Helmet

Help secure Express apps by setting HTTP response headers.

import helmet from "helmet";

const app = express();

app.use(helmet());

Helmet sets the following headers by default:

• Content-Security-Policy: A powerful allow-list of what can happen on your page which mitigates many attacks
• Cross-Origin-Opener-Policy: Helps process-isolate your page
• Cross-Origin-Resource-Policy: Blocks others from loading your resources cross-origin
• Origin-Agent-Cluster: Changes process isolation to be origin-based
• Referrer-Policy: Controls the [Referer][Referer] header
• Strict-Transport-Security: Tells browsers to prefer HTTPS
• X-Content-Type-Options: Avoids [MIME sniffing]
• X-DNS-Prefetch-Control: Controls DNS prefetching
• X-Download-Options: Forces downloads to be saved (Internet Explorer only)
• X-Frame-Options: Legacy header that mitigates [clickjacking] attacks
• X-Permitted-Cross-Domain-Policies: Controls cross-domain behavior for Adobe products, like Acrobat
• X-Powered-By: Info about the web server. Removed because it could be used in simple attacks
• X-XSS-Protection: Legacy header that tries to mitigate [XSS attacks][XSS], but makes things worse, so Helmet disables it

Each header can be configured. For example, here's how you configure the Content-Security-Policy header:

// Configure the Content-Security-Policy header.
app.use(
  helmet({
    contentSecurityPolicy: {
      directives: {
        "script-src": ["'self'", "example.com"],
      },
    },
  }),
);

Headers can also be disabled. For example, here's how you disable the Content-Security-Policy and X-Download-Options headers:

// Disable the Content-Security-Policy and X-Download-Options headers
app.use(
  helmet({
    contentSecurityPolicy: false,
    xDownloadOptions: false,
  }),
);

Reference

<details id="content-security-policy"> <summary><code>Content-Security-Policy</code></summary>

Default:

Content-Security-Policy: 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 Content-Security-Policy header mitigates a large number of attacks, such as [cross-site scripting][XSS]. 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.

// Sets all of the defaults, but overrides `script-src`
// and disables the default `style-src`.
app.use(
  helmet({
    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(
  helmet({
    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(
  helmet({
    contentSecurityPolicy: {
      useDefaults: false,
      directives: {
        defaultSrc: ["'self'"],
        scriptSrc: ["'self'", "example.com"],
        objectSrc: ["'none'"],
        upgradeInsecureRequests: [],
      },
    },
  }),
);

You can get the default directives object with helmet.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 helmet.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(
  helmet({
    contentSecurityPolicy: {
      directives: {
        /* ... */
      },
      reportOnly: true,
    },
  }),
);

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

To disable the Content-Security-Policy header:

app.use(
  helmet({
    contentSecurityPolicy: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.contentSecurityPolicy()).

</details> <details id="cross-origin-embedder-policy"> <summary><code>Cross-Origin-Embedder-Policy</code></summary>

This header is not set by default.

The Cross-Origin-Embedder-Policy header helps control what resources can be loaded cross-origin. See MDN's article on this header for more.

// Helmet does not set Cross-Origin-Embedder-Policy
// by default.
app.use(helmet());

// Sets "Cross-Origin-Embedder-Policy: require-corp"
app.use(helmet({ crossOriginEmbedderPolicy: true }));

// Sets "Cross-Origin-Embedder-Policy: credentialless"
app.use(helmet({ crossOriginEmbedderPolicy: { policy: "credentialless" } }));

You can use this as standalone middleware with app.use(helmet.crossOriginEmbedderPolicy()).

</details> <details id="cross-origin-opener-policy"> <summary><code>Cross-Origin-Opener-Policy</code></summary>

Default:

Cross-Origin-Opener-Policy: same-origin

The Cross-Origin-Opener-Policy header helps process-isolate your page. For more, see MDN's article on this header.

// Sets "Cross-Origin-Opener-Policy: same-origin"
app.use(helmet());

// Sets "Cross-Origin-Opener-Policy: same-origin-allow-popups"
app.use(
  helmet({
    crossOriginOpenerPolicy: { policy: "same-origin-allow-popups" },
  }),
);

To disable the Cross-Origin-Opener-Policy header:

app.use(
  helmet({
    crossOriginOpenerPolicy: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.crossOriginOpenerPolicy()).

</details> <details id="cross-origin-resource-policy"> <summary><code>Cross-Origin-Resource-Policy</code></summary>

Default:

Cross-Origin-Resource-Policy: same-origin

The Cross-Origin-Resource-Policy header blocks others from loading your resources cross-origin in some cases. For more, see "Consider deploying Cross-Origin Resource Policy" and MDN's article on this header.

// Sets "Cross-Origin-Resource-Policy: same-origin"
app.use(helmet());

// Sets "Cross-Origin-Resource-Policy: same-site"
app.use(helmet({ crossOriginResourcePolicy: { policy: "same-site" } }));

To disable the Cross-Origin-Resource-Policy header:

app.use(
  helmet({
    crossOriginResourcePolicy: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.crossOriginResourcePolicy()).

</details> <details id="origin-agent-cluster"> <summary><code>Origin-Agent-Cluster</code></summary>

Default:

Origin-Agent-Cluster: ?1

The Origin-Agent-Cluster header provides a mechanism to allow web applications to isolate their origins from other processes. Read more about it in the spec.

This header takes no options and is set by default.

