How to Receive Mailgun Webhooks in Next.js

App Router Route Handler with raw body access

Next.js is the most-deployed full-stack JavaScript framework in 2026. The App Router's Route Handlers are a natural fit for webhook endpoints, but the default body parsing strips the raw bytes you need for HMAC verification — you have to read the request stream yourself. This guide walks through the Next.js setup for Mailgun webhooks end to end: capturing the raw body, verifying the signature, handling retries idempotently, and iterating locally without redeploying. Cross-reference the Mailgun Webhooks overview for the event catalog and sample payload.

1. Set Up the Next.js Endpoint

The endpoint needs to do three things, in this order: read the raw body, verify the signature against those exact bytes, and only then parse the JSON for your business logic.

// app/api/webhooks/[service]/route.ts
import { NextRequest } from "next/server";

export const runtime = "nodejs"; // signature verification needs Node, not Edge
export const dynamic = "force-dynamic"; // always run, never cache

export async function POST(req: NextRequest) {
  // IMPORTANT: read the raw body BEFORE you parse JSON.
  // HMAC verification has to run against the bytes the sender signed.
  const rawBody = await req.text();
  const signature = req.headers.get("x-signature-header") ?? "";

  // 1. Verify the signature against the raw body
  // 2. Parse JSON only after verification passes
  // 3. Process the event idempotently (use the event id as your key)

  const event = JSON.parse(rawBody);
  console.log("Verified webhook:", event.type ?? event);

  return new Response("ok", { status: 200 });
}

2. Verify the Mailgun Signature

Node.js verification

import crypto from 'node:crypto';
import express from 'express';

const app = express();

app.post(
  '/webhooks/mailgun',
  express.json(), // Mailgun sends JSON
  (req, res) => {
    const { timestamp, token, signature } = req.body.signature ?? {};
    if (!timestamp || !token || !signature) {
      return res.status(401).send('missing signature fields');
    }

    const expected = crypto
      .createHmac('sha256', process.env.MAILGUN_WEBHOOK_SIGNING_KEY!)
      .update(timestamp + token)
      .digest('hex');

    if (
      signature.length !== expected.length ||
      !crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))
    ) {
      return res.status(403).send('invalid signature');
    }
    res.json({ ok: true, event: req.body['event-data']?.event });
  },
);

Wire this verification call into the Next.js handler from section 1. The pattern is identical across Next.js versions: read raw body, verify, parse JSON, dispatch.

See Mailgun's official signing docs for the canonical reference, or the cross-service signature verification guide for the same pattern in Ruby and other languages.

3. Make the Handler Idempotent

Mailgun can — and will — send the same event twice. Network blips, your server returning a 5xx mid-processing, deploy windows: any of these triggers a retry, and your handler will see the same event id again. Build for that on day one rather than chasing duplicate-charge bugs in production.

The simplest pattern is a unique constraint on the event id in your database. The handler does the work inside a transaction, and the insert into the events table is the last step — if a retry arrives, the unique-constraint violation tells you the event already committed and you can return 200 without re-running the side effects.

4. Test Locally Without Deploying

The fastest iteration loop for any webhook handler is: capture a real Mailgun event with HookRay, then replay that captured request against your local Next.js server until the verification + business logic both pass. No need to retrigger the event in Mailgun, no need to redeploy.

1. Get a free webhook URL at hookray.com — no signup.
2. Paste the URL into your Mailgundashboard's webhook settings.
3. Trigger a test event. HookRay shows the headers, raw body, and parsed payload in real time.
4. Use HookRay's replay feature to send the captured request against http://localhost:3000/api/webhooks/mailgun (or wherever your Next.js app is listening) — iterate on your code without re-poking the Mailgun dashboard.

Deploying the Next.js Handler

Vercel runs Route Handlers on either the Node.js or Edge runtime. Pin runtime = "nodejs" for any handler that does HMAC verification — Edge's Web Crypto subset has surprising gaps for older signing schemes.

Need a host that boots quickly enough to absorb webhook bursts? DigitalOcean droplets stay warm, support raw-body proxies cleanly, and avoid the cold-start traps of some serverless runtimes.