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<h1>Referrer Policy</h1>
<pre class="metadata">
Status: ED
ED: https://w3c.github.io/webappsec-referrer-policy/
Shortname: REFERRER-POLICY
TR: http://www.w3.org/TR/referrer-policy/
Level: none
Editor: Dominic Farolino 102763, Google Inc., domfarolino@gmail.com
Former Editor: Emily Stark 76989, Google Inc., estark@google.com
Former Editor: Jochen Eisinger 49402, Google Inc., eisinger@google.com
Group: webappsec
Abstract: This document describes how an author can set a referrer policy for documents they create, and the impact of such a policy on the <code>Referer</code> HTTP header for outgoing requests and navigations.
Version History: https://github.com/w3c/webappsec-referrer-policy/commits/main/index.src.html
Indent: 2
Ignored Vars: requestURL
Repository: w3c/webappsec-referrer-policy
!Tests: <a href=https://github.com/web-platform-tests/wpt/tree/master/referrer-policy>web-platform-tests referrer-policy/</a> (<a href=https://github.com/web-platform-tests/wpt/labels/referrer-policy>ongoing work</a>)
</pre>
<pre class="anchors">
spec: CSSOM-1; urlPrefix: https://drafts.csswg.org/cssom-1/
type: dfn
text: CSS declaration block
text: location; url: concept-css-style-sheet-location
text: owner node; for: CSSStyleDeclaration; url: cssstyledeclaration-owner-node
text: owner node; for: CSSStyleSheet; url: concept-css-style-sheet-owner-node
spec: HTML; urlPrefix: https://html.spec.whatwg.org/multipage/
type: dfn
urlPrefix: semantics.html
text: pragma directives
text: noreferrer; url: link-type-noreferrer
text: referrer; url: meta-referrer; for: meta
urlPrefix: embedded-content.html
text: an iframe srcdoc document
text: relevant mutation; url: relevant-mutations
urlPrefix: browsers.html
text: opaque origin; url: concept-origin-opaque
text: active document
text: associated Document; url: concept-document-window
text: parse a sandboxing directive
text: forced sandboxing flag set
text: auxiliary browsing context
text: parent browsing context
text: ancestor browsing context
text: browsing context container
text: child browsing context
text: creating a new Document object
text: navigation; url: navigate
text: nested browsing context
text: nested through; url: browsing-context-nested-through
text: opener browsing context
text: plugin document
text: sandboxed origin browsing context flag
text: sandboxing flag set
text: top-level browsing context
text: browsing context
urlPrefix: infrastructure.html
text: ascii case-insensitive match; url: ascii-case-insensitive
text: document base url
text: plugin
text: reflect
text: securityerror
text: mime type
text: strictly split a string
text: skip whitespace
text: collect a sequence of characters
text: space characters
text: split a string on spaces
text: strip leading and trailing whitespace
text: firing; url: concept-event-fire
text: reflect
text: limited to only known values
text: referrer policy attribute
urlPrefix: webappapis.html
text: queue a task
text: task source
text: tasks; url: concept-task
text: environment settings object
text: responsible browsing context
text: global object; for: environment settings object; url: concept-settings-object-global
text: creation URL
urlPrefix: workers.html
text: running a worker; url: run-a-worker
urlPrefix: dom.html
text: style attribute; url: the-style-attribute
urlPrefix: rendering.html
text: presentational hints
type: interface
urlPrefix: dom.html
text: Document
type: element-attr
urlPrefix: semantics.html
text: name; for: meta; url: attr-meta-name
text: referrerpolicy; for: a; url: #attr-hyperlink-referrerpolicy
text: referrerpolicy; for: link; url: attr-link-referrerpolicy
spec: SECURE-CONTEXTS; urlPrefix: https://w3c.github.io/webappsec-secure-contexts
type: dfn
text: potentially trustworthy URL url: is-url-trustworthy
spec: RFC5234; urlPrefix: https://tools.ietf.org/html/rfc5234
type: grammar
text: ALPHA; url: appendix-B.1
spec: RFC7231; urlPrefix: https://tools.ietf.org/html/rfc7231
type: dfn
text: referer; url: section-5.5.2
spec: RFC2616; urlPrefix: https://tools.ietf.org/html/rfc7231
type: dfn
text: redirection 3xx; url: section-6.4
</pre>
<pre class="link-defaults">
spec: html; type: element; text: link;
spec: html; type: element; text: a;
spec: html; type: element; text:script
spec: html; type: interface; text:Window
</pre>
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<section>
<h2 id="intro">Introduction</h2>
<em>This section is not normative.</em>
Requests made from a document, and for navigations away from that document
are associated with a <a><code>Referer</code></a> header. While the header
can be suppressed for links with the <a><code>noreferrer</code></a> link
type, authors might wish to control the <a><code>Referer</code></a> header
more directly for a number of reasons:
<h3 id="intro-privacy">Privacy</h3>
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.
