How to rescue OmniAuth::Strategies::OAuth2::CallbackError?

You can set the on_failure proc in the omniauth initializer in an even cleaner fashion:

OmniAuth.config.on_failure = UsersController.action(:oauth_failure)

This happens because the authentication happens in a middleware so your controller is not involved in it. This is where the exception is raised and the called code is this

I think you can handle this kind of error by defining a callback in OmniAuth initializer with this kind of code

OmniAuth.config do |config|
  config.on_failure do
    # your handling code invoked in the context of a rack app
  end
end

Otherwise there is a commit of three months ago which introduce this behavior

def redirect_to_failure
  message_key = env['omniauth.error.type']
  new_path = "#{env['SCRIPT_NAME']}#{OmniAuth.config.path_prefix}/failure?message=#{message_key}"
  Rack::Response.new(["302 Moved"], 302, 'Location' => new_path).finish
end

which states that on errors your user is redirected to /auth/failure with an error message, so you should be able to define a route for that path and handle it in your app. Keep in mind that this won't happen in development mode so you need to try it in other envs. If this doesn't happen in production try to upgrade your omniauth gem to version 1.1.0

I have solved this problem with the Fabio's first suggestion.

OmniAuth.config.on_failure = Proc.new do |env|
  UsersController.action(:omniauth_failure).call(env)
  #this will invoke the omniauth_failure action in UsersController.
end

In my UsersController

class UsersController < ActionController::Base
  def omniauth_failure
    redirect_to init_sign_in_users_path
    #redirect wherever you want.
  end
end