How to Secure Your Vercel App

1. Audit NEXT_PUBLIC_ Variables

The NEXT_PUBLIC_ prefix embeds variables into client-side JavaScript at build time. Search your .env files for any secrets using this prefix — they're visible to every user.

# DANGEROUS — embedded in client bundle:
NEXT_PUBLIC_STRIPE_SECRET=sk_live_abc123

# SAFE — server-side only:
STRIPE_SECRET=sk_live_abc123
NEXT_PUBLIC_STRIPE_PUBLISHABLE=pk_live_abc123

2. Secure Server Actions

Server Actions are exposed as POST endpoints that anyone can call directly — the UI button is not a security boundary. Add auth and authorization checks inside every Server Action.

// Every Server Action needs this pattern:
'use server'
async function deleteAccount(formData: FormData) {
  const session = await getServerSession();
  if (!session) throw new Error('Unauthorized');
  // Verify ownership before acting
  const userId = session.user.id;
  await db.delete(accounts).where(eq(accounts.userId, userId));
}

3. Add Security Headers via Next.js Config

Add CSP, HSTS, and other security headers through the Next.js headers() function — this is the Vercel-native way to configure them.

// next.config.js
headers: async () => [
  {
    source: '/(.*)',
    headers: [
      { key: 'X-Frame-Options', value: 'DENY' },
      { key: 'X-Content-Type-Options', value: 'nosniff' },
      { key: 'Strict-Transport-Security', value: 'max-age=31536000; includeSubDomains' },
    ],
  },
]

4. Scope Environment Variables per Environment

Vercel lets you set different values for Production, Preview, and Development. Without scoping, preview deployments use production API keys — meaning any PR can trigger production actions.

5. Enable Deployment Protection

Every git push generates a public preview URL. Enable Deployment Protection to require Vercel authentication, password, or trusted IPs before accessing previews.

6. Use Edge Middleware for Auth and Scan

Add middleware.ts to enforce authentication before requests reach your pages. Then run vas on the deployed site to catch exposed secrets and header gaps.

// middleware.ts
import { NextResponse } from 'next/server';

export function middleware(request) {
  const token = request.cookies.get('session');
  if (!token && request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
}