# Helmet
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[](https://david-dm.org/helmetjs/helmet)
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Helmet helps you secure your Express apps by setting various HTTP headers. _It's not a silver bullet_, but it can help!
## Quick start
First, run `npm install helmet --save` for your app. Then, in an Express app:
```js
const express = require("express");
const helmet = require("helmet");
const app = express();
app.use(helmet());
// ...
```
## How it works
Helmet is [Connect](https://github.com/senchalabs/connect)-style middleware, which is compatible with frameworks like [Express](https://expressjs.com/). (If you need support for Koa, see [`koa-helmet`](https://github.com/venables/koa-helmet).)
The top-level `helmet` function is a wrapper around 11 smaller middlewares.
In other words, these two things are equivalent:
```js
// This...
app.use(helmet());
// ...is equivalent to this:
app.use(helmet.contentSecurityPolicy());
app.use(helmet.dnsPrefetchControl());
app.use(helmet.expectCt());
app.use(helmet.frameguard());
app.use(helmet.hidePoweredBy());
app.use(helmet.hsts());
app.use(helmet.ieNoOpen());
app.use(helmet.noSniff());
app.use(helmet.permittedCrossDomainPolicies());
app.use(helmet.referrerPolicy());
app.use(helmet.xssFilter());
```
To set custom options for one of the middleware, add options like this:
```js
// This sets custom options for the `referrerPolicy` middleware.
app.use(
helmet({
referrerPolicy: { policy: "no-referrer" },
})
);
```
You can also disable a middleware:
```js
// This disables the `contentSecurityPolicy` middleware but keeps the rest.
app.use(
helmet({
contentSecurityPolicy: false,
})
);
```
## Reference
helmet(options)
Helmet is the top-level middleware for this module, including all 11 others.
All 11 middlewares are enabled by default.
```js
// Includes all 11 middlewares
app.use(helmet());
```
If you want to disable one, pass options to `helmet`. For example, to disable `frameguard`:
```js
// Includes 10 middlewares, skipping `helmet.frameguard`
app.use(
helmet({
frameguard: false,
})
);
```
Most of the middlewares have options, which are documented in more detail below. For example, to pass `{ action: "deny" }` to `frameguard`:
```js
// Includes all 11 middlewares, setting an option for `helmet.frameguard`
app.use(
helmet({
frameguard: {
action: "deny",
},
})
);
```
Each middleware's name is listed below.
helmet.contentSecurityPolicy(options)
`helmet.contentSecurityPolicy` sets the `Content-Security-Policy` header which helps mitigate cross-site scripting attacks, among other things. See [MDN's introductory article on Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP).
This middleware performs very little validation. You should rely on CSP checkers like [CSP Evaluator](https://csp-evaluator.withgoogle.com/) instead.
`options.directives` is an object. Each key is a directive name in camel case (such as `defaultSrc`) or kebab case (such as `default-src`). Each value is an iterable (usually an array) of strings or functions for that directive. If a function appears in the iterable, it will be called with the request and response.
`options.reportOnly` is a boolean, defaulting to `false`. If `true`, [the `Content-Security-Policy-Report-Only` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy-Report-Only) will be set instead.
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 fetch this default with `helmet.contentSecurityPolicy.getDefaultDirectives()`.
