diff --git a/src/lib/integrations/facebook/webhook-manager.ts b/src/lib/integrations/facebook/webhook-manager.ts
index 9341b2c5..48a05a13 100644
--- a/src/lib/integrations/facebook/webhook-manager.ts
+++ b/src/lib/integrations/facebook/webhook-manager.ts
@@ -15,6 +15,7 @@
 
 import { createFacebookClient, FacebookAPIError } from './graph-api-client';
 import { FACEBOOK_CONFIG } from './constants';
+import * as crypto from 'crypto';
 import { decrypt } from './encryption';
 import { prisma } from '@/lib/prisma';
 
@@ -318,7 +319,7 @@ export class WebhookManager {
     verifyToken: string
   ): Promise<boolean> {
     try {
-      const challenge = Math.random().toString(36).substring(7);
+      const challenge = crypto.randomBytes(8).toString('hex');
       const url = new URL(callbackUrl);
       url.searchParams.set('hub.mode', 'subscribe');
       url.searchParams.set('hub.challenge', challenge);
diff --git a/src/lib/subscription/billing-service.ts b/src/lib/subscription/billing-service.ts
index ef65d0c5..69da6bbb 100644
--- a/src/lib/subscription/billing-service.ts
+++ b/src/lib/subscription/billing-service.ts
@@ -7,6 +7,7 @@
 
 import { prisma } from '@/lib/prisma';
 import { getAppUrl } from '@/lib/utils';
+import * as crypto from 'crypto';
 import { getRemainingDays, getExpiryDate, isExpiringSoon } from './state-machine';
 import { getEffectiveFeatureLimits, getUsageStats } from './feature-enforcer';
 import { getPaymentGateway } from './payment-gateway';
@@ -746,13 +747,14 @@ function getRetryDelay(retryCount: number): number {
   return delays[Math.min(retryCount, delays.length - 1)];
 }
 
-async function createInvoiceForPayment(
+export async function createInvoiceForPayment(
   subscriptionId: string,
   amount: number,
   periodStart: Date,
   periodEnd: Date
 ): Promise<void> {
-  const invoiceNumber = `INV-${Date.now()}-${Math.random().toString(36).slice(2, 7).toUpperCase()}`;
+  const randomPart = crypto.randomBytes(4).toString('base64url').toUpperCase().slice(0, 5);
+  const invoiceNumber = `INV-${Date.now()}-${randomPart}`;
 
   await prisma.invoice.create({
     data: {
diff --git a/src/test/services/billing-service.test.ts b/src/test/services/billing-service.test.ts
new file mode 100644
index 00000000..d5f579a4
--- /dev/null
+++ b/src/test/services/billing-service.test.ts
@@ -0,0 +1,56 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { prisma } from '@/lib/prisma';
+import { createInvoiceForPayment } from '@/lib/subscription/billing-service';
+
+// Mock prisma
+vi.mock('@/lib/prisma', () => ({
+  prisma: {
+    invoice: {
+      create: vi.fn().mockResolvedValue({ id: 'inv_123' }),
+    },
+  },
+}));
+
+describe('BillingService', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  describe('createInvoiceForPayment', () => {
+    it('should generate an invoice number with the correct format', async () => {
+      const subscriptionId = 'sub_123';
+      const amount = 1000;
+      const periodStart = new Date();
+      const periodEnd = new Date();
+
+      await createInvoiceForPayment(subscriptionId, amount, periodStart, periodEnd);
+
+      expect(prisma.invoice.create).toHaveBeenCalledTimes(1);
+      const call = vi.mocked(prisma.invoice.create).mock.calls[0][0];
+      const invoiceNumber = call.data.invoiceNumber;
+
+      // Pattern: INV-TIMESTAMP-RANDOM
+      // INV- (4 chars)
+      // TIMESTAMP (digits)
+      // - (1 char)
+      // RANDOM (5 chars ALPHANUMERIC-ish)
+      expect(invoiceNumber).toMatch(/^INV-\d+-[0-9A-Z_-]{5}$/);
+    });
+
+    it('should generate different invoice numbers for each call', async () => {
+      const subscriptionId = 'sub_123';
+      const amount = 1000;
+      const periodStart = new Date();
+      const periodEnd = new Date();
+
+      await createInvoiceForPayment(subscriptionId, amount, periodStart, periodEnd);
+      await createInvoiceForPayment(subscriptionId, amount, periodStart, periodEnd);
+
+      expect(prisma.invoice.create).toHaveBeenCalledTimes(2);
+      const firstCall = vi.mocked(prisma.invoice.create).mock.calls[0][0];
+      const secondCall = vi.mocked(prisma.invoice.create).mock.calls[1][0];
+
+      expect(firstCall.data.invoiceNumber).not.toBe(secondCall.data.invoiceNumber);
+    });
+  });
+});