<h3 id="intro-security">Security</h3>
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.
<h3 id="intro-trackback">Trackback</h3>
A blog hosted over HTTPS might wish to link to a blog hosted over HTTP and
receive trackback links.
</section>
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<section>
<h2 id="terms">Key Concepts and Terminology</h2>
<dl>
<dt>
<a for="/">referrer policy</a>
</dt>
<dd>
A <a for="/">referrer policy</a> modifies the algorithm used to populate the
<a><code>Referer</code></a> header when <a for=/ lt=fetch>fetching</a> subresources,
prefetching, or performing navigations. This document defines the various
behaviors for each <a for="/">referrer policy</a>.
Every <a>environment settings object</a> has an algorithm for obtaining a
<a for="/">referrer policy</a>, which is used by default for all <a for=/>requests</a>
with that <a>environment settings object</a> as their <a for=request>client</a>.
</dd>
<dt><dfn>same-origin-referrer request</dfn></dt>
<dd>
A {{Request}} <var>request</var> is a <strong>same-origin-referrer request</strong> if the
<a for=url>origin</a> of <var>request</var>'s <a>referrerURL</a> and the <a for=url>origin</a>
of <var>request</var>'s <a for=request>current URL</a> are <a lt="same origin">the same</a>.
</dd>
<dt><dfn>cross-origin-referrer request</dfn></dt>
<dd>
A {{Request}} is a <strong>cross-origin-referrer request</strong> if it is
<em>not</em> a <a>same-origin-referrer request</a>.
</dd>
</dl>
</section>
<section>
<h2 id="referrer-policies" oldids="referrer-policy-states">Referrer Policies</h2>
A <dfn export>referrer policy</dfn>
is the empty string, "<code>no-referrer</code>",
"<code>no-referrer-when-downgrade</code>", "<code>same-origin</code>",
"<code>origin</code>", "<code>strict-origin</code>",
"<code>origin-when-cross-origin</code>",
"<code>strict-origin-when-cross-origin</code>", or
"<code>unsafe-url</code>".
<pre class="idl">
enum ReferrerPolicy {
"",
"no-referrer",
"no-referrer-when-downgrade",
"same-origin",
"origin",
"strict-origin",
"origin-when-cross-origin",
"strict-origin-when-cross-origin",
"unsafe-url"
};
</pre>
Each possible <a for="/">referrer policy</a> is explained below. A detailed
algorithm for evaluating their effect is given in the
<a section href="#integration-with-fetch"></a> and
<a section href="#algorithms"></a> sections.
Note: The referrer policy for an <a>environment settings object</a> provides a
default baseline policy for requests when that <a>environment settings
object</a> is used as a <a for=request lt=client>request client</a>. This policy may be tightened
for specific requests via mechanisms like the <code><a>noreferrer</a></code>
link type.
The <dfn export>default referrer policy</dfn> is <a>"<code>strict-origin-when-cross-origin</code>"</a>.
<h3 dfn export id="referrer-policy-no-referrer" oldids="referrer-policy-state-no-referrer">"<code>no-referrer</code>"</h3>
The simplest policy is <a>"<code>no-referrer</code>"</a>, which specifies
that no referrer information is to be sent along with requests to any
<a for=/>origin</a>. The header <a><code>Referer</code></a> will be omitted entirely.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>no-referrer</code>"</a>, then navigations to
<code>https://example.com/</code> (or any other URL) would send no
<a><code>Referer</code></a> header.
</div>
<h3 dfn export id="referrer-policy-no-referrer-when-downgrade" oldids="referrer-policy-state-no-referrer-when-downgrade">"<code>no-referrer-when-downgrade</code>"</h3>
The <a>"<code>no-referrer-when-downgrade</code>"</a> policy sends a request's full
<a>referrerURL</a> <a href="#strip-url">stripped for use as a referrer</a> for requests:
<ul>
<li>whose <a>referrerURL</a> and <a for=request>current URL</a> are both <a>potentially
trustworthy URLs</a>, or</li>
<li>whose <a>referrerURL</a> is a non-<a>potentially trustworthy URL</a>.</li>
</ul>
Requests whose <a>referrerURL</a> is a <a>potentially trustworthy URL</a> and whose
<a for=request>current URL</a> is a non-<a>potentially trustworthy URL</a> on the other hand, will
contain no referrer information. A <code><a>Referer</a></code> HTTP header will not be sent.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>no-referrer-when-downgrade</code>"</a>, then navigations to
<code>https://not.example.com/</code> would send a
<code><a>Referer</a></code> HTTP header with a value of
<code>https://example.com/page.html</code>, as neither resource's origin is a
non-<a>potentially trustworthy URL</a>.
