Next.js Middleware: Auth Guards, Rate Limiting, and Edge Functions Explained

Next.js Middleware runs on every request before it hits your routes. It's the right place for auth guards, rate limiting, geo-redirects, and A/B testing -- all at the Edge.

Here's how to use it correctly without tanking performance.

What Middleware Is

Middleware runs in the Edge Runtime -- a lightweight V8 environment that starts instantly worldwide. It intercepts requests before they reach your Next.js routes.

// middleware.ts (at the project root, not inside /app)
import { NextRequest, NextResponse } from 'next/server'

export function middleware(request: NextRequest) {
  // Runs before every matched request
  return NextResponse.next()
}

export const config = {
  matcher: ['/((?!_next/static|_next/image|favicon.ico).*)'],
}

The matcher config controls which routes trigger middleware. Be specific -- running on static assets wastes cycles.

Auth Guard Pattern

Protect routes without touching every page component:

import { NextRequest, NextResponse } from 'next/server'
import { verifyToken } from '@/lib/auth'

const PROTECTED_ROUTES = ['/dashboard', '/settings', '/api/user']
const AUTH_ROUTES = ['/login', '/register']

export async function middleware(request: NextRequest) {
  const { pathname } = request.nextUrl
  const token = request.cookies.get('auth-token')?.value

  const isProtected = PROTECTED_ROUTES.some(r => pathname.startsWith(r))
  const isAuthRoute = AUTH_ROUTES.some(r => pathname.startsWith(r))

  if (isProtected) {
    if (!token) {
      return NextResponse.redirect(new URL('/login', request.url))
    }
    try {
      await verifyToken(token)
    } catch {
      const response = NextResponse.redirect(new URL('/login', request.url))
      response.cookies.delete('auth-token')
      return response
    }
  }

  // Redirect logged-in users away from auth pages
  if (isAuthRoute && token) {
    try {
      await verifyToken(token)
      return NextResponse.redirect(new URL('/dashboard', request.url))
    } catch { /* token invalid, let them through */ }
  }

  return NextResponse.next()
}

export const config = {
  matcher: ['/dashboard/:path*', '/settings/:path*', '/api/user/:path*', '/login', '/register']
}

Rate Limiting at the Edge

Use Upstash Redis for stateful rate limiting (works in Edge Runtime):

npm install @upstash/ratelimit @upstash/redis

import { Ratelimit } from '@upstash/ratelimit'
import { Redis } from '@upstash/redis'
import { NextRequest, NextResponse } from 'next/server'

const ratelimit = new Ratelimit({
  redis: Redis.fromEnv(),
  limiter: Ratelimit.slidingWindow(10, '10 s'), // 10 req per 10s
  analytics: true,
})

export async function middleware(request: NextRequest) {
  const ip = request.ip ?? request.headers.get('x-forwarded-for') ?? 'anonymous'

  const { success, limit, reset, remaining } = await ratelimit.limit(ip)

  if (!success) {
    return new NextResponse('Too Many Requests', {
      status: 429,
      headers: {
        'X-RateLimit-Limit': limit.toString(),
        'X-RateLimit-Remaining': remaining.toString(),
        'X-RateLimit-Reset': reset.toString(),
        'Retry-After': Math.ceil((reset - Date.now()) / 1000).toString(),
      }
    })
  }

  return NextResponse.next()
}

export const config = {
  matcher: '/api/:path*'
}

Injecting Headers Downstream

Middleware can add headers that your route handlers read:

export async function middleware(request: NextRequest) {
  const token = request.cookies.get('auth-token')?.value
  const response = NextResponse.next()

  if (token) {
    const payload = await verifyToken(token)
    // Downstream routes can read these
    response.headers.set('x-user-id', payload.userId)
    response.headers.set('x-user-role', payload.role)
  }

  // Nonce for CSP (see CSP article)
  const nonce = crypto.randomUUID()
  response.headers.set('x-nonce', nonce)
  response.headers.set('Content-Security-Policy', `script-src 'nonce-${nonce}'`)

  return response
}

In your route handler:

import { headers } from 'next/headers'

export async function GET() {
  const userId = headers().get('x-user-id')
  // userId was set by middleware -- no need to re-verify token here
}

Geo-Based Redirects

export function middleware(request: NextRequest) {
  const country = request.geo?.country ?? 'US'
  const url = request.nextUrl.clone()

  if (country === 'DE' && !url.pathname.startsWith('/de')) {
    url.pathname = `/de${url.pathname}`
    return NextResponse.redirect(url)
  }

  return NextResponse.next()
}

What You Can't Do in Middleware

The Edge Runtime is intentionally limited:

• No Node.js APIs (fs, path, crypto module)
• No Prisma client (uses Node.js drivers)
• No heavy computations (adds latency to every request)
• No long-running processes

For anything requiring Node.js or the database, do it in route handlers, not middleware.

Performance Tips

• Match only the routes that need middleware -- never match static assets
• Keep auth checks fast -- JWT verification is fine, database lookups are not
• Cache rate limit decisions in a Redis key with TTL
• If middleware logic is complex, consider moving some to route-level handlers

Starter Kit With Middleware Pre-Wired

The AI SaaS Starter includes middleware configured with:

• Auth guard protecting all /dashboard routes
• Rate limiting on /api routes
• CSP nonce injection
• User ID header forwarding

AI SaaS Starter Kit -- $99 one-time -- Next.js 14, Stripe, NextAuth, all pre-wired. Clone and deploy.

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