Referrer Policy

1. Introduction

This section is not normative.

Requests made from a document, and for navigations away from that document are associated with a Referer header. While the header can be suppressed for links with the noreferrer link type, authors might wish to control the Referer header more directly for a number of reasons:

1.1. Privacy

A social networking site has a profile page for each of its users, and users add hyperlinks from their profile page to their favorite bands. The social networking site might not wish to leak the user’s profile URL to the band web sites when other users follow those hyperlinks (because the profile URLs might reveal the identity of the owner of the profile).

Some social networking sites, however, might wish to inform the band web sites that the links originated from the social networking site but not reveal which specific user’s profile contained the links.

1.2. Security

A web application uses HTTPS and a URL-based session identifier. The web application might wish to link to HTTPS resources on other web sites without leaking the user’s session identifier in the URL.

Alternatively, a web application may use URLs which themselves grant some capability. Controlling the referrer can help prevent these capability URLs from leaking via referrer headers. [CAPABILITY-URLS]

Note that there are other ways for capability URLs to leak, and controlling the referrer is not enough to control all those potential leaks.

1.3. Trackback

A blog hosted over HTTPS might wish to link to a blog hosted over HTTP and receive trackback links.

2. Key Concepts and Terminology

referrer policy

A referrer policy modifies the algorithm used to populate the Referer header when fetching subresources, prefetching, or performing navigations. This document defines the various behaviors for each referrer policy.

Every environment settings object has an algorithm for obtaining a referrer policy, which is used by default for all requests with that environment settings object as their client.

same-origin request

A Request request is a same-origin request if request’s origin and the origin of request’s current url are the same.

cross-origin request

A Request is a cross-origin request if it is not same-origin.

3. Referrer Policies

A referrer policy is the empty string, "no-referrer", "no-referrer-when-downgrade", "same-origin", "origin", "strict-origin", "origin-when-cross-origin", "strict-origin-when-cross-origin", or "unsafe-url".

enum ReferrerPolicy {
  "",
  "no-referrer",
  "no-referrer-when-downgrade",
  "same-origin",
  "origin",
  "strict-origin",
  "origin-when-cross-origin",
  "strict-origin-when-cross-origin",
  "unsafe-url"
};

Each possible referrer policy is explained below. A detailed algorithm for evaluating their effect is given in the § 5 Integration with Fetch and § 8 Algorithms sections.

Note: The referrer policy for an environment settings object provides a default baseline policy for requests when that environment settings object is used as a request client. This policy may be tightened for specific requests via mechanisms like the noreferrer link type.

3.1. "no-referrer"

The simplest policy is "no-referrer", which specifies that no referrer information is to be sent along with requests made from a particular request client to any origin. The header will be omitted entirely.

3.2. "no-referrer-when-downgrade"

The "no-referrer-when-downgrade" policy sends a full URL along with requests from environment settings objects whose HTTPS state is "`modern`" to a potentially trustworthy URL, and requests from clients whose HTTPS state is not "`modern`" to any origin.

Requests from clients whose HTTPS state is "`modern`" to non-potentially trustworthy URLs, on the other hand, will contain no referrer information. A Referer HTTP header will not be sent.

This is a user agent’s default behavior, if no policy is otherwise specified.

3.3. "same-origin"

The "same-origin" policy specifies that a full URL, stripped for use as a referrer, is sent as referrer information when making same-origin requests from a particular client.

Cross-origin requests, on the other hand, will contain no referrer information. A Referer HTTP header will not be sent.

3.4. "origin"

The "origin" policy specifies that only the ASCII serialization of the origin of the request client is sent as referrer information when making both same-origin requests and cross-origin requests from a particular client.

Note: The serialization of an origin looks like https://example.com. To ensure that a valid URL is sent in the `Referer` header, user agents will append a U+002F SOLIDUS ("/") character to the origin (e.g. https://example.com/).

Note: The "origin" policy causes the origin of HTTPS referrers to be sent over the network as part of unencrypted HTTP requests. The "strict-origin" policy addresses this concern.

3.5. "strict-origin"

The "strict-origin" policy sends the ASCII serialization of the origin of the request client when making requests:

• from an environment settings object whose HTTPS state is "`modern`" to a potentially trustworthy URL, and
• from environment settings objects whose HTTPS state is not "`modern`" to any origin.

Requests from request clients whose HTTPS state is "`modern`" to non-potentially trustworthy URLs, on the other hand, will contain no referrer information. A Referer HTTP header will not be sent.

3.6. "origin-when-cross-origin"

The "origin-when-cross-origin" policy specifies that a full URL, stripped for use as a referrer, is sent as referrer information when making same-origin requests from a particular request client, and only the ASCII serialization of the origin of the request client is sent as referrer information when making cross-origin requests from a particular client.

Note: For the "origin-when-cross-origin" policy, we also consider protocol upgrades, e.g. requests from http://example.com/ to https://example.com/, to be cross-origin requests.

Note: The "origin-when-cross-origin" policy causes the origin of HTTPS referrers to be sent over the network as part of unencrypted HTTP requests. The "strict-origin-when-cross-origin" policy addresses this concern.