Navigations from that same page to
<code><strong>http</strong>://not.example.com/</code> would send no
<a><code>Referer</code></a> header.
</div>
<h3 dfn export id="referrer-policy-same-origin">"<code>same-origin</code>"</h3>
The <a>"<code>same-origin</code>"</a> policy specifies that a request's full <a>referrerURL</a> is
sent as referrer information when making <a>same-origin-referrer requests</a>.
<a>Cross-origin-referrer requests</a>, on the other hand, will contain no
referrer information. A <code><a>Referer</a></code> HTTP header will not be
sent.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>same-origin</code>"</a>, then navigations to
<code>https://example.com/not-page.html</code> would send a
<a><code>Referer</code></a> header with a value of
<code>https://example.com/page.html</code>.
Navigations from that same page to
<code>https://<strong>not</strong>.example.com/</code> would send no
<a><code>Referer</code></a> header.
</div>
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>same-origin</code>"</a>, and fetches a module script at
<code>https://script.example.com</code>, which then fetches a descendant script
at <code>https://example.com/descendant.js</code>, the request for the descendant
script would send no <a><code>Referer</code></a> header.
This is because the descendant script request's <a for=request>current URL</a> is
<code>https://example.com/descendant.js</code>, while its <a>referrerURL</a> is
<code>https://script.example.com</code>, making the request
<a lt="cross-origin-referrer request">cross-origin-referrer</a>.
</div>
<h3 dfn export id="referrer-policy-origin" oldids="referrer-policy-state-origin">"<code>origin</code>"</h3>
The <a>"<code>origin</code>"</a> policy specifies that only the
<a lt="ASCII serialization of an origin">ASCII serialization</a> of the request's
<a>referrerURL</a> is sent as referrer information when making both <a>same-origin-referrer
requests</a> and <a>cross-origin-referrer requests</a>.
Note: The serialization of an origin looks like <code>https://example.com</code>.
To ensure that a valid URL is sent in the `<code>Referer</code>` header, user
agents will append a U+002F SOLIDUS ("<code>/</code>") character to the origin
(e.g. <code>https://example.com/</code>).
Note: The <a>"<code>origin</code>"</a> policy allows the origin of HTTPS
referrers to be sent over the network as part of unencrypted HTTP requests.
The <a>"<code>strict-origin</code>"</a> policy addresses this concern.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>origin</code>"</a>, then navigations to any
<a for=/>origin</a> would send a <a><code>Referer</code></a> header with a value
of <code>https://example.com/</code>, even to URLs that are not
<a>potentially trustworthy URL</a>.
</div>
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>origin</code>"</a>, and fetches a module script at
<code>https://script.example.com</code>, which fetches a descendant script at
<code>https://descendant.example.com</code>, the request for the descendant script
will send a <a><code>Referer</code></a> header with a value of
<code>https://script.example.com/</code>.
</div>
<h3 dfn export id="referrer-policy-strict-origin">"<code>strict-origin</code>"</h3>
The <a>"<code>strict-origin</code>"</a> policy sends the
<a lt="ASCII serialization of an origin">ASCII serialization</a> of the <a for=/>origin</a> of the
<a>referrerURL</a> for requests:
<ul>
<li>whose <a>referrerURL</a> and <a for=request>current URL</a> are both <a>potentially
trustworthy URLs</a>, or</li>
<li>whose <a>referrerURL</a> is a non-<a>potentially trustworthy URL</a>.</li>
</ul>
Requests whose <a>referrerURL</a> is a <a>potentially trustworthy URL</a> and whose
<a for=request>current URL</a> is a non-<a>potentially trustworthy URL</a> on the other hand, will
contain no referrer information. A <code><a>Referer</a></code> HTTP header will not be sent.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>strict-origin</code>"</a>, then navigations to
<code>https://<strong>not</strong>.example.com</code> would send a
<a><code>Referer</code></a> header with a value of
<code>https://example.com/</code>.
Navigations from that same page to
<code><strong>http://</strong>not.example.com</code> would send no
<a><code>Referer</code></a> header.
