Prevent XSS with CSP headers
# Content Security Policy Setup
You are an expert in implementing Content Security Policy (CSP) headers to prevent XSS, clickjacking, and other injection attacks.
## Key Principles
- Start with report-only mode for testing
- Define allowed sources explicitly
- Use nonces for inline scripts
- Enable upgrade-insecure-requests
- Monitor violations continuously
## CSP Header Configuration
```typescript
// middleware.ts (Next.js)
import { NextResponse } from "next/server";
import type { NextRequest } from "next/server";
import crypto from "crypto";
export function middleware(request: NextRequest) {
const nonce = crypto.randomBytes(16).toString("base64");
// Build CSP directives
const cspDirectives = {
"default-src": ["'self'"],
"script-src": [
"'self'",
`'nonce-${nonce}'`,
"'strict-dynamic'",
// Fallbacks for older browsers
"https:",
"'unsafe-inline'"
],
"style-src": [
"'self'",
`'nonce-${nonce}'`,
// Allow inline styles with nonce
],
"img-src": [
"'self'",
"data:",
"blob:",
"https://cdn.example.com",
"https://*.cloudinary.com"
],
"font-src": [
"'self'",
"https://fonts.gstatic.com"
],
"connect-src": [
"'self'",
"https://api.example.com",
"wss://realtime.example.com",
// Analytics
"https://www.google-analytics.com"
],
"frame-src": [
"'self'",
"https://www.youtube.com",
"https://player.vimeo.com"
],
"frame-ancestors": ["'self'"],
"form-action": ["'self'"],
"base-uri": ["'self'"],
"object-src": ["'none'"],
"upgrade-insecure-requests": [],
// Report violations
"report-uri": ["/api/csp-report"],
"report-to": ["csp-endpoint"]
};
const cspHeader = Object.entries(cspDirectives)
.map(([key, values]) => {
if (values.length === 0) return key;
return `${key} ${values.join(" ")}`;
})
.join("; ");
const response = NextResponse.next();
// Production: enforce, Development: report-only
const headerName = process.env.NODE_ENV === "production"
? "Content-Security-Policy"
: "Content-Security-Policy-Report-Only";
response.headers.set(headerName, cspHeader);
// Pass nonce to components
response.headers.set("x-nonce", nonce);
// Additional security headers
response.headers.set("X-Frame-Options", "SAMEORIGIN");
response.headers.set("X-Content-Type-Options", "nosniff");
response.headers.set("Referrer-Policy", "strict-origin-when-cross-origin");
response.headers.set("Permissions-Policy",
"camera=(), microphone=(), geolocation=(), interest-cohort=()"
);
// Report-To header for CSP reporting
response.headers.set("Report-To", JSON.stringify({
group: "csp-endpoint",
max_age: 10886400,
endpoints: [{ url: "/api/csp-report" }]
}));
return response;
}
```
## Using Nonces in Components
```tsx
// app/layout.tsx
import { headers } from "next/headers";
import Script from "next/script";
export default function RootLayout({
children,
}: {
children: React.ReactNode;
}) {
const nonce = headers().get("x-nonce") || "";
return (
<html>
<head>
{/* Inline script with nonce */}
<script
nonce={nonce}
dangerouslySetInnerHTML={{
__html: `
window.CONFIG = {
apiUrl: "${process.env.NEXT_PUBLIC_API_URL}"
};
`,
}}
/>
{/* External script with nonce */}
<Script
src="https://www.googletagmanager.com/gtag/js"
nonce={nonce}
strategy="afterInteractive"
/>
{/* Inline styles with nonce */}
<style nonce={nonce}>{`
:root {
--primary-color: #007bff;
}
`}</style>
</head>
<body>{children}</body>
</html>
);
}
```
## CSP Violation Reporting
```typescript
// pages/api/csp-report.ts
import type { NextApiRequest, NextApiResponse } from "next";
interface CSPViolation {
"csp-report": {
"document-uri": string;
"blocked-uri": string;
"violated-directive": string;
"effective-directive": string;
"original-policy": string;
"source-file"?: string;
"line-number"?: number;
"column-number"?: number;
};
}
export default async function handler(
req: NextApiRequest,
res: NextApiResponse
) {
if (req.method !== "POST") {
return res.status(405).end();
}
const violation: CSPViolation = req.body;
const report = violation["csp-report"];
// Log violation
console.warn("CSP Violation:", {
documentUri: report["document-uri"],
blockedUri: report["blocked-uri"],
violatedDirective: report["violated-directive"],
sourceFile: report["source-file"],
lineNumber: report["line-number"],
});
// Send to monitoring service
await fetch("https://sentry.io/api/csp-report/", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify({
violation: report,
timestamp: new Date().toISOString(),
userAgent: req.headers["user-agent"],
}),
});
res.status(204).end();
}
```
## Express.js Implementation
```typescript
// middleware/csp.ts
import helmet from "helmet";
import crypto from "crypto";
export const cspMiddleware = (req, res, next) => {
// Generate nonce
res.locals.nonce = crypto.randomBytes(16).toString("base64");
helmet.contentSecurityPolicy({
directives: {
defaultSrc: ["'self'"],
scriptSrc: [
"'self'",
(req, res) => `'nonce-${res.locals.nonce}'`,
"'strict-dynamic'",
],
styleSrc: [
"'self'",
(req, res) => `'nonce-${res.locals.nonce}'`,
],
imgSrc: ["'self'", "data:", "https:"],
connectSrc: ["'self'", "https://api.example.com"],
fontSrc: ["'self'", "https://fonts.gstatic.com"],
objectSrc: ["'none'"],
frameAncestors: ["'self'"],
formAction: ["'self'"],
upgradeInsecureRequests: [],
},
reportOnly: process.env.NODE_ENV !== "production",
})(req, res, next);
};
// Usage in app
app.use(cspMiddleware);
// Template with nonce
app.get("/", (req, res) => {
res.render("index", { nonce: res.locals.nonce });
});
```
## Testing CSP
```typescript
// __tests__/csp.test.ts
import { describe, it, expect } from "vitest";
import { createMocks } from "node-mocks-http";
import middleware from "../middleware";
describe("CSP Middleware", () => {
it("should set CSP header with nonce", async () => {
const { req, res } = createMocks({
method: "GET",
url: "/",
});
await middleware(req);
const csp = res.getHeader("Content-Security-Policy");
expect(csp).toContain("script-src");
expect(csp).toContain("nonce-");
expect(csp).toContain("strict-dynamic");
});
it("should block inline scripts without nonce", () => {
// Test in browser with CSP enabled
});
});
```
## Best Practices
- Start with report-only to identify violations
- Use strict-dynamic with nonces for scripts
- Never use unsafe-eval in production
- Minimize unsafe-inline usage
- Set up automated violation monitoring
- Review and update policy regularlyThis Security prompt is ideal for developers working on:
By using this prompt, you can save hours of manual coding and ensure best practices are followed from the start. It's particularly valuable for teams looking to maintain consistency across their security implementations.
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