Webhooks Security: Verifying Signatures and Handling Retries

Webhooks are HTTP callbacks — anyone can POST to your endpoint and fake an event. Signature verification is non-negotiable.

How Signature Verification Works

The sender (Stripe, GitHub, etc.) computes an HMAC of the request body using a shared secret. You verify the same HMAC:

import crypto from 'crypto';

function verifyWebhookSignature(
  payload: Buffer,
  signature: string,
  secret: string
): boolean {
  const expectedSig = crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex');

  // Use timingSafeEqual to prevent timing attacks
  return crypto.timingSafeEqual(
    Buffer.from(signature, 'hex'),
    Buffer.from(expectedSig, 'hex')
  );
}

Stripe Webhooks in Practice

import express from 'express';
import Stripe from 'stripe';

const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!);

// Must use raw body — not parsed JSON
app.post('/webhooks/stripe',
  express.raw({ type: 'application/json' }),
  async (req, res) => {
    const sig = req.headers['stripe-signature']!;

    let event: Stripe.Event;
    try {
      event = stripe.webhooks.constructEvent(
        req.body, sig, process.env.STRIPE_WEBHOOK_SECRET!
      );
    } catch (err) {
      return res.status(400).send(`Webhook Error: ${(err as Error).message}`);
    }

    switch (event.type) {
      case 'checkout.session.completed':
        await handleCheckoutComplete(event.data.object);
        break;
      case 'customer.subscription.deleted':
        await handleSubscriptionCancelled(event.data.object);
        break;
    }

    res.json({ received: true });
  }
);

Idempotency: Handle Retries Safely

Webhook providers retry on failure. Your handler must be idempotent:

async function handleCheckoutComplete(session: Stripe.CheckoutSession) {
  // Check if already processed
  const existing = await db.orders.findUnique({
    where: { stripeSessionId: session.id }
  });
  if (existing) return; // Already processed, skip

  await db.orders.create({
    data: {
      stripeSessionId: session.id,
      userId: session.metadata!.userId,
      status: 'paid',
    }
  });
}

Timestamp Validation

Reject webhooks older than 5 minutes to prevent replay attacks:

const tolerance = 5 * 60; // 5 minutes in seconds
stripe.webhooks.constructEvent(body, sig, secret, tolerance);

Stripe webhook handling — including signature verification, idempotency, and subscription lifecycle — is fully implemented in the AI SaaS Starter Kit.

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