must be a client record in the database. Registration provides the client application with a client ID and secret. The client uses these to authenticate itself.

The client provides its display name, site URL and image URL. These should be shown to the end-user to let them know which client application they're granting access to.

Clients can also register a redirect URL. This is optional but highly recommended for better security, preventing other applications from hijackin the client's ID/secret.

You can register clients using the command line tool oauth2-server:

$ oauth2-server register --db my_db

Or you can register clients using the Web-based OAuth console, see below.

Programatically, registering a new client is as simple as:

$ ./script/console
Loading development environment (Rails 2.3.8)
> uber_client = Rack::OAuth2::Server::Client.create(:display_name=>"UberClient",
> puts "Your client identifier: #{}"
> puts "Your client secret: #{client.secret}"

Step 6: Pimp Your API

I'll let you figure that one for yourself.

Using With Curl

The premise of OAuth 2.0 is that you can use it straight from the command line. Let's start by creating an access token. Aside from the UI authorization flow, OAuth 2.0 allows you to authenticate with username/password. You'll need to register an authenticator, see step 2 above for details.

Now make a request using the client credentials and your account username/password, e.g.:

$ curl -i http://localhost:3000/oauth/access_token \
  -F grant_type=password \
  -F client_id=4dca20453e4859cb000007 \
  -F client_secret=981fa734e110496fcf667cbf52fbaf03 \
  -F "scope=read write" \
  -F \
  -F password=not.telling

This will spit out a JSON document, something like this:

{"scope":"import discover contacts lists",

Grab the access_token value and use it. The access token is good until you delete it from the database. Making a request using the access token:

$ curl -i http://localhost:3000/api/read -H "Authorization: OAuth e57807eb99f8c29f60a27a75a80fec6e"

Although not recommended, you can also pass the token as a query parameter, or when making POST request, as a form field:

$ curl -i http://localhost:3000/api/read?oauth_token=e57807eb99f8c29f60a27a75a80fec6e
$ curl -i http://localhost:3000/api/update -F name=Superman -F oauth_token=e57807eb99f8c29f60a27a75a80fec6e

You'll need to set the option param_authentication to true. Watch out, since this query parameter could conflict with OAuth 1.0 authorization responses that also use oauth_token for a different purpose.

Here's a neat trick. You can create a .curlrc file and load it using the -K option:

$ cat .curlrc
header = "Authorization: OAuth e57807eb99f8c29f60a27a75a80fec6e"
$ curl -i http://localhost:3000/api/read -K .curlrc

If you create .curlrc in your home directory, curl will automatically load it. Convenient, but dangerous, you might end up sending the access token to any server you curl. Useful for development, testing, just don't use it with any production access tokens.

OAuth Web Console

We haz it, and it's pretty rad:

To get the console running, you'll need to do the following. First, you'll need to register a new client application that can access the OAuth console, with a redirect_uri that points to where you plan the Web console to live. This URL must end with “/admin”, for example, “”.

The easiest way to do this is to run the oauth2-sever command line tool:

$ oauth2-server setup --db my_db

Next, in your application, make sure to ONLY AUTHORIZE ADMINISTRATORS to access the console, by granting them access to the oauth-admin scope. For example:

def authorize
  # Only admins allowed to authorize the scope oauth-admin
  if oauth.scope.include?("oauth-admin") && !current_user.admin?
    oauth.deny! oauth.authorization

Make sure you do that, or you'll allow anyone access to the OAuth Web console.

Next, mount the OAuth Web console as part of your application, and feed it the client ID/secret. For example, for Rails add this to config/environment.rb: do |config|
  . . .
  config.middleware.use Rack::OAuth2::Server::Admin.mount
  Rack::OAuth2::Server::Admin.set :client_id, "4dca20453e4859cb000007"
  Rack::OAuth2::Server::Admin.set :client_secret, "981fa734e110496fcf667cbf52fbaf03"

For Sinatra, Padrino and other Rack-based applications, you'll want to mount like so (e.g. in do
  map("/oauth/admin") { run Rack::OAuth2::Server::Admin }
  map("/") { run MyApp }
Rack::OAuth2::Server::Admin.set :client_id, "4dca20453e4859cb000007"
Rack::OAuth2::Server::Admin.set :client_secret, "981fa734e110496fcf667cbf52fbaf03"

Next, open your browser to, or wherever you mounted the console.

The OAuth Web console is a single-page client application that operates by accessing the OAuth API. The API is mounted at /oauth/admin/api (basically /api relative to the console), you can access it yourself if you have an access token with the scope oauth-admin.

To get the access token, the console first redirects you to the path /oauth/authorize, which it expects to be the OAuth 2.0 authorization endpoint. If you need to use a different endpoint, simply set the :authorize option to that URL.

Mandatory ASCII Diagram

This is briefly what the authorization flow looks like, how the workload is split between Rack::OAuth2::Server and your application, and the protocol the two use to control the authorization flow:

              -----------------------    -----------------------
Client app    | /oauth/authorize    |    | Set request.env     |
redirect   -> |                     | -> |                     | ->
              | authenticate client |    | oauth.authorization |
              -----------------------    -----------------------

                                  Your code 
   --------------------     ----------------------    -----------------------
   | Authenticate user |    | Ask user to grant/ |    | Set response        |
-> |                   | -> | deny client access | -> |                     | ->
   |                   |    | to their account   |    | oauth.authorization |
   |                   |    |                    |    | oauth.identity      |
   --------------------     ----------------------    -----------------------

   | Create access grant |    
-> | or access token for | -> Redirect back
   | oauth.identity      |    to client app

Understanding the Models


The {Rack::OAuth2::Server::Client} model represents the credentials of a client application. There are two pairs: the client identifier and secret, which the client uses to identify itself to the authorization server, and the display name and URL, which the client uses to identify itself to the end user.

The client application is not tied to a single Client record. Specifically, if the client credentials are compromised, you'll want to revoke it and create a new Client with new pair of identifier/secret. You can leave the revoked instance around.

Calling revoke! on the client revokes access using these credential pair, and also revokes any outstanding authorization requests, access grants and access tokens created using these credentials.

You may also want to register a redirect URI. If registered, the client is only able to request authorization that redirect back to that redirect URI.

Authorization Request

The authorization process may involve multiple requests, and the application must maintain the authorization request details from beginning to end.

To keep the application simple, all the necessary information for a single authorization request is stored in the {Rack::OAuth2::Server::AuthRequest} model. The application only needs to keep track of the authorization request identifier.

Granting an authorization request (by calling grant!) creates an access grant or access token, depending on the requested response type, and associates it with the identity.

Access Grant

An access grant ({Rack::OAuth2::Server::AccessGrant}) is a nonce use to generate access token. This model keeps track of the nonce (the “authorization code”) and all the data it needs to create an access token.

Access Token

An access token allows the client to access the resource with the given scope on behalf of a given identity. It keeps track of the account identifier (supplied by the application), client identifier and scope (both supplied by the client).

An {Rack::OAuth2::Server::AccessToken} is created by copying values from an AuthRequest or AccessGrant, and remains in effect until revoked. (OAuth 2.0 access tokens can also expire, but we don't support expiration at the moment)


Rack::OAuth2::Server was written to provide authorization/authentication for the new Flowtown API. Thanks to Flowtown for making it happen and allowing it to be open sourced.

Rack::OAuth2::Server is available under the MIT license.

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