Why doesn't adding CORS headers to an OPTIONS route allow browsers to access my API?

Question

I am trying to support CORS in my Node.js application that uses the Express.js web framework. I have read a Google group discussion about how to handle this, and read a few articles about how CORS works. First, I did this (code is written in CoffeeScript syntax):

app.options "*", (req, res) ->
  res.header 'Access-Control-Allow-Origin', '*'
  res.header 'Access-Control-Allow-Credentials', true
  # try: 'POST, GET, PUT, DELETE, OPTIONS'
  res.header 'Access-Control-Allow-Methods', 'GET, OPTIONS'
  # try: 'X-Requested-With, X-HTTP-Method-Override, Content-Type, Accept'
  res.header 'Access-Control-Allow-Headers', 'Content-Type'
  # ...

It doesn't seem to work. It seems like my browser (Chrome) is not sending the initial OPTIONS request. When I just updated the block for the resource I need to submit a cross-origin GET request to:

app.get "/somethingelse", (req, res) ->
  # ...
  res.header 'Access-Control-Allow-Origin', '*'
  res.header 'Access-Control-Allow-Credentials', true
  res.header 'Access-Control-Allow-Methods', 'POST, GET, PUT, DELETE, OPTIONS'
  res.header 'Access-Control-Allow-Headers', 'Content-Type'
  # ...

It works (in Chrome). This also works in Safari.

I have read that...

In a browser implementing CORS, each cross-origin GET or POST request is preceded by an OPTIONS request that checks whether the GET or POST is OK.

So my main question is, how come this doesn't seem to happen in my case? Why isn't my app.options block called? Why do I need to set the headers in my main app.get block?

Answer 1

To answer your main question, the CORS spec only requires the OPTIONS call to precede the POST or GET if the POST or GET has any non-simple content or headers in it.

Content-Types that require a CORS pre-flight request (the OPTIONS call) are any Content-Type except the following:

1. application/x-www-form-urlencoded
2. multipart/form-data
3. text/plain

Any other Content-Types apart from those listed above will trigger a pre-flight request.

As for Headers, any Request Headers apart from the following will trigger a pre-flight request:

1. Accept
2. Accept-Language
3. Content-Language
4. Content-Type
5. DPR
6. Save-Data
7. Viewport-Width
8. Width

Any other Request Headers will trigger the pre-flight request.

So, you could add a custom header such as: x-Trigger: CORS, and that should trigger the pre-flight request and hit the OPTIONS block.

See MDN Web API Reference - CORS Preflighted requests

Answer 2

I found the easiest way is to use the node.js package cors. The simplest usage is:

var cors = require('cors')

var app = express()
app.use(cors())

There are, of course many ways to configure the behaviour to your needs; the page linked above shows a number of examples.

Answer 3

Try passing control to the next matching route. If Express is matching app.get route first, then it won't continue onto the options route unless you do this (note use of next):

app.get('somethingelse', function(req, res, next) {
    //..set headers etc.

    next();
});

In terms of organising the CORS stuff, I put it in a middleware which is working well for me:

//CORS middleware
var allowCrossDomain = function(req, res, next) {
    res.header('Access-Control-Allow-Origin', 'example.com');
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
    res.header('Access-Control-Allow-Headers', 'Content-Type');

    next();
}

//...
app.configure(function() {
    app.use(express.bodyParser());
    app.use(express.cookieParser());
    app.use(express.session({ secret: 'cool beans' }));
    app.use(express.methodOverride());
    app.use(allowCrossDomain);
    app.use(app.router);
    app.use(express.static(__dirname + '/public'));
});

Answer 4

To stay in the same idea of routing. I use this code :

app.all('/*', function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "X-Requested-With");
  next();
});

Similar to http://enable-cors.org/server_expressjs.html example

Answer 5

do

npm install cors --save

and just add these lines in your main file where your request going (keep it before any route).

const cors = require('cors');
const express = require('express');
let app = express();
app.use(cors());
app.options('*', cors());

Answer 6

I have made a more complete middleware suitable for express or connect. It supports OPTIONS requests for preflight checking. Note that it will allow CORS access to anything, you might want to put in some checks if you want to limit access.

