How to Receive Plaid Webhooks in Express

Raw-body middleware ordering for HMAC-friendly handlers

Express is still the default Node.js HTTP framework and powers an enormous amount of production webhook infrastructure. Its body-parsing middleware order is the single most common cause of 'signature mismatch' tickets — fix the ordering once and verification 'just works' across every provider. This guide walks through the Express setup for Plaid webhooks end to end: capturing the raw body, verifying the signature, handling retries idempotently, and iterating locally without redeploying. Cross-reference the Plaid Webhooks overview for the event catalog and sample payload.

1. Set Up the Express 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.

// server.ts
import express from "express";

const app = express();

// 1. Mount raw-body parser on the webhook route ONLY, BEFORE express.json().
//    The "verify" callback hands you the unparsed Buffer to keep alongside req.body.
app.post(
  "/api/webhooks/:service",
  express.raw({ type: "application/json" }),
  (req, res) => {
    const rawBody = req.body as Buffer; // Buffer, not parsed JSON
    const signature = req.header("x-signature-header") ?? "";

    // 1. Verify HMAC over rawBody.toString("utf8")
    // 2. Parse JSON only after verification passes
    // 3. Process the event idempotently (use the event id as your key)

    const event = JSON.parse(rawBody.toString("utf8"));
    console.log("Verified webhook:", event.type ?? event);

    res.status(200).send("ok");
  },
);

// 2. JSON middleware comes AFTER the webhook route, so it doesn't consume
//    your webhook bodies before your handler sees them.
app.use(express.json());

app.listen(3000);

2. Verify the Plaid Signature

Node.js verification

// npm i jsonwebtoken jwk-to-pem
import jwt from 'jsonwebtoken';
import jwkToPem from 'jwk-to-pem';
import crypto from 'node:crypto';
import express from 'express';
import { PlaidApi } from 'plaid';

const plaidClient = new PlaidApi(/* ... */);
const app = express();

app.post(
  '/webhooks/plaid',
  express.raw({ type: 'application/json' }),
  async (req, res) => {
    const token = req.headers['plaid-verification'] as string | undefined;
    if (!token) return res.status(401).send('missing token');

    // Inspect JWT header to get kid.
    const [headerB64] = token.split('.');
    const header = JSON.parse(Buffer.from(headerB64, 'base64').toString());
    if (header.alg !== 'ES256') return res.status(401).send('wrong alg');

    // Fetch JWK from Plaid using the kid.
    const { data } = await plaidClient.webhookVerificationKeyGet({ key_id: header.kid });
    const pem = jwkToPem(data.key as jwkToPem.JWK);

    // Verify the JWT signature + iat freshness (5 min tolerance).
    const decoded = jwt.verify(token, pem, {
      algorithms: ['ES256'],
      maxAge: '5m',
    }) as { request_body_sha256: string };

    // Check the body hash matches what was signed.
    const bodyHash = crypto.createHash('sha256').update(req.body).digest('hex');
    if (
      bodyHash.length !== decoded.request_body_sha256.length ||
      !crypto.timingSafeEqual(
        Buffer.from(bodyHash),
        Buffer.from(decoded.request_body_sha256),
      )
    ) {
      return res.status(403).send('body hash mismatch');
    }
    res.json({ ok: true });
  },
);

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

See Plaid'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

Plaid 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 Plaid event with HookRay, then replay that captured request against your local Express server until the verification + business logic both pass. No need to retrigger the event in Plaid, no need to redeploy.

1. Get a free webhook URL at hookray.com — no signup.
2. Paste the URL into your Plaiddashboard'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/plaid (or wherever your Express app is listening) — iterate on your code without re-poking the Plaid dashboard.

Deploying the Express Handler

Express works equally well on a long-running container (Render, Fly.io, ECS, GCP Cloud Run) or a serverless adapter (Vercel's Express adapter, AWS Lambda + serverless-http). For webhooks specifically, prefer a long-running runtime — cold starts on first request can push past the sender's retry timeout.

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.