No 'Access-Control-Allow-Origin' header is present on the requested resource—when trying to get data from a REST API
cors, fetch-api, javascript
Solution
This answer covers a lot of ground, so it’s divided into three parts:
- How to use a CORS proxy to avoid “No Access-Control-Allow-Origin header” problems
- How to avoid the CORS preflight
- How to fix “Access-Control-Allow-Origin header must not be the wildcard” problems
How to use a CORS proxy to avoid “No Access-Control-Allow-Origin header” problems
If you don’t control the server your frontend code is sending a request to, and the problem with the response from that server is just the lack of the necessary `Access-Control-Allow-Origin` header, you can still get things to work—by making the request through a CORS proxy.
You can easily run your own proxy with code from https://github.com/Rob--W/cors-anywhere/. You can also easily deploy your own proxy to Heroku in just 2-3 minutes, with 5 commands:
git clone https://github.com/Rob--W/cors-anywhere.git
cd cors-anywhere/
npm install
heroku create
git push heroku master
After running those commands, you’ll end up with your own CORS Anywhere server running at, e.g., `https://cryptic-headland-94862.herokuapp.com/`.
Now, prefix your request URL with the URL for your proxy:
https://cryptic-headland-94862.herokuapp.com/https://example.com
Adding the proxy URL as a prefix causes the request to get made through your proxy, which:
- Forwards the request to `https://example.com`.
- Receives the response from `https://example.com`.
- Adds the `Access-Control-Allow-Origin` header to the response.
- Passes that response, with that added header, back to the requesting frontend code.
The browser then allows the frontend code to access the response, because that response with the `Access-Control-Allow-Origin` response header is what the browser sees.
This works even if the request is one that triggers browsers to do a CORS preflight `OPTIONS` request, because in that case, the proxy also sends the `Access-Control-Allow-Headers` and `Access-Control-Allow-Methods` headers needed to make the preflight succeed.
How to avoid the CORS preflight
The code in the question triggers a CORS preflight—since it sends an `Authorization` header.
https://developer.mozilla.org/docs/Web/HTTP/Access_control_CORS#Preflighted_requests
Even without that, the `Content-Type: application/json` header will also trigger a preflight.
What “preflight” means: before the browser tries the `POST` in the code in the question, it first sends an `OPTIONS` request to the server, to determine if the server is opting-in to receiving a cross-origin `POST` that has `Authorization` and `Content-Type: application/json` headers.
It works pretty well with a small curl script - I get my data.
To properly test with `curl`, you must emulate the preflight `OPTIONS` the browser sends:
curl -i -X OPTIONS -H "Origin: http://127.0.0.1:3000" \
-H 'Access-Control-Request-Method: POST' \
-H 'Access-Control-Request-Headers: Content-Type, Authorization' \
"https://the.sign_in.url"
…with `https://the.sign_in.url` replaced by whatever your actual `sign_in` URL is.
The response the browser needs from that `OPTIONS` request must have headers like this:
Access-Control-Allow-Origin: http://127.0.0.1:3000
Access-Control-Allow-Methods: POST
Access-Control-Allow-Headers: Content-Type, Authorization
If the `OPTIONS` response doesn’t include those headers, the browser will stop right there and never attempt to send the `POST` request. Also, the HTTP status code for the response must be a 2xx—typically 200 or 204. If it’s any other status code, the browser will stop right there.
The server in the question responds to the `OPTIONS` request with a 501 status code, which apparently means it’s trying to indicate it doesn’t implement support for `OPTIONS` requests. Other servers typically respond with a 405 “Method not allowed” status code in this case.
So you’ll never be able to make `POST` requests directly to that server from your frontend JavaScript code if the server responds to that `OPTIONS` request with a 405 or 501 or anything other than a 200 or 204 or if doesn’t respond with those necessary response headers.
The way to avoid triggering a preflight for the case in the question would be:
- if the server didn’t require an `Authorization` request header but instead, e.g., relied on authentication data embedded in the body of the `POST` request or as a query param
- if the server didn’t require the `POST` body to have a `Content-Type: application/json` media type but instead accepted the `POST` body as `application/x-www-form-urlencoded` with a parameter named `json` (or whatever) whose value is the JSON data
How to fix “Access-Control-Allow-Origin header must not be the wildcard” problems
I am getting another error message:
The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin '`http://127.0.0.1:3000`' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.
For requests that have credentials, browsers won’t let your frontend JavaScript code access the response if the value of the `Access-Control-Allow-Origin` header is `*`. Instead the value in that case must exactly match your frontend code’s origin, `http://127.0.0.1:3000`.
See Credentialed requests and wildcards in the MDN HTTP access control (CORS) article.
If you control the server you’re sending the request to, a common way to deal with this case is to configure the server to take the value of the `Origin` request header, and echo/reflect that back into the value of the `Access-Control-Allow-Origin` response header; e.g., with nginx:
add_header Access-Control-Allow-Origin $http_origin
But that’s just an example; other (web) server systems have similar ways to echo origin values.
I am using Chrome. I also tried using that Chrome CORS Plugin
That Chrome CORS plugin apparently just simplemindedly injects an `Access-Control-Allow-Origin: *` header into the response the browser sees. If the plugin were smarter, what it would be doing is setting the value of that fake `Access-Control-Allow-Origin` response header to the actual origin of your frontend JavaScript code, `http://127.0.0.1:3000`.
So avoid using that plugin, even for testing. It’s just a distraction. To test what responses you get from the server with no browser filtering them, you’re better off using `curl -H` as above.
As far as the frontend JavaScript code for the `fetch(…)` request in the question:
headers.append('Access-Control-Allow-Origin', 'http://localhost:3000');
headers.append('Access-Control-Allow-Credentials', 'true');
Remove the lines above. The `Access-Control-Allow-*` headers are response headers. You never want to send them in requests. The only effect of that is to trigger a browser to execute a preflight.
Problem
I'm trying to fetch some data from the REST API of HP Alm. It works pretty well with a small curl script—I get my data. Now doing that with JavaScript, fetch and ES6 (more or less) seems to be a bigger issue. I keep getting this error message: Fetch API cannot load . Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:3000' is therefore not allowed access. The response had HTTP status code 501. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled. I understand that this is because I am trying to fetch that data from within my localhost and the solution should be using Cross-Origin Resource Sharing (CORS). I thought I actually did that, but somehow it either ignores what I write in the header or the problem is something else. So, is there an implementation issue? Am I doing it wrong? I can't check the server logs unfortunately. I'm really a bit stuck here. ``` function performSignIn() { let headers = new Headers(); headers.append('Content-Type', 'application/json'); headers.append('Accept', 'application/json'); headers.append('Access-Control-Allow-Origin', 'http://localhost:3000'); headers.append('Access-Control-Allow-Credentials', 'true'); headers.append('GET', 'POST', 'OPTIONS'); headers.append('Authorization', 'Basic ' + base64.encode(username + ":" + password)); fetch(sign_in, { //mode: 'no-cors', credentials: 'include', method: 'POST', headers: headers }) .then(response => response.json()) .then(json => console.log(json)) .catch(error => console.log('Authorization failed : ' + error.message)); } ``` I am using Chrome. I also tried using that Chrome CORS Plugin, but then I am getting another error message: The value of the 'Access-Control-Allow-Origin' header in the response must not be the wildcard '*' when the request's credentials mode is 'include'. Origin 'http://127.0.0.1:3000' is therefore not allowed access. The credentials mode of requests initiated by the XMLHttpRequest is controlled by the withCredentials attribute.