Example Nginx configuration for adding cross-origin resource sharing (CORS) support to reverse proxied APIs

nginx.cors.conf

#
# CORS header support
#
# One way to use this is by placing it into a file called "global/cors.conf"
# under your Nginx configuration directory and placing the following
# statement inside your **location** block(s):
#
#   include global/cors.conf;
#
# As of Nginx 1.7.5, add_header supports an "always" parameter which
# allows CORS to work if the backend returns 4xx or 5xx status code.
#
# For more information on CORS, please see: http://enable-cors.org/
# Forked from this Gist: https://gist.github.com/michiel/1064640
#

# If you prefer to use map, you can use it as follow
# ------------------------------------------------------

map $http_origin $cors {

  # indicates all map values are hostnames and should be parsed as such
  hostnames;

  # default value
  default 'false';

  # all your domains
  localhost          'true';
  www.yourdomain.com 'true';
  www.yourother.com  'true';
}

server {
   # .... server configuration section
}

# --------------------------------------------------
# Put this in /etc/nginx/global/cors.conf
# --------------------------------------------------

set $cors '';
if ($http_origin ~ '^https?://(localhost|www\.yourdomain\.com|www\.yourotherdomain\.com)$') {
        set $cors 'true';
}

# The above is used only if map is not used
# --------------------------------------------------

if ($cors = 'true') {
        add_header 'Access-Control-Allow-Origin' "$http_origin" 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;
        # required to be able to read Authorization header in frontend
        #add_header 'Access-Control-Expose-Headers' 'Authorization' always;
}

if ($request_method = 'OPTIONS') {
        # Tell client that this pre-flight info is valid for 20 days
        add_header 'Access-Control-Max-Age' 1728000;
        add_header 'Content-Type' 'text/plain charset=UTF-8';
        add_header 'Content-Length' 0;
        return 204;
}

# Include the cors configuration in the location section
# ------------------------------------------------------

server {

        root /var/www/htdocs;
        index index.php index.html index.htm;

        server_name yourserver.com www.yourserver.com;

        location / {
...
                include global/cors.conf;
        }   
}


RESOURCES 

* The map use: https://www.digitalocean.com/community/tutorials/how-to-use-nginx-s-map-module-on-ubuntu-16-04
* The gist source: https://gist.github.com/Stanback/7145487

You have repeated the critical vulnerability from Stanback's gist. Please re-read my comment: https://gist.github.com/Stanback/7145487#gistcomment-2044821

You are right! I forgot to update that part of the code. Thank you!