app.use(function(req, res, next) {
    var oneof = false;
    if(req.headers.origin) {
        res.header('Access-Control-Allow-Origin', req.headers.origin);
        oneof = true;
    }
    if(req.headers['access-control-request-method']) {
        res.header('Access-Control-Allow-Methods', req.headers['access-control-request-method']);
        oneof = true;
    }
    if(req.headers['access-control-request-headers']) {
        res.header('Access-Control-Allow-Headers', req.headers['access-control-request-headers']);
        oneof = true;
    }
    if(oneof) {
        res.header('Access-Control-Max-Age', 60 * 60 * 24 * 365);
    }

    // intercept OPTIONS method
    if (oneof && req.method == 'OPTIONS') {
        res.send(200);
    }
    else {
        next();
    }
});

Answer 7

Do something like this:

app.use(function(req, res, next) {
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
});

Answer 8

install cors module of expressjs. you can follow these steps >

Installation

npm install cors

Simple Usage (Enable All CORS Requests)

var express = require('express');
var cors = require('cors');
var app = express();
app.use(cors());

for more details go to https://github.com/expressjs/cors

Answer 9

Testing done with express + node + ionic running in differente ports.

Localhost:8100

Localhost:5000

// CORS (Cross-Origin Resource Sharing) headers to support Cross-site HTTP requests

app.all('*', function(req, res, next) {
       res.header("Access-Control-Allow-Origin", "*");
       res.header("Access-Control-Allow-Headers", "X-Requested-With");
       res.header('Access-Control-Allow-Headers', 'Content-Type');
       next();
});

Answer 10

first simply install cors in your project. Take terminal(command prompt) and cd to your project directory and run the below command:

npm install cors --save

Then take the server.js file and change the code to add the following in it:

var cors = require('cors');


var app = express();

app.use(cors());

app.use(function(req, res, next) {
   res.header("Access-Control-Allow-Origin", "*");
   res.header('Access-Control-Allow-Methods', 'DELETE, PUT, GET, POST');
   res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
   next();
});

This worked for me..

Answer 11

Some time ago, I faced this problem so I did this to allow CORS in my nodejs app:

First you need to install cors by using below command :

npm install cors --save

Now add the following code to your app starting file like ( app.js or server.js)

var express = require('express');
var app = express();

var cors = require('cors');
var bodyParser = require('body-parser');

//enables cors
app.use(cors({
  'allowedHeaders': ['sessionId', 'Content-Type'],
  'exposedHeaders': ['sessionId'],
  'origin': '*',
  'methods': 'GET,HEAD,PUT,PATCH,POST,DELETE',
  'preflightContinue': false
}));

require('./router/index')(app);

Answer 12

This works for me, as its an easy implementation inside the routes, im using meanjs and its working fine, safari, chrome, etc.

app.route('/footer-contact-form').post(emailer.sendFooterMail).options(function(req,res,next){ 
        res.header('Access-Control-Allow-Origin', '*'); 
        res.header('Access-Control-Allow-Methods', 'GET, POST');
        res.header('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');
        return res.send(200);

    });

Answer 13

If you want to make it controller specific, you can use:

res.setHeader('X-Frame-Options', 'ALLOWALL');
res.setHeader('Access-Control-Allow-Origin', '*');
res.setHeader('Access-Control-Allow-Methods', 'POST, GET');
res.setHeader('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');

Please note that this will also allow iframes.

Answer 14

Can refer the code below for the same. Source: Academind/node-restful-api

const express = require('express');
const app = express();

//acts as a middleware
//to handle CORS Errors
app.use((req, res, next) => { //doesn't send response just adjusts it
    res.header("Access-Control-Allow-Origin", "*") //* to give access to any origin
    res.header(
        "Access-Control-Allow-Headers",
        "Origin, X-Requested-With, Content-Type, Accept, Authorization" //to give access to all the headers provided
    );
    if(req.method === 'OPTIONS'){
        res.header('Access-Control-Allow-Methods', 'PUT, POST, PATCH, DELETE, GET'); //to give access to all the methods provided
        return res.status(200).json({});
    }
    next(); //so that other routes can take over
})

Answer 15

The easiest answer is to just use the cors package.

const cors = require('cors');

const app = require('express')();
app.use(cors());

That will enable CORS across the board. If you want to learn how to enable CORS without outside modules, all you really need is some Express middleware that sets the 'Access-Control-Allow-Origin' header. That's the minimum you need to allow cross-request domains from a browser to your server.

