Develop Reference

Window exe API cors issue when calling from any webapp. C++

We've created one EXE file using the CPP language and create one API like http://localhost:5800/get-id/. when I open in browser return me the perfect output.
When I used fetch in HTML > script page, then getting No "Access-Control-Allow-Origin" header is present on the requested resource.
Code1:
fetch("http://localhost:5800/get-id/", {method: 'GET').then(function(response) {
console.log(response.text());
}).catch(function(error) {
console.log('Request failed', error)
});
After research, I've added the mode: no-cors error lost but getting an empty response.
Code2:
fetch("http://localhost:5800/get-id/", {method: 'GET', mode: 'no-cors'}).then(function(response) {
console.log(response.text());
}).catch(function(error) {
console.log('Request failed', error)
});
If I use code2 in any inspect console then getting an empty body but when I open http://localhost:5800/get-id/ in the browser and try to hit code2 in the console then getting the perfect parameter.
It means, localhost domain it's working fine but when it's fetched from any domain through my error.
What is the proper solution for it? In C/CPP language how can we allow cors?
Strange:
when I hit from console, it's show me empty
For same request I checked network tab, show 200 OK with proper response / preview data

CORS is a complex topic, I usually use CORS middleware to handle it in Node.JS in Express server (maybe the code will be useful to solve this).
It's goal is to allow API on domain api-domain to list web applications that can use it, for example your application is on webapp-domain domain.
When application calls fetch('http://api-domain/get-id/') to another domain it is referred to as cross-origin call.
All browser do CORS preflight call like this to check for allowance:
OPTIONS /get-id/
Access-Control-Request-Method: GET
Access-Control-Request-Headers: origin, x-requested-with
Origin: http://webapp-domain
(please note it's an OPTION request to the API, not GET)
And response should list webapp-domain as allowed (and specify which HTTP methods are allowed)
HTTP/1.1 204 No Content
Connection: keep-alive
Access-Control-Allow-Origin: http://webapp-domain
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE
Access-Control-Max-Age: 86400
After the successful preflight call like this the browser will continue with fetch, for example, it will send GET request to http://api-domain/get-id/
P.S.
One of the ways to skip CORS is to set HTTP proxy in webapp-domain which will call api-domain on server-side and is not limited by CORS. See this answer for details.

Replicating a browser https request in javascript

I'm trying to hit an API that needs a cookie to return data.
If i hit the url directly in the browser I get the data i want. The protocol is https.
However, whenever I try to fetch the data using window.fetch I run into CORS errors. I think this is happening because I cant get the cookie in the client request, which is causing the server to redirect to an auth server that is not sending back a CORS header.
I have tried using { credentials: 'include' } to no avail.
I was assuming that because the cookie exists in the browser it will be part of the request.
Any fundamental knowledge I'm missing here?

hmm this is a bit weird, by default fetch uses { credentials: 'same-origin' } which will allow cookies to be sent { credentials: 'include' } also will send cookies even if its not the same origin and that's the only difference.
what i am thinking here is that either somewhere in your code the cookie gets deleted somehow or your request is firing before the cookie is set, or the server doesn't allow CORS or it doesn't allow the OPTIONS method if its a different origin.

POSTing to external API throws CORS but it works from Postman

I am using the imgur api to upload images via a node js app.
I am converting images to base64 strings and sending them via Postman works great.
I use node-fetch to make api calls.
const fetch = require('node-fetch')
...
async uploadImage(base64image) {
try {
const url = 'https://api.imgur.com/3/image'
const res = await fetch(url,
{
method: 'POST',
body: { image: base64image },
headers: {
'content-type': 'application/json',
'Authorization': 'Client-ID [my-client-id]',
'Access-Control-Allow-Headers': 'Content-Type, Authorization, Access-Control-Allow-Headers',
'Access-Control-Allow-Methods': 'POST',
}
}
)
console.log(res)
} catch(err) {
console.log(err)
}
}
Error:
Access to fetch at 'https://api.imgur.com/3/image' from origin 'http://localhost:3000' has been blocked by CORS policy: Request header field Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response.
I have tried many 'Access-Control-Allow-xxx' headers but none of them worked..
I assume it must be something simple that I am missing. I have been stuck on this for hours please help me.

