A request that doesn’t trigger a CORS preflight-a so-called “simple request”-is one that meets all the following conditions: Those are called “simple requests” in this article, though the Fetch spec (which defines CORS) doesn’t use that term. Some requests don’t trigger a CORS preflight. The JavaScript snippets included in these sections (and running instances of the server-code that correctly handles these cross-site requests) can be found "in action" at, and will work in browsers that support cross-site XMLHttpRequest.Ī discussion of Cross-Origin Resource Sharing from a server perspective (including PHP code snippets) can be found in the Server-Side Access Control (CORS) article. All of these examples use the XMLHttpRequest object, which can be used to make cross-site invocations in any supporting browser. Here, we present three scenarios that illustrate how Cross-Origin Resource Sharing works. Subsequent sections discuss scenarios, as well as provide a breakdown of the HTTP headers used. Servers can also notify clients whether "credentials" (including Cookies and HTTP Authentication data) should be sent with requests. Additionally, for HTTP request methods that can cause side-effects on server's data (in particular, for HTTP methods other than GET, or for POST usage with certain MIME types), the specification mandates that browsers "preflight" the request, soliciting supported methods from the server with an HTTP OPTIONS request method, and then, upon "approval" from the server, sending the actual request with the actual HTTP request method. The Cross-Origin Resource Sharing standard works by adding new HTTP headers that allow servers to describe the set of origins that are permitted to read that information using a web browser. This article is a general discussion of Cross-Origin Resource Sharing and includes a discussion of the necessary HTTP headers. Images/video frames drawn to a canvas using drawImage.Web Fonts (for cross-domain font usage in within CSS), so that servers can deploy TrueType fonts that can only be cross-site loaded and used by web sites that are permitted to do so. Invocations of the XMLHttpRequest or Fetch APIs in a cross-site manner, as discussed above.This cross-origin sharing standard is used to enable cross-site HTTP requests for: Another article for server developers discussing cross-origin sharing from a server perspective (with PHP code snippets) is supplementary reading. But this new standard means servers have to handle new request and response headers. Modern browsers handle the client-side components of cross-origin sharing, including headers and policy enforcement. This article is for web administrators, server developers, and front-end developers. Modern browsers use CORS in an API container - such as XMLHttpRequest or Fetch - to mitigate risks of cross-origin HTTP requests. The Cross-Origin Resource Sharing ( CORS) mechanism gives web servers cross-domain access controls, which enable secure cross-domain data transfers. To improve web applications, developers asked browser vendors to allow cross-domain requests. So, a web application using XMLHttpRequest or Fetch could only make HTTP requests to its own domain. For example, XMLHttpRequest and Fetch follow the same-origin policy. Many pages on the web today load resources like CSS stylesheets, images, and scripts from separate domains.įor security reasons, browsers restrict cross-origin HTTP requests initiated from within scripts. For example, an HTML page served from makes an src request for. A resource makes a cross-origin HTTP request when it requests a resource from a different domain, protocol, or port to its own.
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