UeberauthCognito

An Ueberauth Strategy for AWS Cognito.

Installation

Add :ueberauth and :ueberauth_cognito to your mix.exs:

defp deps do
  [
    # ...
    {:ueberauth, "~> 0.6"},
    {:ueberauth_cognito, "~> 0.1"}
  ]
end

Configure Ueberauth to use this strategy:

config :ueberauth, Ueberauth,
  providers: [
    cognito: {Ueberauth.Strategy.Cognito, []}
  ]

and configure the required values:

config :ueberauth, Ueberauth.Strategy.Cognito,
  auth_domain: {System, :get_env, ["COGNITO_DOMAIN"]},
  client_id: {System, :get_env, ["COGNITO_CLIENT_ID"]},
  client_secret: {System, :get_env, ["COGNITO_CLIENT_SECRET"]},
  user_pool_id: {System, :get_env, ["COGNITO_USER_POOL_ID"]},
  aws_region: {System, :get_env, ["COGNITO_AWS_REGION"]} # e.g. "us-east-1"

The values can be configured with an MFA, or simply a string.

Add the routes to the router:

scope "/auth", SignsUiWeb do
  pipe_through([:redirect_prod_http, :browser])
  get("/:provider", AuthController, :request)
  get("/:provider/callback", AuthController, :callback)
end

and create the corresponding controller:

defmodule SignsUiWeb.AuthController do
  use SignsUiWeb, :controller
  plug(Ueberauth)

  def callback(%{assigns: %{ueberauth_failure: _fails}} = conn, _params) do
    # what to do if sign in fails
  end

  def callback(%{assigns: %{ueberauth_auth: auth}} = conn, _params) do
    # sign the user in or something.
    # auth is a `%Ueberauth.Auth{}` struct, with Cognito token info
    send_resp(conn, 200, "Welcome, #{auth.uid}")
  end
end

Note that the entry in the router defines the authentication callback URL, and will need to be whitelisted in the AWS Cognito User Pools settings.

Refreshing access tokens

Cognito supports using refresh tokens to automatically obtain new access tokens for users whose access tokens expire. If your application has a refresh token handy, you can redirect to the callback URL with the refresh_token param set, and Ueberauth will attempt to use the given refresh token value to obtain a fresh access token.

The refresh token for an access token is issued at the same time as the access token, and is available in the Ueberauth.Auth.Credentials struct. Whether to use refresh tokens, and where to store them if used, is an implementation detail. Your first instinct might be to store them in a cookie, but they can sometimes exceed the maximum possible cookie size. An alternative is to store an identifier (which could be a username, or simply a random nonce) in a cookie, and create a GenServer that maps these identifiers to refresh tokens. This has the advantage of not revealing the refresh tokens to the end user, if this is a consideration.