As of Drupal 8.2, it's possible to opt in a particular site to enable CORS for responses served by Drupal.
(This is particularly helpful for fully decoupled Drupal sites which have JS that needs to talk to a Drupal 8 site's REST API. In such cases, that Drupal 8 instance often runs on a separate domain. Due to the same origin policy those requests will be blocked by the browser.)
This is not enabled by default because there are security consequences.
# Configure Cross-Site HTTP requests (CORS). # Read https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS # for more information about the topic in general. # Note: By default the configuration is disabled. cors.config: enabled: false # Specify allowed headers, like 'x-allowed-header'. allowedHeaders:  # Specify allowed request methods, specify ['*'] to allow all possible ones. allowedMethods:  # Configure requests allowed from specific origins. allowedOrigins: ['*'] # Sets the Access-Control-Expose-Headers header. exposedHeaders: false # Sets the Access-Control-Max-Age header. maxAge: false # Sets the Access-Control-Allow-Credentials header. supportsCredentials: false
Note particular that
enabled: false key-value pair!