// Sets "Origin-Agent-Cluster: ?1"
app.use(helmet());

To disable the Origin-Agent-Cluster header:

app.use(
  helmet({
    originAgentCluster: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.originAgentCluster()).

</details> <details id="referrer-policy"> <summary><code>Referrer-Policy</code></summary>

Default:

Referrer-Policy: no-referrer

The Referrer-Policy header which controls what information is set in [the Referer request header][Referer]. See "Referer header: privacy and security concerns" and the header's documentation on MDN for more.

// Sets "Referrer-Policy: no-referrer"
app.use(helmet());

policy is a string or array of strings representing the policy. If passed as an array, it will be joined with commas, which is useful when setting a fallback policy. It defaults to no-referrer.

// Sets "Referrer-Policy: no-referrer"
app.use(
  helmet({
    referrerPolicy: {
      policy: "no-referrer",
    },
  }),
);

// Sets "Referrer-Policy: origin,unsafe-url"
app.use(
  helmet({
    referrerPolicy: {
      policy: ["origin", "unsafe-url"],
    },
  }),
);

To disable the Referrer-Policy header:

app.use(
  helmet({
    referrerPolicy: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.referrerPolicy()).

</details> <details id="strict-transport-security"> <summary><code>Strict-Transport-Security</code></summary>

Default:

Strict-Transport-Security: max-age=15552000; includeSubDomains

The Strict-Transport-Security header tells browsers to prefer HTTPS instead of insecure HTTP. See the documentation on MDN for more.

// Sets "Strict-Transport-Security: max-age=15552000; includeSubDomains"
app.use(helmet());

maxAge is the number of seconds browsers should remember to prefer HTTPS. If passed a non-integer, the value is rounded down. It defaults to 15552000, which is 180 days.

includeSubDomains is a boolean which dictates whether to include the includeSubDomains directive, which makes this policy extend to subdomains. It defaults to true.

preload is a boolean. If true, it adds the preload directive, expressing intent to add your HSTS policy to browsers. See the "Preloading Strict Transport Security" section on MDN for more. It defaults to false.

// Sets "Strict-Transport-Security: max-age=123456; includeSubDomains"
app.use(
  helmet({
    strictTransportSecurity: {
      maxAge: 123456,
    },
  }),
);

// Sets "Strict-Transport-Security: max-age=123456"
app.use(
  helmet({
    strictTransportSecurity: {
      maxAge: 123456,
      includeSubDomains: false,
    },
  }),
);

// Sets "Strict-Transport-Security: max-age=123456; includeSubDomains; preload"
app.use(
  helmet({
    strictTransportSecurity: {
      maxAge: 63072000,
      preload: true,
    },
  }),
);

To disable the Strict-Transport-Security header:

app.use(
  helmet({
    strictTransportSecurity: false,
  }),
);

You may wish to disable this header for local development, as it can make your browser force redirects from http://localhost to https://localhost, which may not be desirable if you develop multiple apps using localhost. See this issue for more discussion.

You can use this as standalone middleware with app.use(helmet.strictTransportSecurity()).

</details> <details id="x-content-type-options"> <summary><code>X-Content-Type-Options</code></summary>

Default:

X-Content-Type-Options: nosniff

The X-Content-Type-Options mitigates MIME type sniffing which can cause security issues. See documentation for this header on MDN for more.

This header takes no options and is set by default.

// Sets "X-Content-Type-Options: nosniff"
app.use(helmet());

To disable the X-Content-Type-Options header:

app.use(
  helmet({
    xContentTypeOptions: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.xContentTypeOptions()).

</details> <details id="x-dns-prefetch-control"> <summary><code>X-DNS-Prefetch-Control</code></summary>

Default:

X-DNS-Prefetch-Control: off

The X-DNS-Prefetch-Control header helps control DNS prefetching, which can improve user privacy at the expense of performance. See documentation on MDN for more.

// Sets "X-DNS-Prefetch-Control: off"
app.use(helmet());

allow is a boolean dictating whether to enable DNS prefetching. It defaults to false.

Examples:

// Sets "X-DNS-Prefetch-Control: off"
app.use(
  helmet({
    xDnsPrefetchControl: { allow: false },
  }),
);

// Sets "X-DNS-Prefetch-Control: on"
app.use(
  helmet({
    xDnsPrefetchControl: { allow: true },
  }),
);

To disable the X-DNS-Prefetch-Control header and use the browser's default value:

app.use(
  helmet({
    xDnsPrefetchControl: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.xDnsPrefetchControl()).

</details> <details id="x-download-options"> <summary><code>X-Download-Options</code></summary>

Default:

X-Download-Options: noopen

The X-Download-Options header is specific to Internet Explorer 8. It forces potentially-unsafe downloads to be saved, mitigating execution of HTML in your site's context. For more, see this old post on MSDN.

This header takes no options and is set by default.

// Sets "X-Download-Options: noopen"
app.use(helmet());

To disable the X-Download-Options header:

app.use(
  helmet({
    xDownloadOptions: false,
  }),
);

You can use this as standalone middleware with app.use(helmet.xDownloadOptions()).

</details> <details id="x-frame-options"> <summary><code>X-Frame-Options</code></summary>

Default:

X-Frame-Options: SAMEORIGIN

The legacy X-Frame-Options header to help you mitigate clickjacking attacks. This header is superseded by the frame-ancestors Content Security Policy directive but is still