helmet

What is helmet?

Helmet is a middleware for Express applications that helps secure your apps by setting various HTTP headers. It's not a silver bullet, but it can help prevent some well-known web vulnerabilities by setting headers appropriately.

What are helmet's main functionalities?

Content Security Policy

Sets the Content-Security-Policy header to help prevent cross-site scripting attacks and other cross-site injections.

app.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'", "https://trustedscripts.example.com"],
    objectSrc: ["'none'"],
    upgradeInsecureRequests: [],
  }
}));

X-DNS-Prefetch-Control

Controls browser DNS prefetching, which can improve user privacy at the expense of performance.

app.use(helmet.dnsPrefetchControl({ allow: false }));

Expect-CT

Sets the Expect-CT header which allows sites to opt in to reporting and/or enforcement of Certificate Transparency requirements.

app.use(helmet.expectCt({
  enforce: true,
  maxAge: 86400
}));

X-Frame-Options

Sets the X-Frame-Options header to control whether the browser should be allowed to render a page in a <frame>, <iframe>, <embed>, or <object>.

app.use(helmet.frameguard({ action: 'deny' }));

X-Powered-By

Removes the X-Powered-By header to make it slightly harder for attackers to see what potentially vulnerable technology powers your site.

app.use(helmet.hidePoweredBy());

Strict-Transport-Security

Sets the Strict-Transport-Security header to enforce secure (HTTP over SSL/TLS) connections to the server.

app.use(helmet.hsts({
  maxAge: 15552000,
  includeSubDomains: true
}));

X-Download-Options

Sets X-Download-Options for IE8+ to prevent others from embedding your site in an iframe.

app.use(helmet.ieNoOpen());

X-Content-Type-Options

Sets the X-Content-Type-Options header to prevent browsers from MIME-sniffing a response away from the declared content-type.

app.use(helmet.noSniff());

Referrer Policy

Sets the Referrer-Policy header to control what information is sent along with the requests.

app.use(helmet.referrerPolicy({ policy: 'no-referrer' }));

X-XSS-Protection

Sets the X-XSS-Protection header to enable the Cross-site scripting (XSS) filter in most recent web browsers.

app.use(helmet.xssFilter());

Other packages similar to helmet

lusca

Lusca is another security middleware for Express applications that offers a variety of security features such as CSRF protection, CSP, X-Frame options, and more. It is similar to Helmet but allows for more granular configuration of security policies.

cors

CORS is a package for providing a Connect/Express middleware that can be used to enable CORS (Cross-Origin Resource Sharing) with various options. While Helmet focuses on securing your app from various web vulnerabilities, CORS specifically provides middleware to enable CORS and manage cross-origin requests.

README

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 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, 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

Content-Security-Policy

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. 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()).

Cross-Origin-Embedder-Policy

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()).

Cross-Origin-Opener-Policy

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()).

Cross-Origin-Resource-Policy

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()).

Origin-Agent-Cluster

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()).

Referrer-Policy

Default:

Referrer-Policy: no-referrer

The Referrer-Policy header which controls what information is set in the Referer request header. 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()).

Strict-Transport-Security

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()).

X-Content-Type-Options

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()).

X-DNS-Prefetch-Control

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()).

X-Download-Options

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()).

X-Frame-Options

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 useful on old browsers or if no CSP is used. For more, see the documentation on MDN.

// Sets "X-Frame-Options: SAMEORIGIN"
app.use(helmet());

action is a string that specifies which directive to use—either DENY or SAMEORIGIN. (A legacy directive, ALLOW-FROM, is not supported by Helmet. Read more here.) It defaults to SAMEORIGIN.

Examples:

// Sets "X-Frame-Options: DENY"
app.use(
  helmet({
    xFrameOptions: { action: "deny" },
  }),
);

// Sets "X-Frame-Options: SAMEORIGIN"
app.use(
  helmet({
    xFrameOptions: { action: "sameorigin" },
  }),
);

To disable the X-Frame-Options header:

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

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

X-Permitted-Cross-Domain-Policies

Default:

X-Permitted-Cross-Domain-Policies: none

The X-Permitted-Cross-Domain-Policies header tells some clients (mostly Adobe products) your domain's policy for loading cross-domain content. See the description on OWASP for more.

// Sets "X-Permitted-Cross-Domain-Policies: none"
app.use(helmet());

permittedPolicies is a string that must be "none", "master-only", "by-content-type", or "all". It defaults to "none".

Examples:

// Sets "X-Permitted-Cross-Domain-Policies: none"
app.use(
  helmet({
    xPermittedCrossDomainPolicies: {
      permittedPolicies: "none",
    },
  }),
);

// Sets "X-Permitted-Cross-Domain-Policies: by-content-type"
app.use(
  helmet({
    xPermittedCrossDomainPolicies: {
      permittedPolicies: "by-content-type",
    },
  }),
);

To disable the X-Permitted-Cross-Domain-Policies header:

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

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

X-Powered-By

Default: the X-Powered-By header, if present, is removed.

Helmet removes the X-Powered-By header, which is set by default in Express and some other frameworks. Removing the header offers very limited security benefits (see this discussion) and is mostly removed to save bandwidth, but may thwart simplistic attackers.

Note: Express has a built-in way to disable the X-Powered-By header, which you may wish to use instead.

The removal of this header takes no options. The header is removed by default.

To disable this behavior:

// Not required, but recommended for Express users:
app.disable("x-powered-by");

// Ask Helmet to ignore the X-Powered-By header.
app.use(
  helmet({
    xPoweredBy: false,
  }),
);

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

X-XSS-Protection

Default:

X-XSS-Protection: 0

Helmet disables browsers' buggy cross-site scripting filter by setting the legacy X-XSS-Protection header to 0. See discussion about disabling the header here and documentation on MDN.

This header takes no options and is set by default.

To disable the X-XSS-Protection header:

// This is not recommended.
app.use(
  helmet({
    xXssProtection: false,
  }),
);

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

Changelog

7.2.0 - 2024-09-28

Changed

• Content-Security-Policy middleware now warns if a directive should have quotes but does not, such as self instead of 'self'. This will be an error in future versions. See #454