Fetch metadata request header

A fetch metadata request header is an HTTP request header that provides additional information about the context from which the request originated. This allows the server to make decisions about whether a request should be allowed based on where the request came from and how the resource will be used.

With this information a server can implement a resource isolation policy, allowing external sites to request only those resources that are intended for sharing, and that are used appropriately. This approach can help mitigate common cross-site web vulnerabilities such as CSRF, Cross-site Script Inclusion('XSSI'), timing attacks, and cross-origin information leaks.

These headers are prefixed with Sec-, and hence have forbidden header names. As such, they cannot be modified from JavaScript.

The fetch metadata request headers are:

• Sec-Fetch-Site
• Sec-Fetch-Mode
• Sec-Fetch-User
• Sec-Fetch-Dest

The following request headers are not strictly "fetch metadata request headers", as they are not in the same specification, but similarly provide information about the context of how a resource will be used. A server might use them to modify its caching behavior, or the information that is returned:

• Sec-Purpose Experimental
• Service-Worker-Navigation-Preload