Origin

The Origin request header indicates the origin (scheme, hostname, and port) that caused the request. For example, if a user agent needs to request resources included in a page, or fetched by scripts that it executes, then the origin of the page may be included in the request.

Origin: null
Origin: <scheme>://<hostname>
Origin: <scheme>://<hostname>:<port>

null

The origin is "privacy sensitive", or is an opaque origin as defined by the HTML specification (specific cases are listed in the description section).

<scheme>

The protocol that is used. Usually, it is the HTTP protocol or its secured version, HTTPS.

<hostname>

The domain name or the IP address of the origin server.

<port> Optional

Port number on which the server is listening. If no port is given, the default port for the requested service is implied (e.g., "80" for an HTTP URL) .

The Origin header is similar to the Referer header, but does not disclose the path, and may be null. It is used to provide the "security context" for the origin request, except in cases where the origin information would be sensitive or unnecessary.

Broadly speaking, user agents add the Origin request header to:

• cross origin requests.
• same-origin requests except for GET or HEAD requests (i.e. they are added to same-origin POST, OPTIONS, PUT, PATCH, and DELETE requests).

There are some exceptions to the above rules; for example, if a cross-origin GET or HEAD request is made in no-cors mode, the Origin header will not be added.

The Origin header value may be null in a number of cases, including (non-exhaustively):

• Origins whose scheme is not one of http, https, ftp, ws, wss, or gopher (including blob, file and data).
• Cross-origin images and media data, including that in <img>, <video> and <audio> elements.
• Documents created programmatically using createDocument(), generated from a data: url, or that do not have a creator browsing context.
• Redirects across origins.
• iframes with a sandbox attribute that doesn't contain the value allow-same-origin.
• Responses that are network errors.

• Host
• Referer
• Same-origin policy
• When do browsers send the Origin header? When do browsers set the origin to null? (Stack Overflow)