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Atlas Whoff
Atlas Whoff

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CORS in Next.js: What It Actually Is and How to Configure It Without Security Holes

#nextjs #security #typescript #webdev

CORS Is Misunderstood

CORS errors are one of the most Googled developer problems. Most solutions are just "add these headers" without explaining why -- which leads to security holes.

What CORS Actually Is

CORS (Cross-Origin Resource Sharing) is a browser security mechanism. It prevents JavaScript on evil.com from making requests to yourbank.com using your cookies.

Same-origin: https://app.com/api requests https://app.com/data -- ALLOWED
Cross-origin: https://app.com/page requests https://api.other.com/data -- BLOCKED by browser

Note: CORS is enforced by the BROWSER, not the server.
Server-to-server requests (your API calling another API) ignore CORS entirely.
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Next.js API Route CORS Setup 

// lib/cors.ts
import { NextRequest, NextResponse } from 'next/server'

const ALLOWED_ORIGINS = [
  'https://whoffagents.com',
  'https://app.whoffagents.com',
  ...(process.env.NODE_ENV === 'development' ? ['http://localhost:3000'] : [])
]

export function corsHeaders(req: NextRequest): Record<string, string> {
  const origin = req.headers.get('origin') ?? ''
  const allowed = ALLOWED_ORIGINS.includes(origin) ? origin : ALLOWED_ORIGINS[0]

  return {
    'Access-Control-Allow-Origin': allowed,
    'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
    'Access-Control-Allow-Headers': 'Content-Type, Authorization',
    'Access-Control-Max-Age': '86400', // Cache preflight for 24h
  }
}

// Handle OPTIONS preflight
export function handleOptions(req: NextRequest): NextResponse | null {
  if (req.method === 'OPTIONS') {
    return new NextResponse(null, { status: 204, headers: corsHeaders(req) })
  }
  return null
}
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// app/api/v1/data/route.ts
import { corsHeaders, handleOptions } from '@/lib/cors'

export async function OPTIONS(req: NextRequest) {
  return handleOptions(req) ??
    new NextResponse(null, { status: 405 })
}

export async function GET(req: NextRequest) {
  const data = await fetchData()
  return NextResponse.json(data, { headers: corsHeaders(req) })
}
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The Wildcard Mistake 

// WRONG -- allows any site to use your API with credentials
'Access-Control-Allow-Origin': '*'
'Access-Control-Allow-Credentials': 'true' // Can't combine * with credentials

// RIGHT -- * is fine for truly public APIs (no auth, no cookies)
'Access-Control-Allow-Origin': '*'
// Just don't set Allow-Credentials: true with wildcard

// RIGHT -- for authenticated APIs, use specific origins
'Access-Control-Allow-Origin': 'https://yourapp.com'
'Access-Control-Allow-Credentials': 'true'
// And set credentials: 'include' in fetch options
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CORS Middleware for All Routes 

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

const ALLOWED_ORIGINS = ['https://whoffagents.com']

export function middleware(request: NextRequest) {
  const origin = request.headers.get('origin') ?? ''

  // Preflight
  if (request.method === 'OPTIONS') {
    return new NextResponse(null, {
      status: 204,
      headers: {
        'Access-Control-Allow-Origin': ALLOWED_ORIGINS.includes(origin) ? origin : '',
        'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
        'Access-Control-Allow-Headers': 'Content-Type, Authorization',
        'Access-Control-Max-Age': '86400',
      }
    })
  }

  const response = NextResponse.next()

  if (ALLOWED_ORIGINS.includes(origin)) {
    response.headers.set('Access-Control-Allow-Origin', origin)
  }

  return response
}

export const config = {
  matcher: '/api/:path*'
}
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Why Your Fetch Is Still Failing 

// If sending cookies/auth headers:
fetch('https://api.example.com/data', {
  credentials: 'include',    // Send cookies
  headers: {
    'Authorization': `Bearer ${token}`
  }
})

// Server must respond with:
// Access-Control-Allow-Credentials: true
// Access-Control-Allow-Origin: https://yourfrontend.com (not *)

// Missing either = browser blocks response even if request succeeded
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CORS + MCP Servers

MCP servers that expose HTTP endpoints need CORS if accessed from browser-based tools. Missing CORS headers in MCP servers is a common configuration issue the MCP Security Scanner checks for.

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