3.7. "strict-origin-when-cross-origin"

The "strict-origin-when-cross-origin" policy specifies that a full URL, stripped for use as a referrer, is sent as referrer information when making same-origin requests from a particular request client, and only the ASCII serialization of the origin of the request client when making cross-origin requests:

• from a environment settings object whose HTTPS state is "`modern`" to a potentially trustworthy URL, and
• from environment settings objects whose HTTPS state is not "`modern`" to any origin.

Requests from clients whose HTTPS state is "`modern`" to non- potentially trustworthy URLs, on the other hand, will contain no referrer information. A Referer HTTP header will not be sent.

3.8. "unsafe-url"

The "unsafe-url" policy specifies that a full URL, stripped for use as a referrer, is sent along with both cross-origin requests and same-origin requests made from a particular client.

Note: The policy’s name doesn’t lie; it is unsafe. This policy will leak origins and paths from secure resources to insecure origins. Carefully consider the impact of setting such a policy for potentially sensitive documents.

3.9. The empty string

The empty string "" corresponds to no referrer policy, causing a fallback to a referrer policy defined elsewhere, or in the case where no such higher-level policy is available, defaulting to "no-referrer-when-downgrade". This defaulting happens in the § 8.3 Determine request’s Referrer algorithm.

4. Referrer Policy Delivery

A request’s referrer policy is delivered in one of five ways:

• Via the Referrer-Policy HTTP header (defined in § 4.1 Delivery via Referrer-Policy header).
• Via a meta element with a name of referrer.
• Via a referrerpolicy content attribute on an a, area, img, iframe, or link element.
• Via the noreferrer link relation on an a, or area element.
• Implicitly, via inheritance.

4.1. Delivery via Referrer-Policy header

The Referrer-Policy HTTP header specifies the referrer policy that the user agent applies when determining what referrer information should be included with requests made, and with browsing contexts created from the context of the protected resource.

The syntax for the name and value of the header are described by the following ABNF grammar. ABNF is defined in [RFC5234], and the #rule ABNF extension used below is defined in Section 7 of [RFC7230].

"Referrer-Policy:" 1#(policy-token / extension-token)

policy-token = "no-referrer" / "no-referrer-when-downgrade" / "strict-origin" / "strict-origin-when-cross-origin" / "same-origin" / "origin" / "origin-when-cross-origin" / "unsafe-url"
extension-token = 1*( ALPHA / "-" )

Note: The header name does not share the HTTP Referer header’s misspelling.

Note: The purpose of extension-token is so that browsers do not fail to parse the entire header field if it includes an unknown policy value. § 11.1 Unknown Policy Values describes in greater detail how new policy values can be deployed.

Note: The quotes in the ABNF above are used to indicate literal strings. Referrer-Policy header values should not be quoted.

§ 5 Integration with Fetch and § 6 Integration with HTML describe how the Referrer-Policy header is processed.

Referrer-Policy: no-referrer

<a href="http://example.com" referrerpolicy="origin">

7. Integration with CSS

The CSS Standard does not specify how it fetches resources referenced from stylesheets. However, implementations should be sure to set the referrer-related properties of any requests initiated by stylesheets as follows:

1. If a CSS style sheet is responsible for the request, and its location is non-null, set the referrer to its location, and the referrer policy to its referrer policy.

   This requires that CSS style sheets process `Referrer-Policy` headers, and store a referrer policy in the same way that Documents do.

2. If a CSS style sheet with a null location is responsible for the request, set the referrer to its owner node’s node document’s URL, and the referrer policy to its owner node’s node document’s referrer policy.
3. Otherwise, a CSS declaration block that was created by the embedder is responsible for the request - either from parsing of an element’s style attribute, or to implement an presentational hint for an element. We assume that in this case the CSS declaration block’s owner node points to that element, and set the referrer to the block’s owner node’s node document’s URL, and the referrer policy to the block’s owner node’s node document’s referrer policy.

Note: Both the value of the request’s referrer and referrer policy are set based on the values at the time a given request is created. If a document’s referrer policy changes during its lifetime, the policy associated with inline stylesheet requests will also change.

8. Algorithms

8.1. Parse a referrer policy from a Referrer-Policy header

Given a response response, the following steps return a referrer policy according to response’s `Referrer-Policy` header:

1. Let policy-tokens be the result of extracting header list values given `Referrer-Policy` and response’s header list.
2. Let policy be the empty string.
3. For each token in policy-tokens, if token is a referrer policy and token is not the empty string, then set policy to token.

   Note: This algorithm loops over multiple policy values to allow deployment of new policy values with fallbacks for older user agents, as described in § 11.1 Unknown Policy Values.

4. Return policy.

8.2. Set request’s referrer policy on redirect

Given a request request and a response actualResponse, this algorithm updates request’s associated referrer policy according to the Referrer-Policy header (if any) in actualResponse.

1. Let policy be the result of executing § 8.1 Parse a referrer policy from a Referrer-Policy header on actualResponse.
2. If policy is not the empty string, then set request’s associated referrer policy to policy.