app.options('*', (req, res) => {
  res.set('Access-Control-Allow-Origin', '*');
  res.send('ok');
});

app.use((req, res) => {
  res.set('Access-Control-Allow-Origin', '*');
});

Answer 16

In my index.js I added:

app.use((req, res, next) => {
   res.header("Access-Control-Allow-Origin", "*");
   res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
   res.header("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS");
   next();
}) 

Answer 17

My simplest solution with Express 4.2.0 (EDIT: Doesn't seem to work in 4.3.0) was:

function supportCrossOriginScript(req, res, next) {
    res.status(200);
    res.header("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Headers", "Content-Type");

    // res.header("Access-Control-Allow-Headers", "Origin");
    // res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    // res.header("Access-Control-Allow-Methods","POST, OPTIONS");
    // res.header("Access-Control-Allow-Methods","POST, GET, OPTIONS, DELETE, PUT, HEAD");
    // res.header("Access-Control-Max-Age","1728000");
    next();
}

// Support CORS
app.options('/result', supportCrossOriginScript);

app.post('/result', supportCrossOriginScript, function(req, res) {
    res.send('received');
    // do stuff with req
});

I suppose doing app.all('/result', ...) would work too...

Answer 18

Below worked for me, hope it helps someone!

const express = require('express');
const cors = require('cors');
let app = express();

app.use(cors({ origin: true }));

Got reference from https://expressjs.com/en/resources/middleware/cors.html#configuring-cors

Answer 19

In typescript, if you want to use the node.js package cors

/**
* app.ts
* If you use the cors library
*/

import * as express from "express";
[...]
import * as cors from 'cors';

class App {
   public express: express.Application;

   constructor() {
       this.express = express();
       [..]
       this.handleCORSErrors();
   }

   private handleCORSErrors(): any {
       const corsOptions: cors.CorsOptions = {
           origin: 'http://example.com',
           optionsSuccessStatus: 200
       };
       this.express.use(cors(corsOptions));
   }
}

export default new App().express;

If you don't want to use third part libraries for cors error handling, you need to change the handleCORSErrors() method.

/**
* app.ts
* If you do not use the cors library
*/

import * as express from "express";
[...]

class App {
   public express: express.Application;

   constructor() {
       this.express = express();
       [..]
       this.handleCORSErrors();
   }

   private handleCORSErrors(): any {
       this.express.use((req, res, next) => {
           res.header("Access-Control-Allow-Origin", "*");
           res.header(
               "Access-Control-ALlow-Headers",
               "Origin, X-Requested-With, Content-Type, Accept, Authorization"
           );
           if (req.method === "OPTIONS") {
               res.header(
                   "Access-Control-Allow-Methods",
                   "PUT, POST, PATCH, GET, DELETE"
               );
               return res.status(200).json({});
           } 
           next(); // send the request to the next middleware
       });
    }
}

export default new App().express;

For using the app.ts file

/**
* server.ts
*/
import * as http from "http";
import app from "./app";

const server: http.Server = http.createServer(app);

const PORT: any = process.env.PORT || 3000;
server.listen(PORT);

Answer 20

Using Express Middleware works great for me. If you are already using Express, just add the following middleware rules. It should start working.

app.all("/api/*", function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "Cache-Control, Pragma, Origin, Authorization, Content-Type, X-Requested-With");
  res.header("Access-Control-Allow-Methods", "GET, PUT, POST");
  return next();
});

app.all("/api/*", function(req, res, next) {
  if (req.method.toLowerCase() !== "options") {
    return next();
  }
  return res.send(204);
});

Reference

Answer 21

Try this in your main js file:

app.use((req, res, next) => {
res.header("Access-Control-Allow-Origin", "*");
res.header(
  "Access-Control-Allow-Headers",
  "Authorization, X-API-KEY, Origin, X-Requested-With, Content-Type, Accept, Access-Control-Allow-Request-Method"
);
res.header("Access-Control-Allow-Methods", "GET, POST, OPTIONS, PUT, DELETE");
res.header("Allow", "GET, POST, OPTIONS, PUT, DELETE");
next();
});

This should solve your problem

Answer 22

I found it to be extremely easy to do this with the npm request package (https://www.npmjs.com/package/request)

Then I based my solution on this post http://blog.javascripting.com/2015/01/17/dont-hassle-with-cors/

