Step 2: Validating webhooks #

Assume that your integration is an online marketplace, and that a customer just placed an order on your site. A few days after the customer initiated their payment, your application receives this webhook.

The topic field of an event holds a description of the event, which is similar the subject of an e-mail message. The webhook itself contains _links to the resource impacted by the event that can be used to retrieve more information about the webhook you have received.

Webhook payload example

{
  "id": "cac95329-9fa5-42f1-a4fc-c08af7b868fb",
  "resourceId": "cdb5f11f-62df-e611-80ee-0aa34a9b2388",
  "topic": "customer_transfer_created",
  "timestamp": "2017-01-20T22:45:12.790Z",
  "_links": {
    "self": {
      "href": "https://api-sandbox.dwolla.com/events/cac95329-9fa5-42f1-a4fc-c08af7b868fb"
    },
    "account": {
      "href": "https://api-sandbox.dwolla.com/accounts/ad5f2162-404a-4c4c-994e-6ab6c3a13254"
    },
    "resource": {
      "href": "https://api-sandbox.dwolla.com/transfers/cdb5f11f-62df-e611-80ee-0aa34a9b2388"
    },
    "customer": {
      "href": "https://api-sandbox.dwolla.com/customers/e358a488-6699-4d79-bbfb-c5bf58100ea4"
    }
  },
  "created": "2017-01-20T22:45:12.790Z"
}

Step A. Authenticating the webhook request

Before we process any data from the webhook we’ll want to validate that the request really came from Dwolla and not someone pretending to be Dwolla. Dwolla signs each webhook request with the secret you passed in when you created the webhook subscription. The signature is contained in the X-Request-Signature-SHA-256 header and is a SHA256 HMAC hash of the request body with the key being your webhook secret.

You can validate the webhook by generating the same SHA256 HMAC hash and comparing it to the signature sent with the payload.

var crypto = require("crypto");
var bufferFrom = require("buffer").Buffer.from;

function verifyGatewaySignature(
  proposed_signature,
  webhook_secret,
  payload_body
) {
  var hash = crypto
    .createHmac("sha256", webhook_secret)
    .update(payload_body)
    .digest("hex");

  return crypto.timingSafeEqual(
    bufferFrom(proposed_signature),
    bufferFrom(hash)
  );
}

Step B. Check for duplicate events

It is important to consider that duplicate webhooks may be fired for the same Event. To avoid doing any business logic twice, you would need to build idempotent webhook event processing in your app by checking if you have already successfully processed a webhook for a particular event. Event IDs are unique, so if you receive a webhook with an event ID that you have already received and acknowledged, then you can ignore that webhook.

To do this, keep a queue of events in a database and check to see if an Event has the same self resource location in _links as another event. If not, process the logic for that event. To illustrate, this is how a developer would implement this using Ruby and the ActiveRecord ORM.

Ruby/ActiveRecord

check_db = ActiveRecord::Base.connection.execute("SELECT * FROM EVENTS WHERE SELF = #{event[:_links][:self].to_s}")

# check_db will be an array of rows returned
unless check_db.length() == 0
    # do something
end