Fetch Metadata Request Headers

1. Introduction

Interesting web applications generally end up with a large number of web-exposed endpoints that might reveal sensitive data about a user, or take action on a user’s behalf. Since users' browsers can be easily convinced to make requests to those endpoints, and to include the users' ambient credentials (cookies, privileged position on an intranet, etc), applications need to be very careful about the way those endpoints work in order to avoid abuse.

Being careful turns out to be hard in some cases ("simple" CSRF), and practically impossible in others (cross-site search, timing attacks, etc). The latter category includes timing attacks based on the server-side processing necessary to generate certain responses, and length measurements (both via web-facing timing attacks and passive network attackers).

It would be helpful if servers could make more intelligent decisions about whether or not to respond to a given request based on the way that it’s made in order to mitigate the latter category. For example, it seems pretty unlikely that a "Transfer all my money" endpoint on a bank’s server would expect to be referenced from an img tag, and likewise unlikely that evil.com is going to be making any legitimate requests whatsoever. Ideally, the server could reject these requests a priori rather than delivering them to the application backend.

Here, we describe a mechanims by which user agents can enable this kind of decision-making by adding additional context to outgoing requests. By delivering metadata to a server in a set of fetch metadata headers, we enable applications to quickly reject requests based on testing a set of preconditions. That work can even be lifted up above the application layer (to reverse proxies, CDNs, etc) if desired.

1.1. Examples

A request generated by a picture element would result in a request containing the following HTTP request headers:

Sec-Fetch-Dest: image
Sec-Fetch-Mode: no-cors
Sec-Fetch-Site: cross-site

A top-level navigation from https://example.com to https://example.com/ caused by a user’s click on an in-page link would result in a request containing the following HTTP request header:

Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: same-origin
Sec-Fetch-User: ?1

2. Fetch Metadata Headers

The following sections define several fetch metadata headers, each of which exposes an interesting request attribute to a server.

2.1. The Sec-Fetch-Dest HTTP Request Header

The Sec-Fetch-Dest HTTP request header exposes a request's destination to a server. It is a Structured Header whose value MUST be a token. [I-D.ietf-httpbis-header-structure] Its ABNF is:

Sec-Fetch-Dest = sh-token

Valid Sec-Fetch-Dest values include the set of valid request destinations defined by [Fetch].

In order to support forward-compatibility with as-yet-unknown request types, servers SHOULD ignore this header if it contains an invalid value.

// fetch()’s destination is the empty string:
Sec-Fetch-Dest: empty

// <img>’s destination is "image"
Sec-Fetch-Dest: image

// new Worker()’s destination is "worker"
Sec-Fetch-Dest: worker

// Top-level navigations' destinations are "document"
Sec-Fetch-Dest: document

// <iframe> navigations' destinations are "iframe"
Sec-Fetch-Dest: iframe

2.2. The Sec-Fetch-Mode HTTP Request Header

The Sec-Fetch-Mode HTTP request header exposes a request's mode to a server. It is a Structured Header whose value is a token. [I-D.ietf-httpbis-header-structure] Its ABNF is:

Sec-Fetch-Mode = sh-token

Valid Sec-Fetch-Mode values include "cors", "navigate", "no-cors", "same-origin", and "websocket". In order to support forward-compatibility with as-yet-unknown request types, servers SHOULD ignore this header if it contains an invalid value.

2.3. The Sec-Fetch-Site HTTP Request Header

The Sec-Fetch-Site HTTP request header exposes the relationship between a request initiator’s origin and its target’s origin. It is a Structured Header whose value is a token. [I-D.ietf-httpbis-header-structure] Its ABNF is:

Sec-Fetch-Site = sh-token

Valid Sec-Fetch-Site values include "cross-site", "same-origin", "same-site", and "none". In order to support forward-compatibility with as-yet-unknown request types, servers SHOULD ignore this header if it contains an invalid value.

