OmniAuth: Standardized Multi-Provider Authentication

An Introduction

OmniAuth is a library that standardizes multi-provider authentication for web applications. It was created to be powerful, flexible, and do as little as possible. Any developer can create strategies for OmniAuth that can authenticate users via disparate systems. OmniAuth strategies have been created for everything from Facebook to LDAP.

In order to use OmniAuth in your applications, you will need to leverage one or more strategies. These strategies are generally released individually as RubyGems, and you can see a community maintained list on the wiki for this project.

One strategy, called Developer, is included with OmniAuth and provides a completely insecure, non-production-usable strategy that directly prompts a user for authentication information and then passes it straight through. You can use it as a placeholder when you start development and easily swap in other strategies later.

Getting Started

Each OmniAuth strategy is a Rack Middleware. That means that you can use it the same way that you use any other Rack middleware. For example, to use the built-in Developer strategy in a Sinatra application you might do this:

require 'sinatra'
require 'omniauth'

class MyApplication < Sinatra::Base
  use Rack::Session::Cookie
  use OmniAuth::Strategies::Developer
end

Because OmniAuth is built for multi-provider authentication, you may want to leave room to run multiple strategies. For this, the built-in OmniAuth::Builder class gives you an easy way to specify multiple strategies. Note that there is no difference between the following code and using each strategy individually as middleware. This is an example that you might put into a Rails initializer at config/initializers/omniauth.rb:

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :developer unless Rails.env.production?
  provider :twitter, ENV['TWITTER_KEY'], ENV['TWITTER_SECRET']
end

You should look to the documentation for each provider you use for specific initialization requirements.

Integrating OmniAuth Into Your Application

OmniAuth is an extremely low-touch library. It is designed to be a black box that you can send your application's users into when you need authentication and then get information back. OmniAuth was intentionally built not to automatically associate with a User model or make assumptions about how many authentication methods you might want to use or what you might want to do with the data once a user has authenticated. This makes OmniAuth incredibly flexible. To use OmniAuth, you need only to redirect users to /auth/:provider, where :provider is the name of the strategy (for example, developer or twitter). From there, OmniAuth will take over and take the user through the necessary steps to authenticate them with the chosen strategy.

Once the user has authenticated, what do you do next? OmniAuth simply sets a special hash called the Authentication Hash on the Rack environment of a request to /auth/:provider/callback. This hash contains as much information about the user as OmniAuth was able to glean from the utilized strategy. You should set up an endpoint in your application that matches to the callback URL and then performs whatever steps are necessary for your application. For example, in a Rails app you would add a line in your routes.rb file like this:

post '/auth/:provider/callback', to: 'sessions#create'

And you might then have a SessionsController with code that looks something like this:

class SessionsController < ApplicationController
  # If you're using a strategy that POSTs during callback, you'll need to skip the authenticity token check for the callback action only. 
  skip_before_action :verify_authenticity_token, only: :create

  def create
    @user = User.find_or_create_from_auth_hash(auth_hash)
    self.current_user = @user
    redirect_to '/'
  end

  protected

  def auth_hash
    request.env['omniauth.auth']
  end
end

The omniauth.auth key in the environment hash provides an Authentication Hash which will contain information about the just authenticated user including a unique id, the strategy they just used for authentication, and personal details such as name and email address as available. For an in-depth description of what the authentication hash might contain, see the Auth Hash Schema wiki page.

Note that OmniAuth does not perform any actions beyond setting some environment information on the callback request. It is entirely up to you how you want to implement the particulars of your application's authentication flow.

Please note: there is currently a CSRF vulnerability which affects OmniAuth (designated CVE-2015-9284) that requires mitigation at the application level. More details on how to do this can be found on the Wiki.

Configuring The origin Param

The origin url parameter is typically used to inform where a user came from and where, should you choose to use it, they'd want to return to.

There are three possible options:

Default Flow:

# /auth/twitter/?origin=[URL]
# No change
# If blank, `omniauth.origin` is set to HTTP_REFERER

Renaming Origin Param:

# /auth/twitter/?return_to=[URL]
# If blank, `omniauth.origin` is set to HTTP_REFERER
provider :twitter, ENV['KEY'], ENV['SECRET'], origin_param: 'return_to'

Disabling Origin Param:

# /auth/twitter
# Origin handled externally, if need be. `omniauth.origin` is not set
provider :twitter, ENV['KEY'], ENV['SECRET'], origin_param: false

Integrating OmniAuth Into Your Rails API

The following middleware are (by default) included for session management in Rails applications. When using OmniAuth with a Rails API, you'll need to add one of these required middleware back in:

• ActionDispatch::Session::CacheStore
• ActionDispatch::Session::CookieStore
• ActionDispatch::Session::MemCacheStore

The trick to adding these back in is that, by default, they are passed session_options when added (including the session key), so you can't just add a session_store.rb initializer, add use ActionDispatch::Session::CookieStore and have sessions functioning as normal.

To be clear: sessions may work, but your session options will be ignored (i.e. the session key will default to _session_id). Instead of the initializer, you'll have to set the relevant options somewhere before your middleware is built (like application.rb) and pass them to your preferred middleware, like this:

application.rb:

config.session_store :cookie_store, key: '_interslice_session'
config.middleware.use ActionDispatch::Cookies # Required for all session management
config.middleware.use ActionDispatch::Session::CookieStore, config.session_options

(Thanks @mltsy)

Logging

OmniAuth supports a configurable logger. By default, OmniAuth will log to STDOUT but you can configure this using OmniAuth.config.logger:

# Rails application example
OmniAuth.config.logger = Rails.logger

Resources

The OmniAuth Wiki has actively maintained in-depth documentation for OmniAuth. It should be your first stop if you are wondering about a more in-depth look at OmniAuth, how it works, and how to use it.

Supported Ruby Versions

OmniAuth is tested under 2.1.10, 2.2.6, 2.3.3, 2.4.0, 2.5.0, and JRuby.

Versioning

This library aims to adhere to Semantic Versioning 2.0.0. Violations of this scheme should be reported as bugs. Specifically, if a minor or patch version is released that breaks backward compatibility, that version should be immediately yanked and/or a new version should be immediately released that restores compatibility. Breaking changes to the public API will only be introduced with new major versions. As a result of this policy, you can (and should) specify a dependency on this gem using the Pessimistic Version Constraint with two digits of precision. For example:

spec.add_dependency 'omniauth', '~> 1.0'

License

Copyright (c) 2010-2017 Michael Bleigh and Intridea, Inc. See LICENSE for details.