Merge pull request #1 from basecamp/ingresses

Accept inbound emails from a variety of ingresses

1 files changed, 58 insertions, 0 deletions

diff --git a/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb b/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb
new file mode 100644
index 0000000000..e878192603
--- /dev/null
+++ b/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb
@@ -0,0 +1,58 @@
+class ActionMailbox::Ingresses::Mailgun::InboundEmailsController < ActionMailbox::BaseController
+  before_action :authenticate
+
+  def create
+    ActionMailbox::InboundEmail.create_and_extract_message_id! params.require("body-mime")
+  end
+
+  private
+    def authenticate
+      head :unauthorized unless authenticated?
+    end
+
+    def authenticated?
+      if key.present?
+        Authenticator.new(
+          key:       key,
+          timestamp: params.require(:timestamp),
+          token:     params.require(:token),
+          signature: params.require(:signature)
+        ).authenticated?
+      else
+        raise ArgumentError, <<~MESSAGE.squish
+          Missing required Mailgun API key. Set action_mailbox.mailgun_api_key in your application's
+          encrypted credentials or provide the MAILGUN_INGRESS_API_KEY environment variable.
+        MESSAGE
+      end
+    end
+
+    def key
+      Rails.application.credentials.dig(:action_mailbox, :mailgun_api_key) || ENV["MAILGUN_INGRESS_API_KEY"]
+    end
+
+    class Authenticator
+      attr_reader :key, :timestamp, :token, :signature
+
+      def initialize(key:, timestamp:, token:, signature:)
+        @key, @timestamp, @token, @signature = key, Integer(timestamp), token, signature
+      end
+
+      def authenticated?
+        signed? && recent?
+      end
+
+      private
+        def signed?
+          ActiveSupport::SecurityUtils.secure_compare signature, expected_signature
+        end
+
+        # Allow for 2 minutes of drift between Mailgun time and local server time.
+        def recent?
+          Time.at(timestamp) >= 2.minutes.ago
+        end
+
+        def expected_signature
+          OpenSSL::HMAC.hexdigest OpenSSL::Digest::SHA256.new, key, "#{timestamp}#{token}"
+        end
+    end
+end