When do browsers send the Origin header? When do browsers set the origin to null?

Question

As you can see from this Bugzilla thread (and also), Firefox does not always send an Origin header in POST requests. The RFC states that it should not be sent in certain undefined "privacy-sensitive" contexts. Mozilla defines those contexts here.

I'd like to know whether these are the only situations in which Firefox will not send the Origin header. As far as I can tell, it also will not send it in cross-origin POST requests (though Chrome and Internet Explorer will), but I can't confirm that in the documentation. Is it enumerated somewhere that I'm missing?

Answer 1

As far as what the relevant specs actually require, the answer has a couple parts:

• When browsers must internally set an origin to a value that’ll get serialized as null
• When browsers must send the Origin header

Here are the details:

When browsers must set origin to a value that’ll get serialized as null

The HTML spec uses the term opaque origin and defines it as an “internal value”:

with no serialization it can be recreated from (it is serialized as "null" per ASCII serialization of an origin), for which the only meaningful operation is testing for equality

In other words everywhere the HTML spec says opaque origin, you can translate that to null.

The HTML spec requires browsers to set an opaque origin or unique origin in these cases:

1. Cross-origin images (including cross-origin img elements)
2. Cross-origin media data (including cross-origin video and audio elements)
3. Any document generated from a data: URL
4. Any iframe with a sandbox attribute that doesn’t contain the value allow-same-origin
5. Any document programmatically created using createDocument(), etc.
6. Any document that does not have a creator browsing context
7. Responses that are network errors
8. The Should navigation response to navigation request of type from source in target be blocked by Content Security Policy? algorithm returns Blocked when executed on a navigate response

The Fetch spec requires browsers to set the origin to a “globally unique identifier” (which basically means the same thing as “opaque origin” which basically means null…) in one case:

8. Redirects across origins

The URL spec requires browsers to set an opaque origin in the following cases:

9. For blob: URLs
10. For file: URLs
11. For any other URLs whose scheme is not one of http, https, ftp, ws, wss, or gopher.

But note that just because the browser has internally set an opaque origin—essentially null—that doesn’t necessarily mean the browser will send an Origin header. So see the next part of this answer for details about when browsers must send the Origin header.

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When browsers must send the Origin header

Browsers send the Origin header for cross-origin requests initiated by a fetch() or XHR call, or by an ajax method from a JavaScript library (axios, jQuery, etc.) — but not for normal page navigations (that is, when you open a web page directly in a browser), and not (normally) for resources embedded in a web page (for example, not for CSS stylesheets, scripts, or images).

But that description is a simplification. There are cases other than cross-origin XHR/fetch/ajax calls when browsers send the Origin header, and cases when browsers send the Origin header for embedded resources. So what follows below is the longer answer.

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In terms of the spec requirements: The spec requires the Origin header to be sent only for any request which the Fetch spec defines as a CORS request:

A CORS request is an HTTP request that includes an Origin header. It cannot be reliably identified as participating in the CORS protocol as the Origin header is also included for all requests whose method is neither GET nor HEAD.

So, what the spec means there is: The Origin header is sent in all cross-origin requests, but it’s also always sent for all POST, PUT, PATCH, and DELETE requests — even for same-origin POST, PUT, PATCH, and DELETE requests (which by definition in Fetch are actually “CORS requests” — even though they’re same-origin).＊

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The other cases when browsers must send the Origin header are any cases where a request is made with the “CORS flag” set — which, as far as HTTP(S) requests, is except when the request mode is navigate, websocket, same-origin, or no-cors.

XHR always sets the mode to cors. But with the Fetch API, those request modes are the ones you can set with the mode field of the init-object argument to the fetch(…) method:

fetch("http://example.com", { mode: 'no-cors' }) // no Origin will be sent 

Font requests always have the mode set to cors and so always have the Origin header.

And for any element with a crossorigin attribute (aka “CORS setting attribute”), the HTML spec requires browsers to set the request mode to cors (and to send the Origin header).

Otherwise, for embedded resources — any elements having attributes with URLs that initiate requests (<script src>, stylesheets, images, media elements) — the mode for the requests defaults to no-cors; and since those requests are GET requests, that means, per-spec, browsers send no Origin header for them.

When HTML form elements initiate POST requests, the mode for those POSTs also defaults to no-cors — in the same way that embedded resources have their mode defaulted to no-cors. However, unlike the no-cors mode GET requests for embedded resources, browsers do send the Origin header for those no-cors mode POSTs initiated from HTML form elements.

The reason for that is, as mentioned earlier in this answer, browsers always send the Origin header in all POST, PUT, PATCH, and DELETE requests.

Also, for completeness here and to be clear: For navigations, browsers send no Origin header. That is, if a user navigates directly to a resource — by pasting a URL into a browser address bar, or by following a link from another web document — then browsers send no Origin header.

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＊ _{The algorithm in the Fetch spec that requires browsers to send the Origin header for all CORS requests is this:}

_{To append a request Origin header, given a request request, run these steps:}

<sub>1. Let serializedOrigin be the result of byte-serializing a request origin with request.
2. If request’s response tainting is "cors" or request’s mode is "websocket", then
    append Origin/serializedOrigin to request’s header list.
3. Otherwise, if request’s method is neither GET nor HEAD,
    then: [also send the Origin header in that case too]</sub>

_{Step 2 there is what requires the Origin header to be sent in all cross-origin requests — because all cross-origin requests have their response tainting set to "cors".}

_{But step 3 there requires the Origin header to also be sent for same-origin POST, PUT, PATCH, and DELETE requests (which by definition in Fetch are actually “CORS requests” — even though they’re same-origin).}

_{The above describes how the Fetch spec currently defines the requirements, due to a change that was made to the spec on 2016-12-09. Up until then the requirements were different:}

  _{•  previously no Origin was sent for a same-origin POST
  •  previously no Origin was sent for cross-origin POST from a <form> (without CORS)}

_{So the Firefox behavior the question describes is what the spec previously required, not what it currently requires.}