CSP Config Generator

To generate a strict Content Security Policy configuration for Next.js applications, follow these steps systematically.

Step 1: Analyze Application Resources

Identify all resource types used in the application.

Discover External Resources

Use Grep to find external resource references:

Scripts:

- "<script.*src="
- "import.*from.*http"
- "next/script"

Stylesheets:

- "<link.*stylesheet"
- "@import.*url"
- "next/font"

Images:

- "<img.*src="
- "next/image"
- "background-image.*url"

Fonts:

- "@font-face"
- "next/font/google"
- "fonts.googleapis.com"

APIs and Connections:

- "fetch("
- "axios"
- "WebSocket"

Media:

- "<video"
- "<audio"
- "<iframe"

Extract Domains

Collect all unique domains used:

• CDNs (cdnjs.cloudflare.com, unpkg.com)
• APIs (api.stripe.com, *.supabase.co)
• Analytics (google-analytics.com, vercel.com)
• Fonts (fonts.googleapis.com, fonts.gstatic.com)
• Images (cloudinary.com, s3.amazonaws.com)

Consult references/csp-directives.md for directive documentation.

Step 2: Determine CSP Strategy

Choose the appropriate CSP implementation strategy:

Strategy A: Nonce-Based CSP (Recommended)

Most secure for Next.js apps with inline scripts:

• Use nonce for inline scripts and styles
• Strict directives
• Works with Next.js App Router

Strategy B: Hash-Based CSP

For static content:

• Use SHA-256 hashes for specific inline scripts
• More restrictive
• Requires rebuilding hashes on changes

Strategy C: Unsafe-Inline (Not Recommended)

Least secure, only for migration:

• Allows all inline scripts
• Use only temporarily while migrating

Recommend Strategy A (nonce-based) for modern Next.js apps.

Step 3: Generate CSP Directives

Build CSP directives based on discovered resources.

Default Directives

const cspDirectives = {
  'default-src': ["'self'"],
  'script-src': [
    "'self'",
    "'nonce-{NONCE}'", // Will be replaced dynamically
    "'strict-dynamic'",
  ],
  'style-src': [
    "'self'",
    "'nonce-{NONCE}'",
    // Add specific domains if needed
  ],
  'img-src': [
    "'self'",
    'data:',
    'blob:',
    // Add CDN domains
  ],
  'font-src': [
    "'self'",
    'data:',
    // Add font provider domains
  ],
  'connect-src': [
    "'self'",
    // Add API domains
  ],
  'frame-src': [
    "'self'",
    // Add allowed iframe sources
  ],
  'object-src': ["'none'"],
  'base-uri': ["'self'"],
  'form-action': ["'self'"],
  'frame-ancestors': ["'none'"],
  'upgrade-insecure-requests': [],
}

Add Discovered Domains

For each resource type, add discovered domains:

// If using Vercel Analytics
cspDirectives['script-src'].push('https://va.vercel-scripts.com')
cspDirectives['connect-src'].push('https://vitals.vercel-insights.com')

// If using Google Fonts
cspDirectives['font-src'].push('https://fonts.gstatic.com')
cspDirectives['style-src'].push('https://fonts.googleapis.com')

// If using Supabase
cspDirectives['connect-src'].push('https://*.supabase.co')

// If using Cloudinary for images
cspDirectives['img-src'].push('https://res.cloudinary.com')

Step 4: Generate Nonce Implementation

Create nonce generation and middleware.

Generate Nonce Utility

// lib/csp/nonce.ts
import { headers } from 'next/headers'

export function generateNonce(): string {
  return Buffer.from(crypto.randomUUID()).toString('base64')
}

export function getNonce(): string | undefined {
  return headers().get('x-nonce') ?? undefined
}

Generate Middleware with Nonce

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

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

  const cspHeader = generateCSPHeader(nonce)

