security: fail-closed webhook auth, constant-time secret comparison, centralized env validation

server/src/config/env.ts

@@ -0,0 +1,37 @@
+import 'dotenv/config'
+
+const requiredEnvVars = ['REVENUECAT_WEBHOOK_SECRET', 'FIREBASE_PROJECT_ID'] as const
+
+type RequiredEnvVar = (typeof requiredEnvVars)[number]
+
+/**
+ * Validates that all required environment variables are set.
+ * Throws an error with details if any are missing.
+ */
+export function validateEnv(): void {
+  const missing: RequiredEnvVar[] = []
+
+  for (const varName of requiredEnvVars) {
+    const value = process.env[varName]
+    if (!value || value.trim() === '') {
+      missing.push(varName)
+    }
+  }
+
+  if (missing.length > 0) {
+    const message = missing.map(v => `  - ${v}`).join('\n')
+    throw new Error(`Missing required environment variables:\n${message}`)
+  }
+}
+
+/**
+ * Safely retrieves a required environment variable.
+ * Throws if not set (after validateEnv has been called).
+ */
+export function getEnv(varName: RequiredEnvVar): string {
+  const value = process.env[varName]
+  if (!value) {
+    throw new Error(`Environment variable ${varName} is not set`)
+  }
+  return value
+}

server/src/index.ts

@@ -4,9 +4,19 @@ import helmet from 'helmet'
 import morgan from 'morgan'
 import { initFirebase } from './config/firebase'
 import { startAnswerListener } from './listeners/answerListener'
+import { validateEnv } from './config/env'
 import healthRouter from './routes/health'
 import webhooksRouter from './routes/webhooks'
 
+// Validate required env vars before starting
+try {
+  validateEnv()
+} catch (err) {
+  console.error('[startup] Env validation failed:')
+  console.error(err)
+  process.exit(1)
+}
+
 initFirebase()
 
 const app = express()

server/src/routes/webhooks.ts

@@ -1,17 +1,29 @@
 import { Router, Request, Response } from 'express'
 import { syncEntitlement } from '../services/entitlement'
 import { RevenueCatEvent } from '../types'
+import { getEnv } from '../config/env'
+import * as crypto from 'crypto'
 
 const router = Router()
 
+/**
+ * Verifies RevenueCat webhook signature using constant-time comparison.
+ * Fails closed: returns false if secret is missing or signature doesn't match.
+ */
 function verifyRevenueCatSecret(req: Request): boolean {
-  const secret = process.env.REVENUECAT_WEBHOOK_SECRET
-  if (!secret) {
-    console.warn('[webhook] REVENUECAT_WEBHOOK_SECRET not set — skipping auth check')
-    return true
-  }
+  const secret = getEnv('REVENUECAT_WEBHOOK_SECRET')
+  
   const auth = req.headers['authorization'] ?? ''
-  return auth === secret
+  
+  // Constant-time comparison to prevent timing attacks
+  const authBuffer = Buffer.from(auth)
+  const secretBuffer = Buffer.from(secret)
+  
+  if (authBuffer.length !== secretBuffer.length) {
+    return false
+  }
+  
+  return crypto.timingSafeEqual(authBuffer, secretBuffer)
 }
 
 router.post('/revenuecat', async (req: Request, res: Response) => {