Content Security Policy middleware

Content Security Policy helps prevent unwanted content being injected into your webpages; this can mitigate XSS vulnerabilities, unintended frames, malicious frames, and more. 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.

Usage:

var csp = require('helmet-csp');

app.use(csp({
  // Specify directives as normal
  defaultSrc: ["'self'", 'default.com'],
  scriptSrc: ["'self'", "'unsafe-inline'"],
  styleSrc: ['style.com'],
  imgSrc: ['img.com', 'data:'],
  sandbox: ['allow-forms', 'allow-scripts'],
  reportUri: '/report-violation',

  // Set to an empty array to allow nothing through
  objectSrc: [],

  // Set to true if you only want browsers to report errors, not block them
  reportOnly: false,

  // Set to true if you want to blindly set all headers: Content-Security-Policy,
  // X-WebKit-CSP, and X-Content-Security-Policy.
  setAllHeaders: false,

  // Set to true if you want to disable CSP on Android.
  disableAndroid: false,

  // Set to true if you want to force buggy CSP in Safari 5.1 and below.
  safari5: false
}));

You can specify keys in a camel-cased fashion (imgSrc) or dashed (img-src); they are equivalent. The following directives are allowed:

• baseUri
• childSrc
• connectSrc
• defaultSrc
• fontSrc
• formAction
• frameAncestors
• frameSrc
• imgSrc
• manifestSrc
• mediaSrc
• objectSrc
• pluginTypes
• reportUri
• sandbox
• scriptSrc
• styleSrc
• upgradeInsecureRequests

There are a lot of inconsistencies in how browsers implement CSP. Helmet sniffs the user-agent of the browser and sets the appropriate header and value for that browser. If no user-agent is matched, it will set all the headers with the 2.0 spec.

Handling CSP violations

If you've specified a reportUri, browsers will POST any CSP violations to your server. Here's a simple example of a route that handles those reports:

// You need a JSON parser first.
app.use(bodyParser.json());

app.post('/report-violation', function (req, res) {
  if (req.body) {
    console.log('CSP Violation: ', req.body);
  } else {
    console.log('CSP Violation: No data received!');
  }
  res.status(204).end();
});

Not all browsers send CSP violations the same.

Note: If you're using a CSRF module like csurf, you might have problems handling these violations without a valid CSRF token. The fix is to put your CSP report route above csurf middleware.