  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
}

function generateCSPHeader(nonce: string): string {
  const csp = [
    { name: 'default-src', values: ["'self'"] },
    { name: 'script-src', values: ["'self'", `'nonce-${nonce}'`, "'strict-dynamic'"] },
    { name: 'style-src', values: ["'self'", `'nonce-${nonce}'`] },
    { name: 'img-src', values: ["'self'", 'data:', 'blob:'] },
    { name: 'font-src', values: ["'self'", 'data:'] },
    { name: 'connect-src', values: ["'self'"] },
    { name: 'frame-src', values: ["'self'"] },
    { name: 'object-src', values: ["'none'"] },
    { name: 'base-uri', values: ["'self'"] },
    { name: 'form-action', values: ["'self'"] },
    { name: 'frame-ancestors', values: ["'none'"] },
  ]

  return csp
    .map(({ name, values }) => `${name} ${values.join(' ')}`)
    .join('; ')
}

export const config = {
  matcher: [
    {
      source: '/((?!api|_next/static|_next/image|favicon.ico).*)',
      missing: [
        { type: 'header', key: 'next-router-prefetch' },
        { type: 'header', key: 'purpose', value: 'prefetch' },
      ],
    },
  ],
}

Update Root Layout with Nonce

// app/layout.tsx
import { getNonce } from '@/lib/csp/nonce'

export default function RootLayout({
  children,
}: {
  children: React.ReactNode
}) {
  const nonce = getNonce()

  return (
    <html lang="en">
      <head nonce={nonce}>
        {/* Head content will use nonce */}
      </head>
      <body nonce={nonce}>{children}</body>
    </html>
  )
}

Update Script Tags

// Use Next.js Script component with nonce
import Script from 'next/script'
import { getNonce } from '@/lib/csp/nonce'

export function AnalyticsScript() {
  const nonce = getNonce()

  return (
    <Script
      src="https://analytics.example.com/script.js"
      strategy="afterInteractive"
      nonce={nonce}
    />
  )
}

Step 5: Generate next.config.ts Configuration

Alternative approach using headers in next.config.ts:

// next.config.ts
import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  async headers() {
    return [
      {
        source: '/(.*)',
        headers: [
          {
            key: 'Content-Security-Policy',
            value: generateCSP(),
          },
        ],
      },
    ]
  },
}

function generateCSP(): string {
  const csp = [
    { name: 'default-src', values: ["'self'"] },
    {
      name: 'script-src',
      values: [
        "'self'",
        "'unsafe-eval'", // Required for React dev mode
        "'unsafe-inline'", // Required for Next.js hydration
        // Add specific domains
        'https://va.vercel-scripts.com',
      ],
    },
    {
      name: 'style-src',
      values: [
        "'self'",
        "'unsafe-inline'", // Required for Next.js styles
        'https://fonts.googleapis.com',
      ],
    },
    {
      name: 'img-src',
      values: ["'self'", 'data:', 'blob:', 'https:'],
    },
    {
      name: 'font-src',
      values: ["'self'", 'data:', 'https://fonts.gstatic.com'],
    },
    {
      name: 'connect-src',
      values: [
        "'self'",
        'https://vitals.vercel-insights.com',
        'https://*.supabase.co',
      ],
    },
    { name: 'frame-src', values: ["'self'"] },
    { name: 'object-src', values: ["'none'"] },
    { name: 'base-uri', values: ["'self'"] },
    { name: 'form-action', values: ["'self'"] },
    { name: 'frame-ancestors', values: ["'none'"] },
  ]

  // Remove 'unsafe-inline' and 'unsafe-eval' in production
  if (process.env.NODE_ENV === 'production') {
    const scriptSrc = csp.find(({ name }) => name === 'script-src')
    if (scriptSrc) {
      scriptSrc.values = scriptSrc.values.filter(
        (value) => value !== "'unsafe-eval'" && value !== "'unsafe-inline'"
      )
    }
  }

  return csp
    .map(({ name, values }) => `${name} ${values.join(' ')}`)
    .join('; ')
}

export default nextConfig

Step 6: Handle Development vs Production

Create environment-specific CSP:

// lib/csp/config.ts
const isDevelopment = process.env.NODE_ENV === 'development'

export function getCSPDirectives() {
  const baseDirectives = {
    'default-src': ["'self'"],
    'script-src': [
      "'self'",
      isDevelopment && "'unsafe-eval'", // React Fast Refresh
    ].filter(Boolean),
    'style-src': [
      "'self'",
      isDevelopment && "'unsafe-inline'", // Development styles
    ].filter(Boolean),
    // ... other directives
  }

  return baseDirectives
}

Step 7: Generate CSP Testing Suite

Create tests to verify CSP configuration:

