Securing your webhooks

Once your server is configured to receive payloads, it'll listen for any payload sent to the endpoint you configured. For security reasons, you probably want to limit requests to those coming from GitHub. There are a few ways to go about this--for example, you could opt to allow requests from GitHub's IP address--but a far easier method is to set up a secret token and validate the information.

The webhook REST APIs enable you to manage repository, organization, and app webhooks. You can use this API to list webhook deliveries for a webhook, or get and redeliver an individual delivery for a webhook, which can be integrated into an external app or service. You can also use the REST API to change the configuration of the webhook. For example, you can modify the payload URL, content type, SSL verification, and secret. For more information, see:

• Repository Webhooks REST API
• Organization Webhooks REST API
• GitHub App Webhooks REST API

Setting your secret token

You'll need to set up your secret token in two places: GitHub and your server.

To set your token on GitHub:

1. Navigate to the repository where you're setting up your webhook.
2. Fill out the Secret textbox. Use a random string with high entropy (e.g., by taking the output of ruby -rsecurerandom -e 'puts SecureRandom.hex(20)' at the terminal).
3. Click Update Webhook.

Next, set up an environment variable on your server that stores this token. Typically, this is as simple as running:

$ export SECRET_TOKEN=your_token

Never hardcode the token into your app!

Validating payloads from GitHub

When your secret token is set, GitHub uses it to create a hash signature with each payload. This hash signature is included with the headers of each request as X-Hub-Signature-256.

For example, if you have a basic server that listens for webhooks, it might be configured similar to this:

require 'sinatra'
require 'json'

post '/payload' do
  request.body.rewind
  push = JSON.parse(request.body.read)
  "I got some JSON: #{push.inspect}"
end

The intention is to calculate a hash using your SECRET_TOKEN, and ensure that the result matches the hash from GitHub. GitHub uses an HMAC hex digest to compute the hash, so you could reconfigure your server to look a little like this:

post '/payload' do
  request.body.rewind
  payload_body = request.body.read
  verify_signature(payload_body)
  push = JSON.parse(payload_body)
  "I got some JSON: #{push.inspect}"
end

def verify_signature(payload_body)
  signature = 'sha256=' + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), ENV['SECRET_TOKEN'], payload_body)
  return halt 500, "Signatures didn't match!" unless Rack::Utils.secure_compare(signature, request.env['HTTP_X_HUB_SIGNATURE_256'])
end

Your language and server implementations may differ from this example code. However, there are a number of very important things to point out:

• No matter which implementation you use, the hash signature starts with sha256=, using the key of your secret token and your payload body.

• Using a plain == operator is not advised. A method like secure_compare performs a "constant time" string comparison, which helps mitigate certain timing attacks against regular equality operators.