# Inbound Webhook Verification

## Objective

Accept provider events without letting anyone forge them.

Signature, timestamp, and replay checks on incoming webhooks, with the real work moved off the request.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Verify the signature against the exact raw request body, captured before any JSON parsing or middleware rewriting. Re-serialized JSON produces a different signature and every event fails.
2. Compare signatures with a constant-time comparison. A byte-by-byte early exit is measurable.
3. Reject requests whose signed timestamp is outside a short tolerance window, and record processed event IDs so a replayed event is a no-op.
4. Acknowledge with a 2xx as soon as verification passes and process asynchronously. Providers time out fast and retry, so slow handlers manufacture duplicates.
5. Support more than one active signing secret so a rotation does not drop events mid-cutover.
6. Do NOT trust any data in the payload beyond the event ID. Fetch the current object from the provider's API before acting on it — payloads arrive out of order and may already be stale.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Events arrive out of order. An update can land before the create. Handle unknown-object events by fetching, not by failing.
- The same event will be delivered more than once. Idempotency is not optional; key on the provider's event ID.
- A malformed or unverifiable body must return an error the provider treats as final, and must be logged with enough context to debug without storing the raw secret.
- Returning a 5xx makes most providers retry with backoff. Return 2xx for events you deliberately ignore, or you will build an accidental retry storm.
- Body-size and content-type limits still apply. An unbounded webhook endpoint is a memory exhaustion vector.
- During secret rotation, verify against both the old and new secret until the old one is retired, then remove it deliberately.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Signatures are verified against the unmodified raw body using a constant-time comparison.
- [ ] Stale timestamps are rejected and repeated event IDs are processed once.
- [ ] The endpoint acknowledges within the provider's timeout and processes work in the background.
- [ ] Two signing secrets can be valid simultaneously during rotation.
- [ ] Ignored events return a success status rather than triggering retries.
- [ ] Handlers refetch object state rather than trusting payload contents.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.