2.4. The Sec-Fetch-User HTTP Request Header

The Sec-Fetch-User HTTP request header exposes whether or not a navigation request was triggered by user activation. It is a Structured Header whose value is a boolean. [I-D.ietf-httpbis-header-structure] Its ABNF is:

Sec-Fetch-User = sh-boolean

Note: The header is delivered only for navigation requests, and only when its value is true. It might be reasonable to expand the headers' scope in the future to include subresource requests generally if we can spell out some use cases that would be improved by exposing that information (and if we can agree on ways to define that status for all the subresource request types we’d be interested in), but for the moment, navigation requests have clear use cases, and seem straightforward to define interoperably.

3. Integration with Fetch and HTML

To support Sec-Fetch-User, request has a user-activation which is false, unless otherwise populated by HTML’s create navigation params by fetching algorithm.

Fetch Metadata headers are appended to outgoing requests from within Fetch’s "HTTP-network-or-cache" algorithm, using the following steps. Consult that specification for integration details [FETCH].

4. Security and Privacy Considerations

4.1. Redirects

The user agent will send a Sec-Fetch-Site header along with each request in a redirect chain. The header’s value will shift in the presence of cross-origin or cross-site redirection in order to mitigate confusion.

The algorithm to set the Sec-Fetch-Site header walks the request's entire url list, and will send cross-site if any URL in the list is cross-site to the request’s current url, same-site only if all URLs in the list are same-site with the request’s current url, and same-origin only if all URLs in the list are same-origin with the request’s current url.

For example, if https://example.com/ requests https://example.com/redirect, the initial request’s Sec-Fetch-Site value would be same-origin. If that response redirected to https://subdomain.example.com/redirect, that request’s Sec-Fetch-Site value would be same-site (as https://subdomain.example.com/ and https://example.com/ have the same registrable domain). If that response redirected to https://example.net/redirect, that request’s Sec-Fetch-Site value would be cross-site (as https://example.net/ is not same-site with https://example.com/ and https://subdomain.example.com/). If that response redirects all the way back to https://example.com/, the final request’s Sec-Fetch-Site value would still be cross-site (as the redirect chain includes https://example.net/, which is still not same-site with the other URLs.

Note: For the special case of Sec-Fetch-Site: None, it seems reasonable to maintain that value through redirects in order to support the common case of copy/pasting shortlinks into the address bar. That is, if a user agent chooses to treat an address-bar navigation to https://sho.rt/link as Sec-Fetch-Site: none, a post-redirect navigation to https://target.com/long/path/goes/here should likewise assert Sec-Fetch-Site: none.

4.2. The Sec- Prefix

Each of the headers defined in this document is prefixed with Sec-, which makes them all forbidden response-header names, and therefore unmodifiable from JavaScript. This will prevent malicious websites from convincing user agents to send forged metadata along with requests, which should give sites a bit more confidence in their ability to respond reasonably to the advertised information.

4.3. Directly User-Initiated Requests

When setting the Sec-Fetch-Site header, user agents are asked to distinguish between navigation requests that are "explicitly caused by a user’s interaction". This somewhat poorly defined phrase is pulled from HTML, which suggests that "A user agent may provide various ways for the user to explicitly cause a browsing context to navigate, in addition to those defined in this specification."

The goal is to distinguish between "webby" navigations that are controlled by a given (potentially malicious!) website (e.g. links, the window.location setter, form submissions, etc.), and those that are not (e.g. user interaction with a user agent’s address bar, bookmarks, etc). The former will be delivered with a Sec-Fetch-Site header whose value is same-origin, same-site, or cross-site, as appropriate. The latter will be distinguished with a value of none, as no specific site is actually responsible for the request, and it makes sense to allow servers to treat them as trusted, as they somehow represent a user’s direct intent.

Each user agent is likely to have a distinct set of interactions which might fall into one or the other category, and it will be hard to share an automated test suite for these cases. Still, it would be ideal to align on behavior for those which are likely to be common. Sme examples follow:

• Navigation from the address bar: In the general case, this kind of navigation should be treated as directly user-initiated, and include Sec-Fetch-Site: none. It may be reasonable for user agents to include heuristics around pasting values into the address bar (especially if the "copy" action can be traced to a specific origin), and to treat such navigations distinctly from those which the user types themselves.

