Snippet

Strict Content Security Policy Nonce Injection via Middleware

In modern Next.js App Router applications, implementing a Strict Content Security Policy (CSP) requires injecting a unique cryptographic nonce per request. Edge Middleware generates a base64-encoded random UUID nonce and forwards it via custom headers to both downstream Server Components and HTTP response headers, preventing Cross-Site Scripting (XSS) attacks.

snippet.js

javascript

import { NextResponse } from 'next/server';
 
export function middleware(request) {
  const nonce = Buffer.from(crypto.randomUUID()).toString('base64');
  const cspHeader = `
    default-src 'self';
    script-src 'self' 'nonce-${nonce}' 'strict-dynamic';
    object-src 'none';
    base-uri 'self';
  `.replace(/\s{2,}/g, ' ').trim();
 
  const requestHeaders = new Headers(request.headers);
  requestHeaders.set('x-nonce', nonce);
  requestHeaders.set('Content-Security-Policy', cspHeader);
 
  const response = NextResponse.next({
    request: { headers: requestHeaders },
  });
  response.headers.set('Content-Security-Policy', cspHeader);
  return response;
}

nextjs

Breakdown

const nonce = Buffer.from(crypto.randomUUID()).toString('base64');

Generates a cryptographically secure random UUID and encodes it as a base64 string to act as a per-request nonce.

requestHeaders.set('x-nonce', nonce);

Passes the generated nonce down to Server Components via request headers.

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Strict Content Security Policy Nonce Injection via Middleware

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