@fastify/cors

@fastify/cors enables the use of CORS in a Fastify application.

Install

npm i @fastify/cors

Compatibility

Please note that if a Fastify version is out of support, then so are the corresponding version(s) of this plugin in the table above. See Fastify's LTS policy for more details.

Usage

Require @fastify/cors and register it as any other plugin, it will add an onRequest hook and a wildcard options route.

import Fastify from 'fastify'
import cors from '@fastify/cors'

const fastify = Fastify()
await fastify.register(cors, { 
  // put your options here
})

fastify.get('/', (req, reply) => {
  reply.send({ hello: 'world' })
})

await fastify.listen({ port: 3000 })

You can use it as is without passing any option or you can configure it as explained below.

Options

• origin: Configures the Access-Control-Allow-Origin CORS header. The value of origin could be of different types:
  • Boolean - set origin to true to reflect the request origin, or set it to false to disable CORS.
  • String - set origin to a specific origin. For example if you set it to "https://example.com" only requests from "https://example.com" will be allowed. The special * value (default) allows any origin.
  • RegExp - set origin to a regular expression pattern that will be used to test the request origin. If it is a match, the request origin will be reflected. For example, the pattern /example\.com$/ will reflect any request that is coming from an origin ending with "example.com".
  • Array - set origin to an array of valid origins. Each origin can be a String or a RegExp. For example ["https://example1.com", /\.example2\.com$/] will accept any request from "https://example1.com" or from a subdomain of "example2.com".
  • Function - set origin to a function implementing some custom logic. The function takes the request origin as the first parameter and a callback as a second (which expects the signature err [Error | null], origin), where origin is a non-function value of the origin option. Async-await and promises are supported as well. The Fastify instance is bound to function call and you may access via this. For example:

  origin: (origin, cb) => {
    const hostname = new URL(origin).hostname
    if(hostname === "localhost"){
      //  Request from localhost will pass
      cb(null, true)
      return
    }
    // Generate an error on other origins, disabling access
    cb(new Error("Not allowed"), false)
  }
  

• methods: Configures the Access-Control-Allow-Methods CORS header. Expects a comma-delimited string (ex: 'GET,PUT,POST') or an array (ex: ['GET', 'PUT', 'POST']); (default: GET,HEAD,PUT,PATCH,POST,DELETE)
• hook: See the section Custom Fastify hook name (default: onRequest)
• allowedHeaders: Configures the Access-Control-Allow-Headers CORS header. Expects a comma-delimited string (ex: 'Content-Type,Authorization') or an array (ex: ['Content-Type', 'Authorization']). If not specified, defaults to reflecting the headers specified in the request's Access-Control-Request-Headers header.
• exposedHeaders: Configures the Access-Control-Expose-Headers CORS header. Expects a comma-delimited string (ex: 'Content-Range,X-Content-Range') or an array (ex: ['Content-Range', 'X-Content-Range']). If not specified, no custom headers are exposed.
• credentials: Configures the Access-Control-Allow-Credentials CORS header. Set to true to pass the header, otherwise it is omitted.
• maxAge: Configures the Access-Control-Max-Age CORS header. In seconds. Set to an integer to pass the header, otherwise it is omitted.
• cacheControl: Configures the Cache-Control header for CORS preflight responses. Set to an integer to pass the header as Cache-Control: max-age=${cacheControl}, or set to a string to pass the header as Cache-Control: ${cacheControl} (fully define the header value), otherwise the header is omitted.
• preflightContinue: Pass the CORS preflight response to the route handler (default: false).
• optionsSuccessStatus: Provides a status code to use for successful OPTIONS requests, since some legacy browsers (IE11, various SmartTVs) choke on 204.
• preflight: if needed you can entirely disable preflight by passing false here (default: true).
• strictPreflight: Enforces strict requirement of the CORS preflight request headers (Access-Control-Request-Method and Origin) as defined by the W3C CORS specification (the current fetch living specification does not define server behavior for missing headers). Preflight requests without the required headers will result in 400 errors when set to true (default: true).
• hideOptionsRoute: hide options route from the documentation built using @fastify/swagger (default: true).

⚠️ DoS attacks

The use of RegExp or a function for the origin parameter might allow an attacker to perform a Denial of Service attack. Craft those with extreme care.

Configuring CORS Asynchronously

const fastify = require('fastify')()

fastify.register(require('@fastify/cors'), (instance) => {
  return (req, callback) => {
    const corsOptions = {
      // This is NOT recommended for production as it enables reflection exploits
      origin: true
    };

    // do not include CORS headers for requests from localhost
    if (/^localhost$/m.test(req.headers.origin)) {
      corsOptions.origin = false
    }

    // callback expects two parameters: error and options
    callback(null, corsOptions)
  }
})

fastify.register(async function (fastify) {
  fastify.get('/', (req, reply) => {
    reply.send({ hello: 'world' })
  })
})

fastify.listen({ port: 3000 })

Custom Fastify hook name

By default, @fastify/cors adds a onRequest hook where the validation and header injection are executed. This can be customized by passing hook in the options. Valid values are onRequest, preParsing, preValidation, preHandler, preSerialization, and onSend.

import Fastify from 'fastify'
import cors from '@fastify/cors'

const fastify = Fastify()
await fastify.register(cors, { 
  hook: 'preHandler',
})

fastify.get('/', (req, reply) => {
  reply.send({ hello: 'world' })
})

await fastify.listen({ port: 3000 })

When configuring CORS asynchronously, an object with delegator key is expected:

const fastify = require('fastify')()

fastify.register(require('@fastify/cors'), {
  hook: 'preHandler',
  delegator: (req, callback) => {
    const corsOptions = {
      // This is NOT recommended for production as it enables reflection exploits
      origin: true
    };

    // do not include CORS headers for requests from localhost
    if (/^localhost$/m.test(req.headers.origin)) {
      corsOptions.origin = false
    }

    // callback expects two parameters: error and options
    callback(null, corsOptions)
  },
})

fastify.register(async function (fastify) {
  fastify.get('/', (req, reply) => {
    reply.send({ hello: 'world' })
  })
})

fastify.listen({ port: 3000 })

Acknowledgements

The code is a port for Fastify of expressjs/cors.

License

Licensed under MIT.
expressjs/cors license