• Navigation from user agent UI (bookmarks, new tab page, etc): A user’s interaction with links in user agent UI should be treated similarly to their input in the user agent’s address bar, including Sec-Fetch-Site: none to demarcate the navigation as user-initiated.

• Navigation from a link’s context menu (e.g. "Open in new window"): as the link’s target is controlled by the page on which the link is present, user agents should treat the navigation as site-controlled, and set the Sec-Fetch-Site header appropriately for the relationship between the site which controls the link and the site which is being opened.

• Ctrl-click on a link: the same arguments and conclusions apply here as apply to a link’s context menu, discussed directly above.

• Navigation through history (e.g. a user agent’s "back" button):

• Drag-and-drop: It seems reasonable to distinguish behavior here based upon the source of the dragged content. If content is dragged from a tab, the user agent should be able to ascertain its origin, and set Sec-Fetch-Site accordingly. If content is dragged from elsewhere (the user agent’s bookmark bar, another app entirely, etc), then Sec-Fetch-Site: none may be appropriate.

4.4. Extension-Initiated Requests

Some user agents support extensions which are capable of making requests, and which can be empowered beyond regular web content in order to give users more control over their web experience. While these are somewhat outside the scope of the web platform, user agents are encouraged to carefully consider how these requests are to be represented to servers. In general, it would be ideal for user agents to satisfy two goals:

1. Extensions without special privileges for a given site cannot cause requests that would bypass that site’s server-side Fetch Metadata logic.

2. Developers can recognize extensions' requests so that they can be exempted from the server’s Fetch Metadata logic if it chooses to do so. This increases developers' ability to confidently deploy Fetch Metadata protections without undermining legitimate user interests.

With these goals in mind, user agents are encouraged to implement the following behaviors:

1. If an extension does not have permission to access to a given URL, its requests to that URL could contain a Sec-Fetch-Site header whose value is cross-site, just as a normal web request would. If the extension does have access to a given URL, the Sec-Fetch-Site value could be same-origin.

2. The Origin header could be included with outgoing requests from extension contexts, with an implementation-defined value that allows servers to distinguish extension-initiated requests from web-initiated requests.

5. Deployment Considerations

5.1. Vary

If a given endpoint’s response depends upon the values the client delivers in a Fetch metadata header, developers should be careful to include an appropriate Vary header [RFC9110], in order to ensure that caches handle the response appropriately. For example, Vary: Accept-Encoding, Sec-Fetch-Site.

5.2. Header Bloat

An earlier version of this document defined a single Sec-Metadata header, whose contents were a dictionary. Subsequent discussion (as well as Mark Nottingham’s excellent [mnot-designing-headers]) shifted the design away from a single dictionary to a series of simple headers, each of which contains only a single token. This design should perform significantly better under HTTP’s current HPACK compression algorithms.

Further discussion on the topic can be found on the review thread in w3ctag/design-reviews#280.

6. IANA Considerations

The permanent message header field registry should be updated with the following registrations for Fetch metadata headers: [RFC3864]

6.1. Sec-Fetch-Dest Registration

Header field name

Sec-Fetch-Dest

Applicable protocol

http

Status

standard

Author/Change controller

Me

Specification document

This specification (See § 2.1 The Sec-Fetch-Dest HTTP Request Header)

6.2. Sec-Fetch-Mode Registration

Header field name

Sec-Fetch-Mode

Applicable protocol

http

Status

standard

Author/Change controller

Me

Specification document

This specification (See § 2.2 The Sec-Fetch-Mode HTTP Request Header)

6.3. Sec-Fetch-Site Registration

Header field name

Sec-Fetch-Site

Applicable protocol

http

Status

standard

Author/Change controller

Me

Specification document

This specification (See § 2.3 The Sec-Fetch-Site HTTP Request Header)

6.4. Sec-Fetch-User Registration

Header field name

Sec-Fetch-User

Applicable protocol

http

Status

standard

Author/Change controller

Me

Specification document

This specification (See § 2.4 The Sec-Fetch-User HTTP Request Header)

7. Acknowledgements

Thanks to Anne van Kesteren, Artur Janc, Dan Veditz, Łukasz Anforowicz, Mark Nottingham, and Roberto Clapis, who all provided substantial support in the design of this mechanism.