The application performs zero validation of the path for which they will redirect users, following authentication. The URL parameter is used to determine where to redirect the user, if the url parameter is not present, the user will be redirect to their home page.

 
					def create
				      path = params[:url].present? ? params[:url] : home_dashboard_index_path    
				      begin
				        # Normalize the email address, why not
				        user = User.authenticate(params[:email].to_s.downcase, params[:password])
				       # @url = params[:url]
				      rescue Exception => e
				      end

				      if user
				        session[:user_id] = user.user_id if User.where(:user_id => user.user_id).exists?
				        redirect_to path
				      else
				        # Removed this code, just doesn't seem specific enough!
				        #  flash[:error] = "Either your username and password is incorrect" 
				        flash[:error] = e.message
				        render "new"
				      end         
				  end
				

Unvalidated Redirects and Forwards - ATTACK

Ensure you are logged out of the application. When requesting the login page, ensure you append a url=. Then, authenticate to the application. Once authenticated, you should be redirected to your test url.

Unvalidated Redirects and Forwards - SOLUTION

To fix this vulnerability, validate the path. In our case, we really only want to redirect users to our site so the TLD is not important. In this case, leveraging URI.parse() can be incredibly helpful. We can change the code to something like:

				  path = home_dashboard_index_path    
				  begin 
				   if params[:url].present?
				   	path = URI.parse(params[:url]).path
				   end
				  rescue 
				  end
			 

Further validation can occur with regular expression. If you must redirect to another application, remember to use URI.parse() and the host, path, and scheme (ssl or not) options FIRST, prior to performing regular expression validation. Additionally, always open and close your validation regexp using Ruby anchor tags \A and \z.

Read the description section, fairly big hint there.