Outgoing Webhooks

Outgoing Webhooks

Customer-supplied URLs you POST to when something happens. Treat the URL as attacker-controlled. Sign the body. Pin DNS to a public IP. Never let a broken subscriber block the action that triggered the event.

Inbound webhooks (Stripe, GitHub) are the opposite direction — don't use this skill for those.

Philosophy

1. SSRF first. Resolve the host, reject private/loopback/link-local, then http.ipaddr = that address so a TTL=0 DNS rebind can't send you to 169.254.169.254.
2. HMAC the payload. Show the secret once. Header like X-Webhook-Signature: sha256=....
3. Delivery is a job with a log row. The user-facing action commits, then perform_later. Failures retry; they don't roll back the event.
4. Auto-disable after a streak of failures. Don't hammer a dead endpoint forever.

Records

class Webhook < ApplicationRecord  # tenanted, or board-scoped in Fizzy
  has_secure_token :signing_secret  # or "whsec_#{SecureRandom.urlsafe_base64(32)}"
  has_many :deliveries, dependent: :destroy

  validates :url, presence: true
  validate :url_is_deliverable
end

class WebhookDelivery < ApplicationRecord
  belongs_to :webhook
  # payload, event, attempts, delivering_at, delivered_at, failed_at,
  # response_status, error_message, signature
end

In activerecord-tenanted apps, Webhook lives in the tenant DB. The job receives a global handle (project slug, tenant key) plus delivery_id, then with_tenant.

Step 1: SSRF

module SsrfProtection
  DISALLOWED = [
    IPAddr.new("0.0.0.0/8"),
    IPAddr.new("10.0.0.0/8"),
    IPAddr.new("100.64.0.0/10"),
    IPAddr.new("127.0.0.0/8"),
    IPAddr.new("169.254.0.0/16"),
    IPAddr.new("172.16.0.0/12"),
    IPAddr.new("192.168.0.0/16"),
    IPAddr.new("198.18.0.0/15"),
    IPAddr.new("::1/128"),
    IPAddr.new("fc00::/7"),
    IPAddr.new("fe80::/10")
  ].freeze

  def self.public_ip(host)
    addrs = Resolv.getaddresses(host.to_s).map { |a| IPAddr.new(a) }
    addrs.find { |ip| DISALLOWED.none? { |net| net.include?(ip.native) } && !ip.loopback? && !ip.private? && !ip.link_local? }
  rescue Resolv::ResolvError, ArgumentError
    nil
  end
end

Validate on create and at delivery time (DNS can change). Production only for the hard block if you need localhost webhooks in dev — Herald does that. Fizzy always pins.

http = Net::HTTP.new(uri.host, uri.port)
http.ipaddr = public_ip.to_s
http.open_timeout = 10
http.read_timeout = 10
http.use_ssl = uri.scheme == "https"

Only http/https. Cap response body (Fizzy: 100 KB) so a subscriber can't fill your disk.

Step 2: Sign

def sign(payload)
  "sha256=#{OpenSSL::HMAC.hexdigest("SHA256", signing_secret, payload)}"
end

Sign the exact bytes you send (request.body). Store the signature on the delivery row so retries don't re-hash a mutated payload. Filter secret / token from logs.

Show the secret once (same trick as API tokens: short-lived signed id on the show URL). Rotating regenerate_secret! is a first-class action.

Step 3: Fire after commit

# after the domain event is committed
delivery = webhook.deliveries.create!(event: "release.published", payload: payload.to_json)
DeliverWebhookJob.perform_later(project.slug, delivery.id)

Job:

• with_tenant first
• Claim the row (delivering_at) so two workers don't double-POST
• Re-check webhook.enabled? and plan entitlements
• Retry 5xx and timeouts (retry_on, polynomially_longer, ~5 attempts)
• 4xx is terminal
• After N consecutive terminal failures, disable the webhook

A failure in this job must not raise into PublishReleaseJob. Herald keeps publish and delivery on separate jobs for that reason.

Step 4: Headers

Content-Type: application/json
User-Agent: App-Webhook/1.0
X-App-Event: release.published
X-App-Delivery: <delivery id>
X-App-Signature: sha256=...

Fizzy also sends X-Webhook-Timestamp. Receivers should verify HMAC, not trust the event name alone.

Tests that catch real bugs

• http://127.0.0.1/ and http://169.254.169.254/ are rejected in production
• Signature header matches HMAC of the stored payload
• Tenanted job without with_tenant cannot find the webhook
• 10 failures auto-disable; a later success (after re-enable) resets the counter
• Publish still succeeds when the endpoint 500s

Anti-Patterns

1. Net::HTTP.post_form(webhook.url) — no SSRF pin, no timeout, no signature.
2. Allowing localhost in production — that's the metadata-service exploit.
3. Signing a Hash then to_json later — key order will not match.
4. Delivering inline in the request — user waits on the subscriber.
5. One job that publishes and fans out webhooks — a bad URL retries the publish.
6. Logging the signing secret or full payload — filter_parameters + truncate response bodies.

Related

• Tenanted jobs: activerecord-tenanted
• Event bus that triggers these (Fizzy): events live in the app; this skill is only delivery

See reference.md for Fizzy vs Herald and the IP allow/deny list.