The simplest is just to hook to 'allowed_http_origin' and call __return_true(), e.g.:

<?php add_filter( 'allowed_http_origin', '__return_true' ); ?>

Or if you want to limit your application to requests from specific domains (a very good idea), hook to 'allowed_http_origins' (note the plural).

The most flexible way is to hook to wp_headers and add all the CORs headers your application requires, e.g.

<?php add_filter( 'wp_headers', array( 'eg_send_cors_headers' ), 11, 1 );

function eg_send_cors_headers( $headers ) 

                   {  $headers['Access-Control-Allow-Origin'] = get_http_origin(); // Can't use wildcard origin for credentials requests, instead set it to the requesting origin

                   $headers['Access-Control-Allow-Credentials'] = 'true';  // Access-Control headers are received during OPTIONS requests

                    if ( 'OPTIONS' == $_SERVER['REQUEST_METHOD'] ) 

                            {  if ( isset( $_SERVER['HTTP_ACCESS_CONTROL_REQUEST_METHOD'] ) ) 

                            {  $headers['Access-Control-Allow-Methods'] = 'GET, POST, OPTIONS';  }

                    if ( isset( $_SERVER['HTTP_ACCESS_CONTROL_REQUEST_HEADERS'] ) )

                              {  $headers['Access-Control-Allow-Headers'] = $_SERVER['HTTP_ACCESS_CONTROL_REQUEST_HEADERS'];  }

 }

 return $headers; }

If you are using json-api wordpress plugin to expose api, then follow these steps:

1:  Go to  your plugin location  (ex:  /wp-content/plugins/json-api)

2:  Go to   /wp-content/plugins/json-api/singletons

3: Find api.php

4: Add   header("Access-Control-Allow-Origin: *"); after <?php tag
ex:

<?php
header("Access-Control-Allow-Origin: *");
class JSON_API {

function __construct() {
$this->query = new JSON_API_Query();
$this->introspector = new JSON_API_Introspector();
$this->response = new JSON_API_Response();
add_action('template_redirect', array(&$this, 'template_redirect'));
add_action('admin_menu', array(&$this, 'admin_menu'));
add_action('update_option_json_api_base', array(&$this, 'flush_rewrite_rules'));
add_action('pre_update_option_json_api_controllers', array(&$this, 'update_controllers'));
}