'use strict'

const express = require('express');
const request = require('request');

let proxyConfig = {
    url : {
        base: 'http://servertoreach.com?id=',
    }
}

/* setting up and configuring node express server for the application */
let server = express();
server.set('port', 3000);


/* methods forwarded to the servertoreach proxy  */
server.use('/somethingElse', function(req, res)
{
    let url = proxyConfig.url.base + req.query.id;
    req.pipe(request(url)).pipe(res);
});


/* start the server */
server.listen(server.get('port'), function() {
    console.log('express server with a proxy listening on port ' + server.get('port'));
});

Answer 23

This is similiar to Pat's answer with the difference that I finish with res.sendStatus(200); instead of next();

The code will catch all the requests of the method type OPTIONS and send back access-control-headers.

app.options('/*', (req, res, next) => {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');
    res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization, Content-Length, X-Requested-With');
    res.sendStatus(200);
});

The code accepts CORS from all origins as requested in the question. However, it would be better to replace the * with a specific origin i.e. http://localhost:8080 to prevent misuse.

Since we use the app.options-method instead of the app.use-method we don't need to make this check:

req.method === 'OPTIONS'

which we can see in some of the other answers.

I found the answer here: http://johnzhang.io/options-request-in-express.

Answer 24

You can use Express middleware, block your domain and methods.

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", process.env.DOMAIN); // update to match the domain you will make the request from
  res.header("Access-Control-Allow-Methods", "GET,PUT,POST,DELETE");
  res.header(
    "Access-Control-Allow-Headers",
    "Origin, X-Requested-With, Content-Type, Accept"
  );
  next();
});

Answer 25

simple is hard:

 let my_data = []
const promise = new Promise(async function (resolve, reject) {
    axios.post('https://cors-anywhere.herokuapp.com/https://maps.googleapis.com/maps/api/directions/json?origin=33.69057660000001,72.9782724&destination=33.691478,%2072.978594&key=AIzaSyApzbs5QDJOnEObdSBN_Cmln5ZWxx323vA'
        , { 'Origin': 'https://localhost:3000' })
        .then(function (response) {
            console.log(`axios response ${response.data}`)
            const my_data = response.data
            resolve(my_data)
        })
        .catch(function (error) {
            console.log(error)
            alert('connection error')
        })
})
promise.then(data => {
    console.log(JSON.stringify(data))
})

Answer 26

using CORS package. and put this parameters:

cors({credentials: true, origin: true, exposedHeaders: '*'})

Answer 27

We can avoid CORS and forward the requests to the other server instead:

// config:
var public_folder = __dirname + '/public'
var apiServerHost = 'http://other.server'

// code:
console.log("starting server...");

var express = require('express');
var app = express();
var request = require('request');

// serve static files
app.use(express.static(public_folder));

// if not found, serve from another server
app.use(function(req, res) {
    var url = apiServerHost + req.url;
    req.pipe(request(url)).pipe(res);
});

app.listen(80, function(){
    console.log("server ready");
});

Answer 28

I used the following steps to my web app and I had success:

Add the cors package to the express:

npm install cors --save

Add following lines after the bodyParser configuration. I had some troubles adding before bodyParser:

 // enable cors to the server
const corsOpt = {
    origin: process.env.CORS_ALLOW_ORIGIN || '*', // this work well to configure origin url in the server
    methods: ['GET', 'PUT', 'POST', 'DELETE', 'OPTIONS'], // to works well with web app, OPTIONS is required
    allowedHeaders: ['Content-Type', 'Authorization'] // allow json and token in the headers
};
app.use(cors(corsOpt)); // cors for all the routes of the application
app.options('*', cors(corsOpt)); // automatic cors gen for HTTP verbs in all routes, This can be redundant but I kept to be sure that will always work.

Answer 29

The simplest approach is install the cors module in your project using:

npm i --save cors

Then in your server file import it using the following:

import cors from 'cors';

Then simply use it as a middleware like this:

app.use(cors());

Hope this helps!

Answer 30

If i were you @OP i would change my programming paradigm.

assuming you are getting these CORS blocked because you are making requests to localhost or something similar.

Eventually if you are going to deploy to production optoins like Google Cloud Platform or Heroku or , you will no have to worry about CORS like allow origin or whatever when in production.

so when testing the server just use postman and you will not get CORS blocked, after that deploy your server and then work on your client.