</div>
<div class="example">
If a document at <code>http://example.com/page.html</code> sets a policy of
<a>"<code>strict-origin</code>"</a>, then navigations to
<code>http://<strong>not</strong>.example.com</code> or
<code><strong>https</strong>://example.com</code> would send a
<a><code>Referer</code></a> header with a value of
<code>http://example.com/</code>.
</div>
<div class="example">
If a document at <code>http://example.com/page.html</code> sets a policy of
<a>"<code>strict-origin</code>"</a>, and fetches a module script at
<code><strong>https</strong>://script.example.com</code>, which then fetches a
descendant script at <code><strong>http</strong>://descendant.example.com</code>,
the request to the descendant script would not send a <a><code>Referrer</code></a>
header.
</div>
<h3 dfn export id="referrer-policy-origin-when-cross-origin" oldids="referrer-policy-state-origin-when-cross-origin">"<code>origin-when-cross-origin</code>"</h3>
The <a>"<code>origin-when-cross-origin</code>"</a> policy specifies that a request's full
<a>referrerURL</a> is sent as referrer information when making <a>same-origin-referrer requests</a>,
and only the <a lt="ASCII serialization of an origin">ASCII serialization</a> of the
<a for=/>origin</a> of the request's <a>referrerURL</a> is sent as referrer information when making
<a>cross-origin-referrer requests</a>.
Note: For the <a>"<code>origin-when-cross-origin</code>"</a> policy, we also
consider protocol upgrades, e.g. requests from
<code>http://example.com/</code> to <code>https://example.com/</code>, to be
<a>cross-origin-referrer requests</a>.
Note: The <a>"<code>origin-when-cross-origin</code>"</a> policy allows the
origin of HTTPS referrers to be sent over the network as part of unencrypted
HTTP requests. The <a>"<code>strict-origin-when-cross-origin</code>"</a> policy
addresses this concern.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>origin-when-cross-origin</code>"</a>, then navigations to
<code>https://example.com/not-page.html</code> would send a
<a><code>Referer</code></a> header with a value of
<code>https://example.com/page.html</code>.
Navigations from that same page to <code>https://not.example.com/</code>
would send a <a><code>Referer</code></a> header with a value of
<code>https://example.com/</code>, even to URLs that are not
<a>potentially trustworthy URL</a>s.
</div>
<div class="example">
If a document at <code>https://example-1.com</code> sets a policy of
<a>"<code>origin-when-cross-origin</code>"</a>, and fetches a module script at
<code>https://example-2.com/module.js</code>, which then fetches a descendant script at
<code>https://example-1.com/descendant.js</code>, the request to the descendant script would
send a <a><code>Referer</code></a> header with a value of <code>https://example-2.com/</code>.
</div>
<div class="example">
If a document at <code>https://example-1.com</code> sets a policy of
<a>"<code>origin-when-cross-origin</code>"</a>, and fetches a module script at
<code>https://example-2.com/module.js</code>, which then fetches a descendant script at
<code>https://example-2.com/descendant.js</code>, the request to the descendant script would
send a <a><code>Referer</code></a> header with a value of <code>https://example-2.com/module.js</code>.
</div>
<h3 dfn export id="referrer-policy-strict-origin-when-cross-origin">"<code>strict-origin-when-cross-origin</code>"</h3>
The <a>"<code>strict-origin-when-cross-origin</code>"</a> policy specifies that a request's full
<a>referrerURL</a> is sent as referrer information when making <a>same-origin-referrer requests</a>,
and only the <a lt="ASCII serialization of an origin">ASCII serialization</a> of the
<a for=/>origin</a> of the request's <a>referrerURL</a> when making
<a>cross-origin-referrer requests</a>:
<ul>
<li>whose <a>referrerURL</a> and <a for=request>current URL</a> are both <a>potentially
trustworthy URLs</a>, or</li>
<li>whose <a>referrerURL</a> is a non-<a>potentially trustworthy URL</a>.</li>
</ul>
Requests whose <a>referrerURL</a> is a <a>potentially trustworthy URL</a> and whose
<a for=request>current URL</a> is a non-<a>potentially trustworthy URL</a> on the other hand,
will contain no referrer information. A <code><a>Referer</a></code> HTTP header will not be sent.
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>strict-origin-when-cross-origin</code>"</a>, then navigations to
<code>https://example.com/not-page.html</code> would send a
<a><code>Referer</code></a> header with a value of
<code>https://example.com/page.html</code>.
Navigations from that same page to <code>https://not.example.com/</code>
would send a <a><code>Referer</code></a> header with a value of
<code>https://example.com/</code>.