Examples:
```js
// Sets "Content-Security-Policy: default-src 'self';script-src 'self' example.com;object-src 'none';upgrade-insecure-requests"
app.use(
helmet.contentSecurityPolicy({
directives: {
defaultSrc: ["'self'"],
scriptSrc: ["'self'", "example.com"],
objectSrc: ["'none'"],
upgradeInsecureRequests: [],
},
})
);
// Sets "Content-Security-Policy: default-src 'self';script-src 'self' example.com;object-src 'none'"
app.use(
helmet.contentSecurityPolicy({
directives: {
"default-src": ["'self'"],
"script-src": ["'self'", "example.com"],
"object-src": ["'none'"],
},
})
);
// Sets all of the defaults, but overrides script-src
app.use(
helmet.contentSecurityPolicy({
directives: {
...helmet.contentSecurityPolicy.getDefaultDirectives(),
"script-src": ["'self'", "example.com"],
},
})
);
// Sets the "Content-Security-Policy-Report-Only" header instead
app.use(
helmet.contentSecurityPolicy({
directives: {
/* ... */
},
reportOnly: true,
})
);
// Sets "Content-Security-Policy: default-src 'self';script-src 'self' 'nonce-e33ccde670f149c1789b1e1e113b0916'"
app.use((req, res, next) => {
res.locals.cspNonce = crypto.randomBytes(16).toString("hex");
next();
});
app.use(
helmet.contentSecurityPolicy({
directives: {
defaultSrc: ["'self'"],
scriptSrc: ["'self'", (req, res) => `'nonce-${res.locals.cspNonce}'`],
},
})
);
```
You can install this module separately as `helmet-csp`.
helmet.expectCt(options)
`helmet.expectCt` sets the `Expect-CT` header which helps mitigate misissued SSL certificates. See [MDN's article on Certificate Transparency](https://developer.mozilla.org/en-US/docs/Web/Security/Certificate_Transparency) and the [`Expect-CT` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expect-CT) for more.
`options.maxAge` is the number of seconds to expect Certificate Transparency. It defaults to `0`.
`options.enforce` is a boolean. If `true`, the user agent (usually a browser) should refuse future connections that violate its Certificate Transparency policy. Defaults to `false`.
`options.reportUri` is a string. If set, complying user agents will report Certificate Transparency failures to this URL. Unset by default.
Examples:
```js
// Sets "Expect-CT: max-age=86400"
app.use(
helmet.expectCt({
maxAge: 86400,
})
);
// Sets "Expect-CT: max-age=86400, enforce, report-uri="https://example.com/report"
app.use(
helmet.expectCt({
maxAge: 86400,
enforce: true,
reportUri: "https://example.com/report",
})
);
```
You can install this module separately as `expect-ct`.
helmet.referrerPolicy(options)
`helmet.referrerPolicy` sets the `Referrer-Policy` header which controls what information is set in [the `Referer` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referer). See ["Referer header: privacy and security concerns"](https://developer.mozilla.org/en-US/docs/Web/Security/Referer_header:_privacy_and_security_concerns) and [the header's documentation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy) on MDN for more.
`options.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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy#Specifying_a_fallback_policy). It defaults to `no-referrer`.
Examples:
```js
// 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"],
})
);
```
You can install this module separately as `referrer-policy`.
helmet.hsts(options)
`helmet.hsts` sets the `Strict-Transport-Security` header which tells browsers to prefer HTTPS over insecure HTTP. See [the documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security) for more.
`options.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.
`options.includeSubDomains` is a boolean which dictates whether to include the `includeSubDomains` directive, which makes this policy extend to subdomains. It defaults to `true`.
`options.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](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security#Preloading_Strict_Transport_Security) for more. It defaults to `false`.
Examples:
```js
// Sets "Strict-Transport-Security: max-age=123456; includeSubDomains"
app.use(
helmet.hsts({
maxAge: 123456,
})
);
// Sets "Strict-Transport-Security: max-age=123456"
app.use(
helmet.hsts({
maxAge: 123456,
includeSubDomains: false,
})
);
// Sets "Strict-Transport-Security: max-age=123456; includeSubDomains; preload"
app.use(
helmet.hsts({
maxAge: 63072000,
preload: true,
})
);
```
You can install this module separately as `hsts`.
helmet.noSniff()
`helmet.noSniff` sets the `X-Content-Type-Options` header to `nosniff`. This mitigates [MIME type sniffing](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types#MIME_sniffing) which can cause security vulnerabilities. See [documentation for this header on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Content-Type-Options) for more.
This middleware takes no options.
Example:
```js
// Sets "X-Content-Type-Options: nosniff"
app.use(helmet.noSniff());
```
You can install this module separately as `dont-sniff-mimetype`.
helmet.dnsPrefetchControl(options)
`helmet.dnsPrefetchControl` sets the `X-DNS-Prefetch-Control` header to help control DNS prefetching, which can improve user privacy at the expense of performance. See [documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-DNS-Prefetch-Control) for more.
