NGINX Reverse Proxy and Access-Control-Allow-Origin issue

Question

I'm configuring an NGINX Reverse Proxy.

On the browser I go to:
client url: https://www.hollywood.com

Then the web page above needs to do requests to:
server url: https://server.hollywood.com/api/auth/login

This is the configuration corresponding to: server.hollywood.com:

server {
    listen 443 ssl;
    server_name             server.hollywood.com;
    # add_header 'Access-Control-Allow-Origin' "https://www.hollywood.com" always;
    add_header 'Access-Control-Allow-Credentials' 'true' always;
    add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
    add_header 'Access-Control-Allow-Headers' 'Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With' always;
    ssl_certificate         ../ssl/lets-encrypt/hollywood.com.crt;
    ssl_certificate_key     ../ssl/lets-encrypt/hollywood.com.key;
    location /
    {
        proxy_pass http://localhost:9201;
        include "../proxy_params.conf";
    }
}

Experiment 1:

With the Access-Control-Allow-Origin line commented out, when I access to:
client url: https://www.hollywood.com

I get the following error on the browser console (Chrome in my case):

POST https://server.hollywood.com/api/auth/login/local 502 (Bad Gateway)
(index):1 Failed to load https://server.hollywood.com/api/auth/login/local: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://www.hollywood.com' is therefore not allowed access. The response had HTTP status code 502.

Experiment 2:

If I enable the Access-Control-Allow-Origin line above, then I get on the browser terminal:

Failed to load https://server.hollywood.com/api/auth/login/local: Response to preflight request doesn't pass access control check: The 'Access-Control-Allow-Origin' header contains multiple values '*, https://www.hollywood.com', but only one is allowed. Origin 'https://www.hollywood.com' is therefore not allowed access.

I don't know why multiple when before that header was not present???

Experiment 3:

In the other hand, if I go directly on the browser to the:
server url: https://server.hollywood.com/api/auth/login with the Access-Control-Allow-Origin line commented out, I get the following (on the Network section):

Response Headers:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 139
Content-Security-Policy: default-src 'self'
Content-Type: text/html; charset=utf-8
Date: Sat, 09 Jun 2018 06:34:00 GMT
Server: nginx/1.13.12
X-Content-Type-Options: nosniff

Before I got: "No 'Access-Control-Allow-Origin' header is present on the requested resource." but now I see above that field is in there on the Response Headers.

Experiment 4:

If I enable again the Access-Control-Allow-Origin line above, then I get the following (on the Network section):

Response Headers:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: Accept,Authorization,Cache-Control,Content-Type,DNT,If-Modified-Since,Keep-Alive,Origin,User-Agent,X-Requested-With
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Origin: https://www.hollywood.com
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 139
Content-Security-Policy: default-src 'self'
Content-Type: text/html; charset=utf-8
Date: Sat, 09 Jun 2018 06:34:58 GMT
Server: nginx/1.13.12
X-Content-Type-Options: nosniff

Now I get two times the field: Access-Control-Allow-Origin.

Do you have any idea why my first 2 experiments are failing getting the errors relative to: Access-Control-Allow-Origin?

Thanks!

Answer 1

It could be that the server behind your proxy_pass was setting the Access-Control-Allow-Origin header as well.

For what it's worth for future readers with a similar problem, I found that my node.js server was passing an Access-Control-Allow-Origin: '*' header for some reason, as well as the actual header I'd set in node.js to restrict CORS. When commenting out my node.js cors middleware, the Access-Control-Allow-Origin: '*' header still remained.

To resolve this, I used the nginx proxy_hide_header directive to remove the header coming from node.js and manually adding it as it should be:

# proxying the
map $http_upgrade $connection_upgrade {
    default upgrade;
    '' close;
}

# local node.js server
upstream websocket {
    server 127.0.0.1:3000;
}

server {
    server_name ...;
    # ...;

    # add the header here
    add_header Access-Control-Allow-Origin https://www.hollywood.com;

    # Websockets config
    location /socket.io/ {
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $connection_upgrade;

        # do not pass the CORS header from the response of the proxied server to the
        # client
        proxy_hide_header 'Access-Control-Allow-Origin';

        proxy_pass http://websocket;
        proxy_http_version 1.1;
    }

    location / {
        # ...
        try_files $uri /index.html;
    }
}

Googling this issue was pretty hard since most people are trying to fix CORS by making the Access-Control wide open! Here was another issue with similar problems:

https://serverfault.com/questions/751678/how-can-i-replace-access-control-allow-origin-header-in-proxy-response-with-ngin

Answer 2

Try this configuration:

server {
  listen 443 ssl;
  server_name server.hollywood.com;

  ssl_certificate ../ssl/lets-encrypt/hollywood.com.crt;
  ssl_certificate_key ../ssl/lets-encrypt/hollywood.com.key;

  add_header 'Access-Control-Allow-Origin' '*';
  add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
  add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';

  location / {
    if ($request_method = 'OPTIONS') {
      add_header 'Access-Control-Max-Age' 1728000;
      add_header 'Content-Type' 'text/plain; charset=utf-8';
      add_header 'Content-Length' 0;
      return 204;
    }
    if ($request_method = 'POST') {
      add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range';
      proxy_pass http://localhost:9201;
      include "../proxy_params.conf";
    }
    if ($request_method = 'GET') {
      add_header 'Access-Control-Expose-Headers' 'Content-Length,Content-Range';
      proxy_pass http://localhost:9201;
      include "../proxy_params.conf";
    }
  }
}

Code based on https://enable-cors.org/server_nginx.html

Hope it helps.