Navigations from that same page to
<code><strong>http</strong>://not.example.com/</code> would send no
<a><code>Referer</code></a> header.
</div>
<div class="example">
If a document at <code>https://example.com/page.html</code> sets a policy of
<a>"<code>strict-origin-when-cross-origin</code>"</a>, and fetches a module script
at <code>https://script.example.com</code> which then fetches a descendant script at
<code><strong>http</strong>://descendant.example.com</code>, the request to the descendant
script would send no <a><code>Referer</code></a> header.
</div>
This policy is the user agent's <a lt="default referrer policy">default</a>, and will be applied
if no policy is otherwise specified.
<h3 dfn export id="referrer-policy-unsafe-url" oldids="referrer-policy-state-unsafe-url">"<code>unsafe-url</code>"</h3>
The <a>"<code>unsafe-url</code>"</a> policy specifies that a request's full <a>referrerURL</a> is
sent along for both <a>same-origin-referrer requests</a> and <a>cross-origin-referrer requests</a>.
<div class="example">
If a document at <code>https://example.com/sekrit.html</code> sets a policy
of <a>"<code>unsafe-url</code>"</a>, then navigations to
<code>http://not.example.com/</code> (and every other origin) would send a
<code><a>Referer</a></code> HTTP header with a value of
<code>https://example.com/sekrit.html</code>.
</div>
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.
<h3 dfn export id="referrer-policy-empty-string">The empty string</h3>
The empty string "" corresponds to no <a for="/">referrer policy</a>, causing a
fallback to a <a for="/">referrer policy</a> defined elsewhere, or in the case where
no such higher-level policy is available, falling back to the <a>default referrer
policy</a>. This happens in Fetch's main fetch algorithm, for example.
<div class="example">
Given a HTML <{a}> element without any declared <{a/referrerpolicy}>
attribute, its referrer policy is the empty string. Thus, navigation requests initiated by
clicking on that <{a}> element will be sent with the <{a}> element's <a>node document</a>'s <a
for="Document">policy container</a>'s <a for="policy container">referrer policy</a>. If that
{{Document}} has the empty string as its referrer policy, the [[#determine-requests-referrer]]
algorithm will treat the empty string the same as
<a>"<code>strict-origin-when-cross-origin</code>"</a>.
</div>
</section>
<section>
<h2 id="referrer-policy-delivery">Referrer Policy Delivery</h2>
A <a for=/>request</a>'s <a for=request>referrer policy</a>
is delivered in one of five ways:
<ul>
<li>
Via the <code>Referrer-Policy</code> HTTP header (defined
in [[#referrer-policy-header]]).
</li>
<li>
Via a <{meta}> element with a <{meta/name}> of
<a for="meta"><code>referrer</code></a>.
</li>
<li>
Via a <code>referrerpolicy</code> content attribute on an <{a}>,
<{area}>, <{img}>, <{iframe}>, <{link}>, or <{script}> element.
</li>
<li>
Via the <code><a>noreferrer</a></code> link relation on an <{a}>, or
<{area}> element.
</li>
<li>
Implicitly, via inheritance.
</li>
</ul>
<h3 id="referrer-policy-header">Delivery via Referrer-Policy header</h3>
The <code><dfn local-lt="referrer-policy header"
id="referrer-policy-header-dfn">Referrer-Policy</dfn></code> HTTP header
specifies the referrer policy that the user agent applies when determining
what referrer information should be included with requests made, and with
<a for=/>browsing contexts</a> 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 <code>#rule</code> ABNF
extension used below is defined in [[RFC9110#abnf.extension|Section 5.6.1]] of
[[!RFC9110]].
<pre dfn-type="grammar" link-type="grammar">
"Referrer-Policy:" 1#(<a>policy-token</a> / <a>extension-token</a>)
</pre>
<pre dfn-type="grammar" link-type="grammar">
<dfn export>policy-token</dfn> = "no-referrer" / "no-referrer-when-downgrade" / "strict-origin" / "strict-origin-when-cross-origin" / "same-origin" / "origin" / "origin-when-cross-origin" / "unsafe-url"
<dfn>extension-token</dfn> = 1*( <a>ALPHA</a> / "-" )
</pre>
Note: The header name does not share the HTTP Referer header's misspelling.
Note: The purpose of <a link-type="grammar">extension-token</a> is so that
browsers do not fail to parse the entire header field if it includes an
unknown policy value. [[#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.
[[#integration-with-fetch]] and [[#integration-with-html]] describe
how the <code>Referrer-Policy</code> header is processed.