`options.allow` is a boolean dictating whether to enable DNS prefetching. It defaults to `false`.
Examples:
```js
// Sets "X-DNS-Prefetch-Control: off"
app.use(
helmet.dnsPrefetchControl({
allow: false,
})
);
// Sets "X-DNS-Prefetch-Control: on"
app.use(
helmet.dnsPrefetchControl({
allow: true,
})
);
```
You can install this module separately as `dns-prefetch-control`.
helmet.ieNoOpen()
`helmet.ieNoOpen` sets the `X-Download-Options` header, which 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](https://docs.microsoft.com/en-us/archive/blogs/ie/ie8-security-part-v-comprehensive-protection).
This middleware takes no options.
Examples:
```js
// Sets "X-Download-Options: noopen"
app.use(helmet.ieNoOpen());
```
You can install this module separately as `ienoopen`.
helmet.frameguard(options)
`helmet.frameguard` sets the `X-Frame-Options` header to help you mitigate [clickjacking attacks](https://en.wikipedia.org/wiki/Clickjacking). This header is superseded by [the `frame-ancestors` Content Security Policy directive](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors) but is still useful on old browsers. For more, see [the documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options).
`options.action` is a string that specifies which directive to use—either `DENY` or `SAMEORIGIN`. (A legacy directive, `ALLOW-FROM`, is not supported by this middleware. [Read more here.](https://github.com/helmetjs/helmet/wiki/How-to-use-X%E2%80%93Frame%E2%80%93Options's-%60ALLOW%E2%80%93FROM%60-directive)) It defaults to `SAMEORIGIN`.
Examples:
```js
// Sets "X-Frame-Options: DENY"
app.use(
helmet.frameguard({
action: "deny",
})
);
// Sets "X-Frame-Options: SAMEORIGIN"
app.use(
helmet.frameguard({
action: "sameorigin",
})
);
```
You can install this module separately as `frameguard`.
helmet.permittedCrossDomainPolicies(options)
`helmet.permittedCrossDomainPolicies` sets the `X-Permitted-Cross-Domain-Policies` header, which tells some clients (mostly Adobe products) your domain's policy for loading cross-domain content. See [the description on OWASP](https://owasp.org/www-project-secure-headers/) for more.
`options.permittedPolicies` is a string that must be `"none"`, `"master-only"`, `"by-content-type"`, or `"all"`. It defaults to `"none"`.
Examples:
```js
// Sets "X-Permitted-Cross-Domain-Policies: none"
app.use(
helmet.permittedCrossDomainPolicies({
permittedPolicies: "none",
})
);
// Sets "X-Permitted-Cross-Domain-Policies: by-content-type"
app.use(
helmet.permittedCrossDomainPolicies({
permittedPolicies: "by-content-type",
})
);
```
You can install this module separately as `helmet-crossdomain`.
helmet.hidePoweredBy()
`helmet.hidePoweredBy` removes the `X-Powered-By` header, which is set by default in some frameworks (like Express). Removing the header offers very limited security benefits (see [this discussion](https://github.com/expressjs/express/pull/2813#issuecomment-159270428)) and is mostly removed to save bandwidth.
This middleware takes no options.
If you're using Express, this middleware will work, but you should use `app.disable("x-powered-by")` instead.
Examples:
```js
// Removes the X-Powered-By header if it was set.
app.use(helmet.hidePoweredBy());
```
You can install this module separately as `hide-powered-by`.
helmet.xssFilter()
`helmet.xssFilter` disables browsers' buggy cross-site scripting filter by setting the `X-XSS-Protection` header to `0`. See [discussion about disabling the header here](https://github.com/helmetjs/helmet/issues/230) and [documentation on MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection).
This middleware takes no options.
Examples:
```js
// Sets "X-XSS-Protection: 0"
app.use(helmet.xssFilter());
```
You can install this module separately as `x-xss-protection`.