Content Security Policy middleware

Content Security Policy (CSP) helps prevent unwanted content from being injected/loaded into your webpages. This can mitigate cross-site scripting (XSS) vulnerabilities, clickjacking, formjacking, malicious frames, unwanted trackers, and other web client-side attacks.

If you want to learn how CSP works, check out the fantastic HTML5 Rocks guide, the Content Security Policy Reference, and the Content Security Policy specification.

This middleware helps set Content Security Policies.

Basic usage:

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

app.use(
  contentSecurityPolicy({
    useDefaults: true,
    directives: {
      defaultSrc: ["'self'", "default.example"],
      scriptSrc: ["'self'", "js.example.com"],
      objectSrc: ["'none'"],
      upgradeInsecureRequests: [],
    },
    reportOnly: false,
  })
);

If no directives are supplied, the following policy is set (whitespace added for readability):

default-src 'self';
base-uri 'self';
block-all-mixed-content;
font-src 'self' https: data:;
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

You can use this default with the useDefaults option. useDefaults is false by default, but will be true in the next major version of this module.

You can also get the default directives object with contentSecurityPolicy.getDefaultDirectives().

You can set any directives you wish. defaultSrc is required, but can be explicitly disabled by setting its value to contentSecurityPolicy.dangerouslyDisableDefaultSrc. Directives can be kebab-cased (like script-src) or camel-cased (like scriptSrc). They are equivalent, but duplicates are not allowed.

The reportOnly option, if set to true, sets the Content-Security-Policy-Report-Only header instead.

This middleware does minimal validation. You should use a more sophisticated CSP validator, like Google's CSP Evaluator, to make sure your CSP looks good.

Recipe: generating nonces

You can dynamically generate nonces to allow inline <script> tags to be safely evaluated. Here's a simple example:

const crypto = require("crypto");

app.use((req, res, next) => {
  res.locals.nonce = crypto.randomBytes(16).toString("hex");
  next();
});

app.use((req, res, next) => {
  csp({
    useDefaults: true,
    directives: {
      scriptSrc: ["'self'", `'nonce-${res.locals.nonce}'`],
    },
  })(req, res, next);
});

app.use((req, res) => {
  res.end(`<script nonce="${res.locals.nonce}">alert(1 + 1);</script>`);
});

See also

• Google's CSP Evaluator tool
• CSP Scanner
• GitHub's CSP journey
• Content Security Policy for Single Page Web Apps