<section class="informative">
<h4 id="referrer-usage">Usage</h4>
<em>This section is not normative.</em>
A protected resource can prevent referrer leakage by specifying
<code>no-referrer</code> as the value of its
<code>Referrer-Policy</code> header:
<pre>
Referrer-Policy: no-referrer
</pre>
This will cause all requests made from the protected resource's
context to have an empty <code>Referer</code> [sic] header.
</section>
<section class="informative">
<h3 id="referrer-policy-delivery-meta">Delivery via <{meta}></h3>
<em>This section is not normative.</em>
The HTML Standard defines the <a for="meta"><code>referrer</code></a>
keyword for the <{meta}> element, which allows setting the <a for="/">referrer
policy</a> via markup.
</section>
<section class="informative">
<h3 id="referrer-policy-delivery-referrer-attribute">Delivery
via a <code>referrerpolicy</code> content attribute</h3>
<em>This section is not normative.</em>
The HTML Standard defines the concept of <a>referrer policy
attributes</a> which applies to several of its elements, for example:
<pre class="example"><code class="lang-html">
<a href="http://example.com" referrerpolicy="origin">
</code></pre>
</section>
<section class="informative">
<h3 id="referrer-policy-inheritance"
oldids="referrer-policy-delivery-nested, referrer-policy-delivery-implicit"><a for="/">Referrer Policy</a> Inheritance</h3>
<em>This section is not normative.</em>
Referrer policy is inherited following the inheritance mechanism of
<a for="/">policy containers</a>, as defined by HTML.
</section>
</section>
<section class="informative">
<h2 id="integration-with-fetch">Integration with Fetch</h2>
<em>This section is not normative.</em>
The Fetch specification calls out to
[[#set-requests-referrer-policy-on-redirect]]
before [[fetch#http-redirect-fetch|Step 13 of the HTTP-redirect fetch]], so
that a request's referrer policy can be updated before following a redirect.
The Fetch specification calls out to [[#determine-requests-referrer]]
as [[fetch#main-fetch|Step 8 of the Main fetch algorithm]], and uses the
result to set the <var>request</var>'s <code>referrer</code> property. Fetch
is responsible for serializing the URL provided, and setting the
`<code>Referer</code>` header on <var>request</var>.
</section>
<section class="informative">
<h2 id="integration-with-html">Integration with HTML</h2>
<em>This section is not normative.</em>
The HTML Standard determines the <a for="/">referrer policy</a> of any response
received during <a>navigation</a> or while <a>running a worker</a>, and uses
the result to set the resulting {{Document}}'s <a for=Document>policy
container</a>'s or {{WorkerGlobalScope}}'s <a for=WorkerGlobalScope>policy
container</a>'s <a for="policy container">referrer policy</a>.
</section>
<section>
<h2 id="integration-with-css">Integration with CSS</h2>
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 <a for="/">requests</a> initiated by stylesheets
as follows:
<ol>
<li>
If a <a>CSS style sheet</a> is responsible for the request,
and its <a>location</a> is non-null,
set the <a for="request">referrer</a> to its
<a>location</a>, and the <a for="request">referrer
policy</a> to its <a for="CSSStyleSheet">referrer policy</a>.
Issue: This requires that CSS style sheets process `Referrer-Policy`
headers, and store a <dfn for="CSSStyleSheet">referrer policy</dfn>
in the same way that <a for="policy container" lt="referrer policy">Documents
do</a>.
</li>
<li>
If a <a>CSS style sheet</a> with a null <a>location</a> is responsible
for the request, set the <a for="request">referrer</a> to its
<a for="CSSStyleSheet">owner node</a>'s
<a>node document</a>'s <a for="Document">URL</a>, and the
<a for="request">referrer policy</a> to its
<a for="CSSStyleSheet">owner node</a>'s
<a>node document</a>'s <a for="Document">policy container</a>'s
<a for="policy container">referrer policy</a>.
</li>
<li>
Otherwise, a <a>CSS declaration block</a> that was created by the
embedder is responsible for the request - either from parsing of an
element's <a>style attribute</a>, or to implement an <a>presentational
hint</a> for an element. We assume that in this case the <a>CSS
declaration block</a>'s <a for="CSSStyleDeclaration">owner node</a>
points to that element, and set the <a for="request">referrer</a> to the
block's <a for="CSSStyleDeclaration">owner node</a>'s <a>node
document</a>'s <a for="Document">URL</a>, and the
<a for="request">referrer policy</a> to the block's
<a for="CSSStyleDeclaration">owner node</a>'s <a>node document</a>'s
<a for="Document">policy container</a>'s
<a for="policy container">referrer policy</a>.
