Implementing Content Security Policy (CSP)

Introduction

A Content Security Policy (CSP) is an added layer of security that helps detect and mitigate certain types of attacks, including Cross-Site Scripting (XSS) and data injection attacks.

A CSP dictates exactly which dynamic resources (like JavaScript, CSS, Images, and Fonts) are allowed to load and execute on a webpage. If an attacker injects a malicious script tag, the browser will block it from executing if it violates the CSP.

Implementing CSP with Helmet

The industry standard for configuring CSP in an Express application is using the helmet package.

npm install helmet

const express = require('express');
const helmet = require('helmet');

const app = express();

app.use(helmet.contentSecurityPolicy({
  directives: {
    // Only allow resources from the same origin by default
    defaultSrc: ["'self'"], 
    
    // Allow scripts from self and trusted CDNs
    scriptSrc: ["'self'", "https://trusted-cdn.com", "https://apis.google.com"],
    
    // Allow styles from self and inline styles (unsafe-inline is often needed for React/Vue)
    styleSrc: ["'self'", "'unsafe-inline'", "https://fonts.googleapis.com"],
    
    // Allow images from anywhere
    imgSrc: ["*", "data:"],
    
    // Enforce HTTPS
    upgradeInsecureRequests: [],
  }
}));

How it works

When a user visits your site, Express sends the Content-Security-Policy HTTP header with the directives you specified. The browser's engine strictly enforces these rules. If a rule is broken (e.g., an inline script tries to execute but 'unsafe-inline' is not present in scriptSrc), the browser blocks it and logs an error to the console.

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