Sec-Fetch-Site

The Sec-Fetch-Site fetch metadata request header indicates the relationship between a request initiator's origin and the origin of the requested resource.

In other words, this header tells a server whether a request for a resource is coming from the same origin, the same site, a different site, or is a "user initiated" request. The server can then use this information to decide if the request should be allowed.

Same-origin requests would usually be allowed by default, but what happens for requests from other origins may further depend on what resource is being requested, or information in other Fetch metadata request headers. By default, requests that are not accepted should be rejected with a 403 response code.

Sec-Fetch-Site: cross-site
Sec-Fetch-Site: same-origin
Sec-Fetch-Site: same-site
Sec-Fetch-Site: none

cross-site

The request initiator and the server hosting the resource have a different site (i.e. a request by "potentially-evil.com" for a resource at "example.com").

same-origin

The request initiator and the server hosting the resource have the same origin (same scheme, host and port).

same-site

The request initiator and the server hosting the resource have the same scheme, domain and/or subdomain, but not necessarily the same port.

none

This request is a user-originated operation. For example: entering a URL into the address bar, opening a bookmark, or dragging-and-dropping a file into the browser window.

A fetch request to https://mysite.example/foo.json originating from a web page on https://mysite.example (with the same port) is a same-origin request. The browser will generate the Sec-Fetch-Site: same-origin header as shown below, and the server will typically allow the request:

GET /foo.json
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-origin

A fetch request to the same URL from another site, for example potentially-evil.com, causes the browser to generate a different header (e.g. Sec-Fetch-Site: cross-site), which the server can choose to accept or reject:

GET /foo.json
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site

• Related headers
  • Sec-Fetch-Mode
  • Sec-Fetch-User
  • Sec-Fetch-Dest
• Protect your resources from web attacks with Fetch Metadata (web.dev)
• Fetch Metadata Request Headers playground (secmetadata.appspot.com)