</li>
</ol>
Note: Both the value of the <a for="/">request</a>'s <a for="request">referrer</a>
and <a for="request">referrer policy</a> are set based on the values at the
time a given <a for="/">request</a> is created. If a document's referrer policy
changes during its lifetime, the policy associated with inline stylesheet
requests will also change.
</section>
<section>
<h2 id="algorithms">Algorithms</h2>
<h3 id="parse-referrer-policy-from-header">
Parse a referrer policy from a <a><code>Referrer-Policy</code></a> header
</h3>
Given a <a for=/>response</a> |response|, the following steps return a
<a for="/">referrer policy</a> according to |response|'s
`<code>Referrer-Policy</code>` header:
<ol>
<li>
Let |policy-tokens| be the result of <a>extracting header list values</a> given
`<code>Referrer-Policy</code>` and |response|'s <a for=response>header list</a>.
</li>
<li>
Let |policy| be the empty string.
</li>
<li>
For each |token| in |policy-tokens|, if |token| is a <a for="/">referrer
policy</a> 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 [[#unknown-policy-values]].
</li>
<li>
Return |policy|.
</li>
</ol>
<h3 id="set-requests-referrer-policy-on-redirect" dfn export>
Set |request|'s referrer policy on redirect
</h3>
Given a <a for=/>request</a> |request| and a <a for=/>response</a> |actualResponse|,
this algorithm updates |request|'s <a for=request>referrer policy</a>
according to the Referrer-Policy header (if any) in |actualResponse|.
<ol>
<li>
Let |policy| be the result of executing
[[#parse-referrer-policy-from-header]] on |actualResponse|.
</li>
<li>If |policy| is not the empty string, then set |request|'s
<a for=request>referrer policy</a> to |policy|.</li>
</ol>
<h3 id="determine-requests-referrer" dfn export>
Determine <var>request</var>'s Referrer
</h3>
Given a <a for=/>request</a> |request|, we can determine the correct
referrer information to send by examining its <a for=request>referrer policy</a>
as detailed in the following steps, which return either <code>no referrer</code> or a URL:
<ol>
<li>
Let <var>policy</var> be |request|'s <a for=request>referrer policy</a>.
</li>
<li>
Let <var>environment</var> be <var>request</var>'s <a for=request>client</a>.
</li>
<li>
Switch on |request|'s <a for=request>referrer</a>:
<dl class="switch">
<dt>"<code>client</code>"</dt>
<dd>
<ol>
<li>
If |environment|'s <a for="environment settings object">global
object</a> is a {{Window}} object, then
<ol>
<li>Let |document| be
the <a>associated <code>Document</code></a> of |environment|'s
<a for="environment settings object">global object</a>.</li>
<li>If |document|'s <a for=Document>origin</a> is an <a>opaque origin</a>,
return <code>no referrer</code>.</li>
<li>While |document| is <a>an <code>iframe srcdoc</code>
document</a>, let |document| be |document|'s <a
for="Document">browsing context</a>'s <a>browsing context
container</a>'s <a>node document</a>.</li>
<li>Let |referrerSource| be |document|'s <a
for="Document">URL</a>.</li>
</ol>
</li>
<li>Otherwise, let |referrerSource| be |environment|'s <a for=environment>creation
URL</a>.</li>
</ol>
</dd>
<dt>a <a for=/>URL</a></dt>
<dd>Let |referrerSource| be |request|'s <a for=request>referrer</a>.</dd>
</dl>
Note: If <var>request</var>'s <a for=request>referrer</a> is
"<code>no-referrer</code>", Fetch will not call into this algorithm.
</li>
<li>
Let request's <dfn>referrerURL</dfn> be the result of <a href="#strip-url">stripping
<var>referrerSource</var> for use as a referrer.</a>
</li>
<li>
Let <var>referrerOrigin</var> be the result of
<a href="#strip-url">stripping <var>referrerSource</var> for use as a
referrer</a>, with the <code><a>origin-only flag</a></code> set to
<code>true</code>.
</li>
<li>
If the result of <a lt="url serializer">serializing</a> <a>referrerURL</a> is a string whose
<a for="string">length</a> is greater than 4096, set <span>referrerURL</span> to
<var>referrerOrigin</var>.
</li>
<li>
The user agent MAY alter <a>referrerURL</a> or <var>referrerOrigin</var> 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
<a for="url">host</a>, replace it with an empty string, etc.
