Content Security Policy middleware

The Content-Security-Policy header mitigates a large number of attacks, such as cross-site scripting. See MDN's introductory article on Content Security Policy.

This header is powerful but likely requires some configuration for your specific app.

To configure this header, pass an object with a nested directives object. Each key is a directive name in camel case (such as defaultSrc) or kebab case (such as default-src). Each value is an array (or other iterable) of strings or functions for that directive. If a function appears in the array, it will be called with the request and response objects.

const contentSecurityPolicy = require("helmet-csp");

// Sets all of the defaults, but overrides `script-src`
// and disables the default `style-src`.
app.use(
  contentSecurityPolicy({
    directives: {
      "script-src": ["'self'", "example.com"],
      "style-src": null,
    },
  }),
);

// Sets the `script-src` directive to
// "'self' 'nonce-e33cc...'"
// (or similar)
app.use((req, res, next) => {
  res.locals.cspNonce = crypto.randomBytes(32).toString("hex");
  next();
});
app.use(
  contentSecurityPolicy({
    directives: {
      scriptSrc: ["'self'", (req, res) => `'nonce-${res.locals.cspNonce}'`],
    },
  }),
);

These directives are merged into a default policy, which you can disable by setting useDefaults to false.

// Sets "Content-Security-Policy: default-src 'self';
// script-src 'self' example.com;object-src 'none';
// upgrade-insecure-requests"
app.use(
  contentSecurityPolicy({
    useDefaults: false,
    directives: {
      defaultSrc: ["'self'"],
      scriptSrc: ["'self'", "example.com"],
      objectSrc: ["'none'"],
      upgradeInsecureRequests: [],
    },
  }),
);

You can get the default directives object with contentSecurityPolicy.getDefaultDirectives(). Here is the default policy (formatted for readability):

default-src 'self';
base-uri 'self';
font-src 'self' https: data:;
form-action 'self';
frame-ancestors 'self';
img-src 'self' data:;
object-src 'none';
script-src 'self';
script-src-attr 'none';
style-src 'self' https: 'unsafe-inline';
upgrade-insecure-requests

The default-src directive can be explicitly disabled by setting its value to contentSecurityPolicy.dangerouslyDisableDefaultSrc, but this is not recommended.

You can set the Content-Security-Policy-Report-Only instead:

// Sets the Content-Security-Policy-Report-Only header
app.use(
  contentSecurityPolicy({
    directives: {/* ... */},
    reportOnly: true,
  }),
);

upgrade-insecure-requests, a directive that causes browsers to upgrade HTTP to HTTPS, is set by default. You may wish to avoid this in development, as you may not be developing with HTTPS. Notably, Safari will upgrade http://localhost to https://localhost, which can cause problems. To work around this, you may wish to disable the upgrade-insecure-requests directive in development. For example:

const isDevelopment = app.get("env") === "development";

app.use(
  contentSecurityPolicy({
    directives: {
      // Disable upgrade-insecure-requests in development.
      "upgrade-insecure-requests": isDevelopment ? null : [],
    },
  }),
);

This module performs very little validation on your CSP. You should rely on CSP checkers like CSP Evaluator instead.