helmet

Security middleware collection for Express/Connect

Helmet

Helmet is a series of middleware that help secure your Express/Connect apps. It's not a silver bullet, but it can help!

First, run npm install helmet --save for your app. Then, in an Express app:

var express = require('express');
var helmet = require('helmet');
 
var app = express();
 
app.use(helmet());
 
// ... 

You can also use them individually:

app.use(helmet.noCache());
app.use(helmet.frameguard());

If you're using Express 3, make sure these middlewares are listed before app.router.

Helmet is really just a collection of 9 smaller middleware functions:

Running app.use(helmet()) will include 7 of the 9, leaving out contentSecurityPolicy and noCache. You can also use each module individually, as documented below.

For each of the middlewares, we'll talk about three things:

  1. What's the attack we're trying to prevent?
  2. How do we use Helmet to help mitigate those issues?
  3. What are the non-obvious limitations of this middleware?

Let's get started.

Trying to prevent: Injecting anything unintended into our page. That could cause XSS vulnerabilities, unintended tracking, malicious frames, and more.

How to use Helmet to mitigate this: Set an appropriate Content Security Policy. If you want to learn how CSP works, check out the fantastic HTML5 Rocks guide and the Content Security Policy Reference.

Usage:

app.use(helmet.contentSecurityPolicy({
  defaultSrc: ["'self'", 'default.com'],
  scriptSrc: ['scripts.com'],
  styleSrc: ['style.com'],
  imgSrc: ['img.com'],
  connectSrc: ['connect.com'],
  fontSrc: ['font.com'],
  objectSrc: ['object.com'],
  mediaSrc: ['media.com'],
  frameSrc: ['frame.com'],
  sandbox: ['allow-forms', 'allow-scripts'],
  reportUri: '/report-violation',
  reportOnly: false, // set to true if you only want to report errors 
  setAllHeaders: false, // set to true if you want to set all headers 
  disableAndroid: false, // set to true if you want to disable Android (browsers can vary and be buggy) 
  safari5: false // set to true if you want to force buggy CSP in Safari 5 
}));

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

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 1.0 spec.

Note: If you're using the reportUri feature and you're using csurf, you might have errors. Check this out for a workaround.

Limitations: CSP is often difficult to tune properly, as it's a whitelist and not a blacklist. It isn't supported on old browsers but is pretty well-supported on non-IE browsers nowadays.

Trying to prevent: Cross-site scripting attacks (XSS), a subset of the above.

How to use Helmet to mitigate this: The X-XSS-Protection HTTP header is a basic protection against XSS. It was originally by Microsoft but Chrome has since adopted it as well. Helmet lets you use it easily:

app.use(helmet.xssFilter());

This sets the X-XSS-Protection header. On modern browsers, it will set the value to 1; mode=block. On old versions of Internet Explorer, this creates a vulnerability (see here and here), and so the header is set to 0 to disable it. To force the header on all versions of IE, add the option:

app.use(helmet.xssFilter({ setOnOldIE: true }));
// This has some security problems for old IE! 

Limitations: This isn't anywhere near as thorough as CSP. It's only properly supported on IE9+ and Chrome; no other major browsers support it at this time. Old versions of IE support it in a buggy way, which we disable by default.

Trying to prevent: Your page being put in a <frame> or <iframe> without your consent. This can result in clickjacking attacks, among other things.

How to use Helmet to mitigate this: The X-Frame HTTP header restricts who can put your site in a frame. It has three modes: DENY, SAMEORIGIN, and ALLOW-FROM. If your app does not need to be framed (and most don't) you can use the default DENY. If your site can be in frames from the same origin, you can set it to SAMEORIGIN. If you want to allow it from a specific URL, you can allow that with ALLOW-FROM and a URL.

Usage:

// Only let me be framed by people of the same origin: 
app.use(helmet.frameguard('sameorigin'));
app.use(helmet.frameguard());  // Same-origin by default. 
 
// Don't allow anyone to put me in a frame. 
app.use(helmet.frameguard('deny'));
 
// Allow from a specific host: 
app.use(helmet.frameguard('allow-from', 'http://example.com'));

Limitations: This has pretty good (but not 100%) browser support: IE8+, Opera 10.50+, Safari 4+, Chrome 4.1+, and Firefox 3.6.9+. It only prevents against a certain class of attack, but does so pretty well. It also prevents your site from being framed, which you might want for legitimate reasons.

Trying to prevent: Users viewing your site on HTTP instead of HTTPS. HTTP is pretty insecure.

How to use Helmet to mitigate this: This middleware adds the Strict-Transport-Security header to the response. This tells browsers, "hey, only use HTTPS for the next period of time". (See the spec for more.)

