Verify Slack webhooks

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

Identifier	'slack'
Import	import { verifySlack } from 'verihook/slack'
Headers	x-slack-signature x-slack-request-timestamp
Signature	HMAC-SHA256 of v0:<timestamp>:<raw body>, sent as v0=<hex>.
Replay window	300 seconds (options.tolerance)
result.eventType	the body’s type (Events API), the slash command, or the interaction payload.type

Get your secret

The app’s Signing Secret: open your app at api.slack.com/apps, then Basic Information → App Credentials. It is not the bot token or the deprecated verification token.

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

Verify a request

import { verifySlack } from 'verihook/slack';

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

  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('slack', 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/slack/route.ts

import { createWebhookHandler } from 'verihook/next';

export const POST = createWebhookHandler('slack', process.env.SLACK_SIGNING_SECRET!, 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/slack', verihookExpress('slack', process.env.SLACK_SIGNING_SECRET!), (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/slack', { preHandler: verihookFastify('slack', process.env.SLACK_SIGNING_SECRET!) }, 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: { SLACK_SIGNING_SECRET: string }; Variables: VerihookVariables }>();

app.post('/webhooks/slack', verihookHono('slack', (c) => c.env.SLACK_SIGNING_SECRET), (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('slack')
  @UseGuards(createVerihookGuard('slack', process.env.SLACK_SIGNING_SECRET!))
  handle(@Req() req: VerihookNestRequest) {
    const { eventType, event } = req.verihook!;
  }
}

Nuxt / h3

server/api/webhooks/slack.post.ts

import { createWebhookHandler } from 'verihook/h3';

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

SvelteKit

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

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

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

Remix / React Router

app/routes/webhooks.slack.ts

import { createWebhookHandler } from 'verihook/remix';

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

Astro

src/pages/api/webhooks/slack.ts

import { createWebhookHandler } from 'verihook/astro';

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

AWS Lambda

import { createWebhookHandler } from 'verihook/lambda';

export const handler = createWebhookHandler('slack', process.env.SLACK_SIGNING_SECRET!, 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

• Slash commands and interactivity are form posts (application/x-www-form-urlencoded). result.event holds the form fields; interactivity puts JSON in the payload field.
• The Events API sends a url_verification request when you set the URL. After verification, respond with the challenge value from the body.

Testing

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

import { signWebhook } from 'verihook/testing';
import { verifySlack } from 'verihook/slack';

const hook = await signWebhook('slack', {
  secret: 'slack_test_secret',
  payload: { id: 'evt_test' },
});

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

From the command line, npx verihook simulate slack --url http://localhost:3000/webhooks/slack 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.

Slack’s webhook documentation