host-validation-middleware

Middleware for validating host headers in requests to protect against DNS rebinding attacks.

[!NOTE] DNS rebinding attacks are not effective against HTTPS sites. Since HTTPS is now commonly used for production environments, this middleware is generally unnecessary for production sites.

Install

npm i -D host-validation-middleware # pnpm add -D host-validation-middleware

Usage

hostValidationMiddleware

This middleware is compatible with Connect and frameworks like Express that support Connect-style middleware.

import connect from 'connect'
import { hostValidationMiddleware } from 'host-validation-middleware'

const app = connect()

app.use(
  hostValidationMiddleware({
    // Values starting with `.` will allow all the subdomains under that domain
    allowedHosts: Object.freeze(['example.com', '.mydomain.com']),
    // Optionally customize the error message:
    generateErrorMessage: hostname => `Access denied for host: ${hostname}`,
    // Optionally set the error response content type:
    errorResponseContentType: 'text/plain'
  })
)

app.use((req, res) => {
  res.end('Hello, world!')
})

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000')
})

If the host header is not in the allowed hosts list, a 403 Forbidden response is sent.

isHostAllowed

You can also use the core host validation logic directly:

import { isHostAllowed } from 'host-validation-middleware'

const allowedHosts = Object.freeze(['example.com', '.mydomain.com'])

console.log(isHostAllowed('example.com', allowedHosts)) // true
console.log(isHostAllowed('sub.mydomain.com', allowedHosts)) // true
console.log(isHostAllowed('evil.com', allowedHosts)) // false

This function will cache the result if the allowedHosts array is frozen.

Allowed Hosts

The host names listed in the allowedHosts options will be allowed. If the host name starts with a dot, the domain without the dot and any subdomain of it will be allowed.

• Example: With allowedHosts: ['example.com', '.mydomain.com']:
  • Requests to example.com are allowed.
  • Requests to mydomain.com, foo.mydomain.com, bar.foo.mydomain.com are also allowed.

Also the following hosts that cannot be used for DNS rebinding attacks are always allowed:

• Any localhost or subdomain of localhost (e.g., localhost, foo.localhost)
• Any IPv4 or IPv6 address (e.g., 127.0.0.1, [::1])
• Any host using the file: or browser extension protocol

Credits

The API interface and the original implementation is based on webpack-dev-server's allowedHosts option.