Stripe Webhook Signature Bypass: The Payment Vulnerability Hiding in Most Vibe-Coded Apps

Most vibe-coded apps that take payments have a broken webhook handler. Not broken in a way you would notice. It processes real payments fine. But it will also process fake ones.

Here is the problem.

What the vulnerable code looks like

When Cursor or Lovable generates a Stripe webhook handler, it usually looks something like this:

export async function POST(req: NextRequest) {
  const body = await req.json();
  const event = body as Stripe.Event;

  if (event.type === "checkout.session.completed") {
    await db.user.update({
      where: { email: event.data.object.customer_email },
      data: { plan: "pro" },
    });
  }

  return NextResponse.json({ received: true });
}

This works. But it trusts whatever lands in the request body. Anyone who knows your webhook URL can POST a fake checkout.session.completed event, set customer_email to their own account, and get upgraded to pro. No payment needed.

Why AI tools generate this

The AI builds the handler to pass the demo. In testing, events come from Stripe and everything works. Signature verification is a production concern, not a demo concern, so the AI skips it.

The fix

Stripe signs every webhook request. You verify the signature using the raw request body and your webhook secret:

export async function POST(req: NextRequest) {
  const rawBody = await req.text();
  const sig = req.headers.get("stripe-signature");

  if (!sig) {
    return NextResponse.json({ error: "Missing signature" }, { status: 400 });
  }

  let event: Stripe.Event;

  try {
    event = stripe.webhooks.constructEvent(
      rawBody,
      sig,
      process.env.STRIPE_WEBHOOK_SECRET!
    );
  } catch {
    return NextResponse.json({ error: "Invalid signature" }, { status: 400 });
  }

  if (event.type === "checkout.session.completed") {
    await db.user.update({
      where: { email: event.data.object.customer_email },
      data: { plan: "pro" },
    });
  }

  return NextResponse.json({ received: true });
}

Two things changed. First, req.text() instead of req.json(). The signature is computed against the raw bytes. If you parse to JSON first, the bytes change and verification breaks even for real Stripe events. Second, stripe.webhooks.constructEvent() checks the signature and throws if it does not match.

One more thing: STRIPE_WEBHOOK_SECRET is not the same as STRIPE_SECRET_KEY. The webhook secret starts with whsec_ and lives in your Stripe dashboard under Developers, then Webhooks, then your endpoint.

Check your own app

Search for req.json() in any file that has "stripe" or "webhook" in the name. If you find it, and there is no call to constructEvent nearby, your handler is probably unprotected.

You can also test it yourself. POST a fake event payload to your webhook URL without a stripe-signature header. If your app processes it as a real event, you have the problem.

If you want us to check your payment flow and the rest of your app, get a free assessment. We flag the most critical issues within 24 hours.

VibeAudits audits apps built with Cursor, Lovable, Bolt, Claude Code, Replit, and other AI tools.