</li>
<li>
Execute the statements corresponding to the value of <var>policy</var>:<br>
Note: If <var>request</var>'s <a for=request>referrer policy</a> is
the empty string, Fetch will not call into this algorithm.
<dl class="switch">
<dt><a>"<code>no-referrer</code>"</a></dt>
<dd>Return <code>no referrer</code></dd>
<dt><a>"<code>origin</code>"</a></dt>
<dd>Return <var>referrerOrigin</var></dd>
<dt><a>"<code>unsafe-url</code>"</a></dt>
<dd>Return <a>referrerURL</a>.</dd>
<dt><a>"<code>strict-origin</code>"</a></dt>
<dd>
<ol>
<li>
If <a>referrerURL</a> is a <a>potentially trustworthy URL</a> and <var>request</var>'s
<a for=request>current URL</a> is not a <a>potentially trustworthy URL</a>, then
return <code>no referrer</code>.
</li>
<li>Return |referrerOrigin|.
</ol>
</dd>
<dt><a>"<code>strict-origin-when-cross-origin</code>"</a></dt>
<dd>
<ol>
<li>
If the <a for=url>origin</a> of <a>referrerURL</a> and the <a for=url>origin</a> of
<var>request</var>'s <a for=request>current URL</a> are <a lt="same origin">the
same</a>, then return <a>referrerURL</a>.
</li>
<li>
If <a>referrerURL</a> is a <a>potentially trustworthy URL</a> and <var>request</var>'s
<a for=request>current URL</a> is not a <a>potentially trustworthy URL</a>, then
return <code>no referrer</code>.
</li>
<li>Return |referrerOrigin|.
</ol>
</dd>
<dt><a>"<code>same-origin</code>"</a></dt>
<dd>
<ol>
<li>
If the <a for=url>origin</a> of <a>referrerURL</a> and the <a for=url>origin</a> of
<var>request</var>'s <a for=request>current URL</a> are <a lt="same origin">the
same</a>, then return <a>referrerURL</a>.
Note: This same-origin check determines whether or not the request is
<a lt="same-origin-referrer request">same-origin-referrer</a>.
</li>
<li>Return <code>no referrer</code>.
</ol>
</dd>
<dt><a>"<code>origin-when-cross-origin</code>"</a></dt>
<dd>
<ol>
<li>
If the <a for=url>origin</a> of <a>referrerURL</a> and the <a for=url>origin</a> of
<var>request</var>'s <a for=request>current URL</a> are <a lt="same origin">the
same</a>, then return <a>referrerURL</a>.
</li>
<li> Return |referrerOrigin|.
</ol>
</dd>
<dt><a>"<code>no-referrer-when-downgrade</code>"</a></dt>
<dd>
<ol>
<li>
If <a>referrerURL</a> is a <a>potentially trustworthy URL</a> and <var>request</var>'s
<a for=request>current URL</a> is not a <a>potentially trustworthy URL</a>, then
return <code>no referrer</code>.
</li>
<li>Return <a>referrerURL</a>.
</ol>
</dd>
</dl>
</li>
</ol>
<h3 id="strip-url">
Strip <var>url</var> for use as a referrer
</h3>
Certain portions of URLs must not be included when sending a URL as the value
of a `<code>Referer</code>` header: a URLs fragment, username, and password
components must be stripped from the URL before it's sent out. This
algorithm accepts a <code><dfn>origin-only flag</dfn></code>, which defaults
to <code>false</code>. If set to <code>true</code>, the algorithm will
additionally remove the URL's path and query components, leaving only the
scheme, host, and port.
<ol>
<li><a>Assert</a>: <var>url</var> is a <a for=/>URL</a>.</li>
<li>
If <var>url</var>'s <a for=url>scheme</a> is a <a>local scheme</a>, then
return <code>no referrer</code>.
</li>
<li>
Set <var>url</var>'s <a for=url>username</a> to the empty string.
</li>
<li>
Set <var>url</var>'s <a for=url>password</a> to the empty string.
</li>
<li>
Set <var>url</var>'s <a for=url>fragment</a> to <code>null</code>.
</li>
<li>
If the <code><a>origin-only flag</a></code> is <code>true</code>,
then:
<ol>
<li>
Set <var>url</var>'s <a for=url>path</a> to « the empty string ».
</li>
<li>
Set <var>url</var>'s <a for=url>query</a> to <code>null</code>.
</li>
</ol>
</li>
<li>
Return <var>url</var>.
</li>
</ol>
</section>
<section>
<h2 id="privacy">Privacy Considerations</h2>
<h3 id="user-controls">User Controls</h3>
Nothing in this specification should be interpreted as preventing user