Browser restricts HTTP requests to be at the same domain as your web page, so you won't be able to hit imgur api directly from the browser without running into CORS issue.
I am converting images to base64 strings and sending them via Postman
works great.
That's because Postman is not a browser, so is not limited by CORS policy.
I have tried many 'Access-Control-Allow-xxx' headers but none of them
worked..
These headers must be returned by the server in response - in your case by the imgur server. You can't set them in the request from browser, so it'll never work.
Error: Access to fetch at 'https://api.imgur.com/3/image' from origin
'http://localhost:3000' has been blocked by CORS policy: Request
header field Access-Control-Allow-Headers is not allowed by
Access-Control-Allow-Headers in preflight response.
Possible solutions to your problem:
If you have access to the backend api you can set the "Access-Control-Allow-Origin" header on the server and let your app access the api - but as you won't have access to the imgur server - you probably can't do that.
Disable CORS in the browser - you can use a plugin like: https://chrome.google.com/webstore/detail/allow-control-allow-origi/nlfbmbojpeacfghkpbjhddihlkkiljbi?hl=en. This workaound should be fine for development. The plugin will disable your CORS settings and you will be able to hit imgur apis.
The third solution is using a proxy. You can setup a small node server using express. You will then hit your own node server, which in turn will hit the imgur api. As node server is not a browser environment, it won't have any CORS issue and you will be able to access imgur API that way. This is also the reason you were able to hit the API from Postman without any issues. As Postman is not a browser environment, it's not limited by CORS policy.

That's because Access-Control-Allow-Headers, Access-Control-Allow-Methods are the headers that is used by the server. The server appends the header by a middleware.
Now, imagine in the server(in this below example an express server) with CORS enabled this kind of (default) headers are getting set:
app.use(function (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, Accept');
});
And you are sending Access-Control-Allow-Headers from the client side, and server sees that as a header that is not whitelisted.
So, in headers just use these:
headers: {
'content-type': 'application/json',
'Authorization': 'Client-ID [my-client-id]'
}
It should work fine.
Btw, I think it is working with postman because:
Postman cannot set certain headers if you don't install that tiny postman capture extension.
Browser security stops the cross origin requests. If you disable the chrome security it will do any CORS request just fine.
Also, according to this:
I believe this might likely be that Chrome does not support
localhost to go through the Access-Control-Allow-Origin -- see
Chrome issue
To have Chrome send Access-Control-Allow-Origin in the header, just
alias your localhost in your /etc/hosts file to some other domain,
like:
127.0.0.1 localhost yourdomain.com
Then if you'd access your script using yourdomain.com instead of
localhost, the call should succeed.
Note: I don't think the content type should be application/json it should be like image/jpeg or something. Or maybe don't include that header if it doesn't work.

I have some few observations in my own app that helped me solve this issue. I have a node app as a backend api service and a VueJS built front end. I set my node app with cors with a list of endpoints that are allowed to access my node app. Working on my local machine doesn't give me any errors until I upload it to my server.
here are my environments
Local Environment
Nodejs: 12.16.1
OS: Windows 10
DB: MySQL
NodeJS Server Framework: ExpressJS
Upload Module: Multer
Production Environment
Nodejs: 12.19.0
OS: Ubuntu 20.0.4
DB: MySQL
NodeJS Server Framework: ExpressJS
Upload Module: Multer
nginx
Here are my observations based on my production built app.
When I upload a form with image [500kb and above] [post or put], the cors error shows up but less than that, it all went fine.
If I use form data to send data to the server, I see 2 requests in my network tab, the OPTIONS and the actual request.
The actual request failed but I saw that my content-length is very high which leads me to the conclusion that my request is rejected due to the large amount of data that the client sent which my server may have limited. I know that may be misleading but the solution I did works so I don't know why cors issue is popping up even though the data limit is the issue.
MY SOLUTION:
In my nginx config file, I increased my client_max_body_size to 100M. I believe that nginx has a default of 1MB
Open /etc/nginx/sites-available/your-server-file where your-server-file can be like www.example.com or default.
Add the following line inside the server block. You can set it to any amount you want other than 100M.
server {
client_max_body_size 100M;
...
}
type in sudo systemctl restart nginx to restart nginx.
type in sudo nginx -t to check if change is successful.
Reload your app if you are using pm2 and you are done.

