Stripe Webhook Security: Signature Verification, Idempotency, and Local Testing

Stripe webhooks are where most payment integrations break. Missed events, duplicate processing, and replay attacks are all common failure modes.

Here's the production-ready webhook handler pattern.

Step 1: Verify the Signature

Every Stripe webhook includes a Stripe-Signature header. Always verify it before doing anything:

// app/api/webhooks/stripe/route.ts
import Stripe from 'stripe'
import { headers } from 'next/headers'

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

export async function POST(request: Request) {
  const body = await request.text() // Must be raw text, not parsed JSON
  const signature = headers().get('stripe-signature')

  if (!signature) {
    return Response.json({ error: 'Missing signature' }, { status: 400 })
  }

  let event: Stripe.Event
  try {
    event = stripe.webhooks.constructEvent(
      body,
      signature,
      process.env.STRIPE_WEBHOOK_SECRET!
    )
  } catch (err) {
    console.error('Webhook signature verification failed:', err)
    return Response.json({ error: 'Invalid signature' }, { status: 400 })
  }

  // Signature verified -- safe to process
  await handleEvent(event)
  return Response.json({ received: true })
}

Critical: use request.text(), not request.json(). Parsing JSON before verification corrupts the raw body Stripe uses to compute the signature.

Step 2: Idempotent Processing

Stripe may deliver the same event multiple times. Your handler must be idempotent:

async function handleEvent(event: Stripe.Event) {
  // Check if already processed
  const existing = await db.stripeEvent.findUnique({
    where: { stripeId: event.id }
  })
  if (existing) {
    console.log(`Event ${event.id} already processed, skipping`)
    return
  }

  // Record before processing (prevents double-processing on crash)
  await db.stripeEvent.create({
    data: { stripeId: event.id, type: event.type, status: 'processing' }
  })

  try {
    await processEvent(event)
    await db.stripeEvent.update({
      where: { stripeId: event.id },
      data: { status: 'completed' }
    })
  } catch (err) {
    await db.stripeEvent.update({
      where: { stripeId: event.id },
      data: { status: 'failed', error: String(err) }
    })
    throw err // Re-throw so Stripe retries
  }
}

Step 3: Handle Key Events

async function processEvent(event: Stripe.Event) {
  switch (event.type) {
    case 'checkout.session.completed': {
      const session = event.data.object as Stripe.Checkout.Session
      await fulfillOrder(session)
      break
    }
    case 'customer.subscription.created':
    case 'customer.subscription.updated': {
      const sub = event.data.object as Stripe.Subscription
      await syncSubscription(sub)
      break
    }
    case 'customer.subscription.deleted': {
      const sub = event.data.object as Stripe.Subscription
      await cancelSubscription(sub.metadata.userId)
      break
    }
    case 'invoice.payment_failed': {
      const invoice = event.data.object as Stripe.Invoice
      await handlePaymentFailure(invoice)
      break
    }
    default:
      console.log(`Unhandled event type: ${event.type}`)
  }
}

Step 4: Fulfill Orders

async function fulfillOrder(session: Stripe.Checkout.Session) {
  const userId = session.metadata?.userId
  const productId = session.metadata?.productId

  if (!userId || !productId) {
    throw new Error('Missing metadata on checkout session')
  }

  await db.purchase.create({
    data: {
      userId,
      productId,
      stripeSessionId: session.id,
      amount: session.amount_total ?? 0,
      status: 'completed'
    }
  })

  // Send delivery email
  await sendProductEmail(userId, productId)
}

Always put fulfillment data in metadata when creating the checkout session:

const session = await stripe.checkout.sessions.create({
  // ...
  metadata: {
    userId: user.id,
    productId: product.id,
  }
})

Step 5: Local Testing

# Install Stripe CLI
brew install stripe/stripe-cli/stripe

# Login
stripe login

# Forward to your local dev server
stripe listen --forward-to localhost:3000/api/webhooks/stripe

# Trigger test events
stripe trigger checkout.session.completed
stripe trigger customer.subscription.created

The CLI gives you a webhook secret for local testing -- store it as STRIPE_WEBHOOK_SECRET in .env.local.

The Prisma Schema

model StripeEvent {
  id        String   @id @default(cuid())
  stripeId  String   @unique
  type      String
  status    String   // 'processing' | 'completed' | 'failed'
  error     String?
  createdAt DateTime @default(now())
}

model Purchase {
  id              String   @id @default(cuid())
  userId          String
  productId       String
  stripeSessionId String   @unique
  amount          Int
  status          String
  createdAt       DateTime @default(now())
  user            User     @relation(fields: [userId], references: [id])
}

Pre-Wired in the Starter Kit

The AI SaaS Starter Kit ships with this exact webhook handler implemented:

• Signature verification
• Idempotency via StripeEvent table
• Subscription sync to user table
• Local testing config

AI SaaS Starter Kit -- $99 one-time -- Stripe fully wired, clone and deploy.

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