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Content Security Policy in Next.js: Preventing XSS in Production

#nextjs #security #typescript #webdev

Content Security Policy in Next.js: Preventing XSS in Production

XSS is the most common web vulnerability. CSP is your last line of defense.
Here's how to implement it correctly in Next.js without breaking your app.

What CSP Does

CSP tells the browser which sources of scripts, styles, and other resources are allowed.
An attacker who injects <script>evil.com/steal.js</script> gets blocked.

Setting Up CSP in Next.js 

// next.config.js
const cspHeader = `
  default-src 'self';
  script-src 'self' 'nonce-{NONCE}';
  style-src 'self' 'unsafe-inline';
  img-src 'self' data: https:;
  font-src 'self';
  connect-src 'self' https://api.stripe.com;
  frame-src https://js.stripe.com;
  object-src 'none';
  base-uri 'self';
  form-action 'self';
`

module.exports = {
  async headers() {
    return [
      {
        source: '/(.*)',
        headers: [
          {
            key: 'Content-Security-Policy',
            value: cspHeader.replace(/\n/g, ''),
          },
        ],
      },
    ]
  },
}
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Nonce-Based CSP (Recommended)

A nonce is a random token generated per request. Only scripts with that nonce run:

// middleware.ts
import { NextResponse } from 'next/server'
import { v4 as uuidv4 } from 'uuid'

export function middleware(request: NextRequest) {
  const nonce = Buffer.from(uuidv4()).toString('base64')

  const cspHeader = [
    `default-src 'self'`,
    `script-src 'self' 'nonce-${nonce}' 'strict-dynamic'`,
    `style-src 'self' 'unsafe-inline'`,
    `img-src 'self' data: https:`,
    `connect-src 'self' https://api.stripe.com`,
    `frame-src https://js.stripe.com`,
    `object-src 'none'`,
    `base-uri 'self'`,
  ].join('; ')

  const requestHeaders = new Headers(request.headers)
  requestHeaders.set('x-nonce', nonce)

  const response = NextResponse.next({ request: { headers: requestHeaders } })
  response.headers.set('Content-Security-Policy', cspHeader)

  return response
}
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// app/layout.tsx — pass nonce to scripts
import { headers } from 'next/headers'

export default function RootLayout({ children }) {
  const nonce = headers().get('x-nonce')

  return (
    <html>
      <head>
        <script
          nonce={nonce}
          dangerouslySetInnerHTML={{
            __html: `window.__ENV__ = ${JSON.stringify({ apiUrl: process.env.NEXT_PUBLIC_API_URL })}`,
          }}
        />
      </head>
      <body>{children}</body>
    </html>
  )
}
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Other Security Headers 

const securityHeaders = [
  {
    key: 'X-DNS-Prefetch-Control',
    value: 'on',
  },
  {
    key: 'Strict-Transport-Security',
    value: 'max-age=63072000; includeSubDomains; preload',
  },
  {
    key: 'X-Frame-Options',
    value: 'SAMEORIGIN',
  },
  {
    key: 'X-Content-Type-Options',
    value: 'nosniff',
  },
  {
    key: 'Referrer-Policy',
    value: 'strict-origin-when-cross-origin',
  },
  {
    key: 'Permissions-Policy',
    value: 'camera=(), microphone=(), geolocation=()',
  },
]
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Testing Your CSP

  1. Open DevTools > Console — CSP violations appear as errors
  2. Use Content-Security-Policy-Report-Only header first (report without blocking)
  3. Check securityheaders.com for a grade 
// Report violations without blocking (testing mode)
response.headers.set(
  'Content-Security-Policy-Report-Only',
  cspHeader
)

// Switch to enforcement when ready:
response.headers.set('Content-Security-Policy', cspHeader)
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