According to this article I used this command in linux and SOME OF(!) cross-origins fixed.
google-chrome --disable-web-security --user-data-dir="/tmp/YOUR_TEMPORARY_PATH"

This will not work if you pass headers from frontend. CORS policy is enabled by browsers. Browser blocks the response when they don't found the headers in response.
Possible Solutions:
You can pass the headers in response (If you have the access of backend or ask the API provider for this)
You can setup a middleware to resolve this.
You can get information from here

How do I fix CORS issue in Fetch API

I'm building a front-end only basic Weather App using reactjs. For API requests I'm using Fetch API.
In my app, I'm getting the current location from a simple API I found
and it gives the location as a JSON object. But when I request it through Fetch API, I'm getting this error.
Failed to load http://ip-api.com/json: Request header field Access-Control-Allow-Origin is not allowed by Access-Control-Allow-Headers in preflight response.
So I searched through and found multiple solutions to fix this.
Enabling CORS in Chrome solves the error but when I deploy the app on heroku, how can I access it through a mobile device without running into the same CORS issue.
I found an proxy API which enables the CORS requests. But as this is a location request, this gives me the location of the proxy server. So it's not a solution.
I've gone through this Stackoverflow question and added the headers to the header in my http request but it doesn't solve the problem. (Still it gives the same error).
So how can I solve the issue permanently ? What's the best solution I can use to solve the CORS issue for http requests in Fetch API ?

if you are making a post, put or patch request, you have to stringify your data with body: JSON.stringify(data)
fetch(URL,
{
method: "POST",
body: JSON.stringify(data),
mode: 'cors',
headers: {
'Content-Type': 'application/json',
}
}
).then(response => response.json())
.then(data => {
....
})
.catch((err) => {
....
})
});

To the countless future visitors:
If my original answer doesn't help you, you may have been looking for:
Trying to use fetch and pass in mode: no-cors
What is an opaque response, and what purpose does it serve?
Regarding the issue faced by the OP...
That API appears to be permissive, responding with Access-Control-Allow-Origin:*
I haven't figured out what is causing your problem, but I don't think it is simply the fetch API.
This worked fine for me in both Firefox and Chrome...
fetch('http://ip-api.com/json')
.then( response => response.json() )
.then( data => console.log(data) )

You should use the proxy solution, but pass it the IP of the client instead of the proxy. Here is an example URL format for the API you specified, using the IP of WikiMedia:
http://ip-api.com/json/208.80.152.201

How do I use window.fetch() with httpOnly cookies or basic auth

I'm playing around with window.fetch() in Firefox and Chrome. For some reasons, fetch() doesn't send any cookies. Now that wouldn't be a problem, as I can send them using
fetch('/something', { headers: { Cookie: document.cookie } })
But this won't work for httpOnly cookies.

Okay, I found out after reading on the Mozilla Developer Network a bit more and trying out the credentials option.
Looks like the credentials option is what I should have looked for.
fetch('/something', { credentials: 'same-origin' }) // or 'include'
Will send the cookies.

There are three points here:
If you're making a cross-origin request, set the Access-Control-Allow-Credentials: true. So that the server accepts your cookies.
Set the credentials options either to include for cross-origin requests or same-origin for same-origin requests
Set the credentials option of fetch on both requests that you retrieve and send the cookie