// tests/csp.test.ts
import { describe, it, expect } from 'vitest'

describe('CSP Configuration', () => {
  it('includes required directives', () => {
    const csp = generateCSPHeader('test-nonce')

    expect(csp).toContain("default-src 'self'")
    expect(csp).toContain("object-src 'none'")
    expect(csp).toContain('nonce-test-nonce')
  })

  it('does not include unsafe-inline in production', () => {
    process.env.NODE_ENV = 'production'
    const csp = generateCSPHeader('test-nonce')

    expect(csp).not.toContain("'unsafe-inline'")
    expect(csp).not.toContain("'unsafe-eval'")
  })

  it('allows development-specific directives', () => {
    process.env.NODE_ENV = 'development'
    const csp = generateCSPHeader('test-nonce')

    // Development may include unsafe-eval for React Fast Refresh
    expect(csp).toBeDefined()
  })
})

Step 8: Generate CSP Violation Reporting

Set up CSP violation reporting:

// app/api/csp-report/route.ts
import { NextResponse } from 'next/server'

export async function POST(request: Request) {
  try {
    const report = await request.json()

    // Log violation
    console.error('CSP Violation:', {
      blockedURI: report['csp-report']['blocked-uri'],
      violatedDirective: report['csp-report']['violated-directive'],
      originalPolicy: report['csp-report']['original-policy'],
      documentURI: report['csp-report']['document-uri'],
    })

    // Optionally send to monitoring service
    // await sendToMonitoring(report)

    return NextResponse.json({ received: true })
  } catch (error) {
    return NextResponse.json({ error: 'Invalid report' }, { status: 400 })
  }
}

Add report-uri to CSP:

const csp = [
  // ... other directives
  { name: 'report-uri', values: ['/api/csp-report'] },
  { name: 'report-to', values: ['csp-endpoint'] },
]

Step 9: Generate Documentation

Create comprehensive CSP documentation using template from assets/csp-documentation-template.md:

# Content Security Policy Configuration

## Overview
This application uses a strict Content Security Policy to prevent XSS attacks.

## Implementation
CSP is implemented via [middleware/next.config] using [nonce-based/hash-based] strategy.

## Directives
- `default-src 'self'` - Only load resources from same origin
- `script-src 'self' 'nonce-{NONCE}'` - Scripts require nonce
- [etc.]

## Adding New Resources
To add a new external resource:
1. Identify the resource type
2. Add domain to appropriate directive
3. Test in development
4. Update documentation

## Troubleshooting
Common CSP violations and fixes...

Step 10: Validate CSP Configuration

Generate validation checklist:

# CSP Validation Checklist

- [ ] All external scripts have nonce or are in script-src
- [ ] All external styles are in style-src
- [ ] All API endpoints are in connect-src
- [ ] object-src is set to 'none'
- [ ] frame-ancestors is configured
- [ ] No 'unsafe-inline' in production script-src
- [ ] CSP violations are being reported
- [ ] Development mode works correctly
- [ ] Production build passes CSP checks

Consulting References

Throughout generation:

• Consult references/csp-directives.md for directive documentation
• Consult references/csp-best-practices.md for security guidelines
• Use templates from assets/csp-config-template.ts
• Use documentation template from assets/csp-documentation-template.md

Output Format

Generate files:

lib/csp/
  nonce.ts
  config.ts
middleware.ts (enhanced)
app/api/csp-report/
  route.ts
docs/
  csp-configuration.md
tests/
  csp.test.ts

Verification Checklist

Before completing:

• All resource types analyzed
• CSP directives generated
• Nonce implementation complete
• Middleware configured
• Development/production handled
• Violation reporting set up
• Documentation created
• Tests generated

Completion

When finished:

1. Display generated CSP configuration
2. List all allowed domains
3. Explain implementation approach
4. Provide testing instructions
5. Offer to implement or adjust configuration