8.3. Determine request’s Referrer

Given a request request, we can determine the correct referrer information to send by examining the referrer policy associated with it, as detailed in the following steps, which return either no referrer or a URL:

1. Let policy be request’s associated referrer policy.
2. Let environment be request’s client.
3. Switch on request’s referrer:

   "client"

   1. If environment’s global object is a Window object, then
      1. Let document be the associated Document of environment’s global object.
      2. If document’s origin is an opaque origin, return no referrer.
      3. While document is an iframe srcdoc document, let document be document’s browsing context’s browsing context container’s node document.
      4. Let referrerSource be document’s URL.
   2. Otherwise, let referrerSource be environment’s creation URL.

   a URL

   Let referrerSource be request’s referrer.

   Note: If request’s referrer is "no-referrer", Fetch will not call into this algorithm.

4. Let referrerURL be the result of stripping referrerSource for use as a referrer.
5. Let referrerOrigin be the result of stripping referrerSource for use as a referrer, with the origin-only flag set to true.
6. If the result of serializing referrerURL is a string whose length is greater than 4096, set referrerURL to referrerOrigin.
7. The user agent MAY alter referrerURL or referrerOrigin at this point to enforce arbitrary policy considerations in the interests of minimizing data leakage. For example, the user agent could strip the URL down to an origin, modify its host, replace it with an empty string, etc.
8. Execute the statements corresponding to the value of policy:

   "no-referrer"

   Return no referrer

   "origin"

   Return referrerOrigin

   "unsafe-url"

   Return referrerURL.

   "strict-origin"

   1. If environment is not null:
      1. If environment’s HTTPS state is "`modern`" and request’s current URL is not a potentially trustworthy URL, then return no referrer.
   2. Return referrerOrigin.

   "strict-origin-when-cross-origin"

   1. If request is a same-origin request, then return referrerURL.
   2. If environment is not null:
      1. If environment’s HTTPS state is "`modern`" and request’s current URL is not a potentially trustworthy URL
         1. Return no referrer.
   3. Return referrerOrigin.

   "same-origin"

   1. If request is a same-origin request, then return referrerURL.
   2. Otherwise, return no referrer.

   "origin-when-cross-origin"

   1. If request is a cross-origin request, then return referrerOrigin.
   2. Otherwise, return referrerURL.

   "no-referrer-when-downgrade"

   1. If environment is not null:
      1. If environment’s HTTPS state is "`modern`" and request’s current URL is not a potentially trustworthy URL, then return no referrer.
   2. Return referrerURL.

   Note: Fetch will ensure request’s referrer policy is not the empty string before calling this algorithm.

8.4. Strip url for use as a referrer

Certain portions of URLs must not be included when sending a URL as the value of a `Referer` header: a URLs fragment, username, and password components must be stripped from the URL before it’s sent out. This algorithm accepts a origin-only flag, which defaults to false. If set to true, the algorithm will additionally remove the URL’s path and query components, leaving only the scheme, host, and port.

1. If url is null, return no referrer.
2. If url’s scheme is a local scheme, then return no referrer.
3. Set url’s username to the empty string.
4. Set url’s password to null.
5. Set url’s fragment to null.
6. If the origin-only flag is true, then:
   1. Set url’s path to null.
   2. Set url’s query to null.
7. Return url.

9. Privacy Considerations

9.1. User Controls

Nothing in this specification should be interpreted as preventing user agents from offering options to users which would change the information sent out via a `Referer` header. For instance, user agents MAY allow users to suppress the referrer header entirely, regardless of the active referrer policy on a page.

10. Security Considerations

10.1. Information Leakage

The referrer policies "origin", "origin-when-cross-origin" and "unsafe-url" might leak the origin and the URL of a secure site respectively via insecure transport.

Those three policies are included in the spec nevertheless to lower the friction of sites adopting secure transport.

Authors wanting to ensure that they do not leak any more information than the default policy should instead use the policy states "same-origin", "strict-origin", "strict-origin-when-cross-origin" or "no-referrer".

10.2. Downgrade to less strict policies

The spec does not forbid downgrading to less strict policies, e.g., from "no-referrer" to "unsafe-url".

On the one hand, it is not clear which policy is more strict for all possible pairs of policies: While "no-referrer-when-downgrade" will not leak any information over insecure transport, and "origin" will, the latter reveals less information across cross-origin navigations.

On the other hand, allowing for setting less strict policies enables authors to define safe fallbacks as described in § 11.1 Unknown Policy Values.

11. Authoring Considerations

11.1. Unknown Policy Values

As described in § 8.1 Parse a referrer policy from a Referrer-Policy header and in the meta referrer algorithm, unknown policy values will be ignored, and when multiple sources specify a referrer policy, the value of the latest one will be used. This makes it possible to deploy new policy values.

Referrer-Policy: no-referrer
Referrer-Policy: unsafe-url

Referrer-Policy: no-referrer,unsafe-url

This behavior does not, however, apply to the referrerpolicy attribute. Authors may dynamically set and get the referrerpolicy attribute to detect whether a particular policy value is supported.

12. Acknowledgements

This specification is based in large part on Adam Barth and Jochen Eisinger’s Meta referrer document.