Stands for "Cross-Origin Resource Sharing." CORS allows scripts on webpages to request resources from other domains. Most web browsers block these types of requests by default for security purposes.

A webpage can request resources from another domain — as long as the requests come from the HTML. For example, the <head> section may reference resources, such as CSS files, fonts, and JS files other domains. Examples include Google Analytics scripts, jQuery libraries, and fonts hosted on another server. Similarly, the <body> can request images from a CDN or other domain. Cross-origin resource requests in the HTML do not require CORS permissions.

When a script or iframe element makes a cross-origin request, CORS is required. For example, an AJAX method – which runs after the page is loaded – cannot request a resource from another domain. CORS overrides this default browser setting and allows the request to go through.

CORS is implemented using "access control" HTTP headers. A server admin can add or modify the response headers, which are sent to a client's browser when a webpage is accessed. These settings, which can be applied to Apache and IIS servers, may be site-specific or server-wide. Below are common request and response headers:

CORS Request Headers:

  • Origin
  • Access-Control-Request-Method
  • Access-Control-Request-Headers

CORS Response Headers:

  • Access-Control-Allow-Origin
  • Access-Control-Allow-Methods
  • Access-Control-Expose-Headers

CORS Example

If a script on requests a resource from using a GET action, it may send the following request headers:

Access-Control-Request-Method: GET

To allow the request, may respond with the following headers:

Access-Control-Allow-Methods: GET

Access-Control-Allow-Origin can be set to specific domains or a wildcard using an asterisk (*). The wildcard setting allows cross-resource requests from all domains, which may be a security risk. Access-Control-Allow-Methods can be set to PUT, POST, DELETE, and others, including a wildcard (*) setting that allows all methods.

Updated November 3, 2020 by Per C.

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A CDN multiplies content from what kind of server?

Default server
Origin server
Simple server
Baseline server
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