Webhooks

Events

You can subscribe to one or more of the following events:

HTTP headers

The following HTTP headers are present in every webhook request, which allow you to identify the event that took place, and to verify the authenticity of the request:

One of either the Token or Signature headers will be present in every webhook request. The token value and header setting can be found under Token in your Webhook Notification service.

Your selection in the Webhook Notification service will determine which is sent:

HTTP request body

Each event's data is sent JSON encoded in the request body. See each event's documentation (agent, build, job, ping) to see which keys are available in the payload. For example:

{
  "event": "build.started",
  "build": {
    "keys": "vals"
  },
  "sender": {
    "keys": "vals"
  }
}

Webhook token

By default, Buildkite will send a token with each webhook in the X-Buildkite-Token header.

The token value and header setting can be found under Token in your Webhook Notification service.

The token is passed in clear text.

Webhook signature

Buildkite can optionally send an HMAC signature in place of a webhook token.

The X-Buildkite-Signature header contains a timestamp and an HMAC signature. The timestamp is prefixed by timestamp= and the signature is prefixed by signature=.

Buildkite generates the signature using HMAC-SHA256; a hash-based message authentication code HMAC used with the SHA-256 hash function and a secret key. The webhook token value is used as the secret key. The timestamp is an integer representation of a UTC timestamp. The raw request body is the signed message.

The token value and header setting can be found under Token in your Webhook Notification service.

Verifying HMAC signatures

When using HMAC signatures, you'll want to verify that the signature is legitimate.

Using the token as the secret along with the timestamp from the webhook, compute the expected signature based on the raw request body. There should be a library available in the programming language you are using that can perform this operation.

Compare the code to the signature received in the webhook. If they do not match, your payload has been altered.

The below example in Ruby verifies the signature and timestamp using the OpenSSL gem's HMAC :

require 'openssl'

class BuildkiteWebhook
  def self.valid?(webhook_request_body, header, secret)
    timestamp, signature = get_timestamp_and_signatures(header)
    expected = OpenSSL::HMAC.hexdigest("sha256", secret, "#{timestamp}.#{webhook_request_body}")
    Rack::Utils.secure_compare(expected, signature)
  end

  def self.get_timestamp_and_signatures(header)
    parts = header.split(",").map { |kv| kv.split("=", 2).map(&:strip) }.to_h
    [parts["timestamp"], parts["signature"]]
  end
end

BuildkiteWebhook.valid?(
  request.body.read,
  request.headers["X-Buildkite-Signature"],
  ENV["BUILDKITE_WEBHOOK_SECRET"]
)

Defending against replay attacks

A replay attack is when an attacker intercepts a valid payload and its signature, then re-transmits them. One way to help mitigate such attacks is to send a timestamp with your payload and only accept them within a short window (for example, 5 minutes).

Buildkite sends a timestamp in the X-Buildkite-Signature header. The timestamp is part of the signed payload so that it is verified by the signature. An attacker will not be able to change the timestamp without invalidating the signature.

To help protect against a replay attack, upon receipt of a webhook:

1. Verify the signature
2. Check the timestamp against the current time

If the webhook's timestamp is within your chosen window of the current time, it can reasonably be assumed to be the original webhook.

Example implementations

The following example repositories show how to receive a webhook event and trigger a LIFX powered build light. You can browse their source, fork them, and deploy them to Heroku directly from their GitHub readmes, or use them as an example to implement webhooks in your tool of choice.

Node webhook example application github.com/buildkite/lifx-buildkite-build-light-node

Ruby webhook example application github.com/buildkite/lifx-buildkite-build-light-ruby

PHP webhook example application github.com/buildkite/lifx-buildkite-build-light-php

Webtask.io webhook example application github.com/buildkite/lifx-buildkite-build-light-webtask

Request logs

The last 20 webhook request and responses are saved, so you can debug and inspect your webhook. Each webhook's request logs are available on the bottom of their settings page.