This will set the Strict Transport Security header, telling browsers to visit by HTTPS for the next ninety days:

var ninetyDaysInMilliseconds = 7776000000;
app.use(helmet.hsts({ maxAge: ninetyDaysInMilliseconds }));

You can also include subdomains. If this is set on example.com, supported browsers will also use HTTPS on my-subdomain.example.com. Here's how you do that:

app.use(helmet.hsts({
  maxAge: 123000,
  includeSubdomains: true
}));

Chrome lets you submit your site for baked-into-Chrome HSTS by adding preload to the header. You can add that with the following code, and then submit your site to the Chrome team at hstspreload.appspot.com.

app.use(helmet.hsts({
  maxAge: 10886400000,     // Must be at least 18 weeks to be approved by Google 
  includeSubdomains: true, // Must be enabled to be approved by Google 
  preload: true
}));

This'll be set if req.secure is true, a boolean auto-populated by Express. If you're not using Express, that value won't necessarily be set, so you have two options:

// Set the header based on silly conditions 
app.use(helmet.hsts({
  maxAge: 1234000,
  setIffunction(reqres) {
    return Math.random() < 0.5;
  }
}));
 
// ALWAYS set the header 
app.use(helmet.hsts({
  maxAge: 1234000,
  force: true
}));

Note that the max age is in milliseconds, even though the spec uses seconds. This will round to the nearest full second.

Limitations: This only works if your site actually has HTTPS. It won't tell users on HTTP to switch to HTTPS, it will just tell HTTPS users to stick around. You can enforce this with the express-enforces-ssl module. It's somewhat well-supported by browsers.

Trying to prevent: Hackers can exploit known vulnerabilities in Express/Node if they see that your site is powered by Express (or whichever framework you use). X-Powered-By: Express is sent in every HTTP request coming from Express, by default.

How to use Helmet to mitigate this: The hidePoweredBy middleware will remove the X-Powered-By header if it is set (which it will be by default in Express).

app.use(helmet.hidePoweredBy());

You can also explicitly set the header to something else, if you want. This could throw people off:

app.use(helmet.hidePoweredBy({ setTo: 'PHP 4.2.0' }));

Note: if you're using Express, you can skip Helmet's middleware if you want:

app.disable('x-powered-by');

Limitations: There might be other telltale signs that your site is Express-based (a blog post about your tech stack, for example). This might prevent hackers from easily exploiting known vulnerabilities in your stack, but that's all it does.

Trying to prevent: Some web applications will serve untrusted HTML for download. By default, some versions of IE will allow you to open those HTML files in the context of your site, which means that an untrusted HTML page could start doing bad things in the context of your pages. For more, see this MSDN blog post.

How to use Helmet to mitigate this: Set the X-Download-Options header to noopen to prevent IE users from executing downloads in your site's context.

app.use(helmet.ieNoOpen());

Limitations: This is pretty obscure, fixing a small bug on IE only. No real drawbacks other than performance/bandwidth of setting the headers, though.

Trying to prevent: Some browsers will try to "sniff" mimetypes. For example, if my server serves file.txt with a text/plain content-type, some browsers can still run that file with <script src="file.txt"></script>. Many browsers will allow file.js to be run even if the content-type isn't for JavaScript. There are some other vulnerabilities, too.

How to use Helmet to mitigate this: Use Helmet's noSniff middleware to keep Chrome, Opera, and IE from doing this sniffing (and Firefox soon). The following example sets the X-Content-Type-Options header to its only option, nosniff:

app.use(helmet.noSniff());

MSDN has a good description of how browsers behave when this header is sent.

Limitations: This only prevents against a certain kind of attack.

Trying to prevent: Users caching your old, buggy resources. It's possible that you've got bugs in an old HTML or JavaScript file, and with a cache, some users will be stuck with those old versions.

How to use Helmet to mitigate this: Use Helmet to disable this kind of caching. This sets a number of HTTP headers that stop caching.

app.use(helmet.noCache());

This will set Cache-Control and Pragma headers to stop caching. It will also set an Expires header of 0, effectively saying "this has already expired."

If you want to crush the ETag header as well, you can:

app.use(helmet.noCache({ noEtag: true }));

Limitations: Caching has some real benefits, and you lose them here (which is why it's disabled in the default configuration). Browsers won't cache resources with this enabled, although some performance is retained if you keep ETag support. It's also possible that you'll introduce new bugs and you'll wish people had old resources cached, but that's less likely.

Trying to prevent: Adobe defines the spec for crossdomain.xml, a policy file that grants some Adobe products (like Flash) read access to resources on your domain. An unrestrictive policy could let others load things off your domain that you don't want.

How to use Helmet to mitigate this: Simply use Helmet's crossdomain middleware to serve up a restrictive policy:

app.use(helmet.crossdomain());

This serves the policy at /crossdomain.xml. By default, this is case-insensitive. To make it case-sensitive:

app.use(helmet.crossdomain({ caseSensitive: true }));
// This will now ONLY match all-lowercase /crossdomain.xml. 

Limitations: This doesn't make you wildly more secure, but it does help to keep Flash from loading things that you don't want it to. You might also want some of this behavior, in which case you should make your own less-restrictive policy and serve it.