Verify Postmark webhooks

Verify that a webhook really came from Postmark before acting on it.

Identifier	'postmark'
Import	import { verifyPostmark } from 'verihook/postmark'
Headers	authorization (Basic)
Signature	No signature. HTTP Basic auth from credentials in the webhook URL.
Replay window	None (the provider sends no timestamp)
result.eventType	the body’s RecordType, e.g. Delivery

Get your secret

The user:pass you put in the webhook URL (https://user:pass@example.com/webhooks/postmark) in the Postmark server’s Webhooks settings, passed as "user:pass".

Store it in an environment variable (POSTMARK_WEBHOOK_CREDENTIALS below) and never commit it.

Verify a request

import { verifyPostmark } from 'verihook/postmark';

export async function POST(request: Request) {
  const result = await verifyPostmark(request, process.env.POSTMARK_WEBHOOK_CREDENTIALS!);

  if (!result.valid) {
    console.warn(result.code, result.reason, result.hint);
    return new Response('Invalid signature', { status: 401 });
  }

  console.log(result.eventType, result.event);
  return Response.json({ received: true });
}

request can be a Fetch Request or { headers, body, url? } with the raw body. verifyWebhook('postmark', request, secret) from verihook does the same. Read why the raw body matters if verification fails behind a body parser.

Framework examples

Next.js

app/api/webhooks/postmark/route.ts

import { createWebhookHandler } from 'verihook/next';

export const POST = createWebhookHandler('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!, async (payload, result) => {
  // Runs only for a valid signature
  console.log(result.eventType, result.event);
});

Express

import express from 'express';
import { verihookExpress, type VerihookRequestAdditions } from 'verihook/express';

const app = express();

// Register before express.json(), or keep express.json() off this route
app.post('/webhooks/postmark', verihookExpress('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!), (req, res) => {
  const { eventType, event } = (req as typeof req & VerihookRequestAdditions).verihook!;
  res.json({ received: true });
});

Fastify

import Fastify from 'fastify';
import { verihookFastify, verihookRawBody, type VerihookFastifyRequest } from 'verihook/fastify';

const app = Fastify();
await app.register(verihookRawBody);

app.post('/webhooks/postmark', { preHandler: verihookFastify('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!) }, async (request) => {
  const { eventType, event } = (request as typeof request & VerihookFastifyRequest).verihook!;
  return { received: true };
});

Hono

import { Hono } from 'hono';
import { verihookHono, type VerihookVariables } from 'verihook/hono';

const app = new Hono<{ Bindings: { POSTMARK_WEBHOOK_CREDENTIALS: string }; Variables: VerihookVariables }>();

app.post('/webhooks/postmark', verihookHono('postmark', (c) => c.env.POSTMARK_WEBHOOK_CREDENTIALS), (c) => {
  const { eventType, event } = c.get('verihook');
  return c.json({ received: true });
});

NestJS

// main.ts: NestFactory.create(AppModule, { rawBody: true })
import { Controller, Post, Req, UseGuards } from '@nestjs/common';
import { createVerihookGuard, type VerihookNestRequest } from 'verihook/nestjs';

@Controller('webhooks')
export class WebhooksController {
  @Post('postmark')
  @UseGuards(createVerihookGuard('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!))
  handle(@Req() req: VerihookNestRequest) {
    const { eventType, event } = req.verihook!;
  }
}

Nuxt / h3

server/api/webhooks/postmark.post.ts

import { createWebhookHandler } from 'verihook/h3';

export default defineEventHandler(
  createWebhookHandler('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!, async (payload, result) => {
    console.log(result.eventType);
  }),
);

SvelteKit

src/routes/webhooks/postmark/+server.ts

import { env } from '$env/dynamic/private';
import { createWebhookHandler } from 'verihook/sveltekit';

export const POST = createWebhookHandler('postmark', () => env.POSTMARK_WEBHOOK_CREDENTIALS, async (payload, result) => {
  console.log(result.eventType);
});

Remix / React Router

app/routes/webhooks.postmark.ts

import { createWebhookHandler } from 'verihook/remix';

export const action = createWebhookHandler('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!, async (payload, result) => {
  console.log(result.eventType);
});

Astro

src/pages/api/webhooks/postmark.ts

import { createWebhookHandler } from 'verihook/astro';

export const prerender = false;
export const POST = createWebhookHandler('postmark', import.meta.env.POSTMARK_WEBHOOK_CREDENTIALS, async (payload, result) => {
  console.log(result.eventType);
});

AWS Lambda

import { createWebhookHandler } from 'verihook/lambda';

export const handler = createWebhookHandler('postmark', process.env.POSTMARK_WEBHOOK_CREDENTIALS!, async (payload, result) => {
  console.log(result.eventType);
});

Each adapter responds 401 to an invalid signature, 413 to a body over maxBodySize (2 MB by default) and 200 to a duplicate when you pass a dedupeStore.

Things to know

• Postmark doesn’t sign webhooks. It also recommends allowlisting its IP addresses.

Testing

signWebhook from verihook/testing builds a correctly signed Postmark request for your tests:

import { signWebhook } from 'verihook/testing';
import { verifyPostmark } from 'verihook/postmark';

const hook = await signWebhook('postmark', {
  secret: 'postmark_user:postmark_pass',
  payload: { id: 'evt_test' },
});

const result = await verifyPostmark(
  { headers: hook.headers, body: hook.body, url: hook.url },
  hook.secret,
);
// result.valid === true

From the command line, npx verihook simulate postmark --url http://localhost:3000/webhooks/postmark sends one to your local server.

See Testing your handlers for supertest and Fetch Request examples.

Troubleshooting

Failed results carry a code and often a hint with the likely cause. See Troubleshooting for each error code.

Postmark’s webhook documentation