Referrer-Policy

The Referrer-Policy HTTP header controls how much referrer information (sent via the Referer header) should be included with requests. Aside from the HTTP header, you can set this policy in HTML.

Referrer-Policy: no-referrer
Referrer-Policy: no-referrer-when-downgrade
Referrer-Policy: origin
Referrer-Policy: origin-when-cross-origin
Referrer-Policy: same-origin
Referrer-Policy: strict-origin
Referrer-Policy: strict-origin-when-cross-origin
Referrer-Policy: unsafe-url

no-referrer

The Referer header will be omitted entirely. No referrer information is sent along with requests.

no-referrer-when-downgrade (default)

This is the default behavior if no policy is specified, or if the provided value is invalid. The origin, path, and querystring of the URL are sent as a referrer when the protocol security level stays the same (HTTP→HTTP, HTTPS→HTTPS) or improves (HTTP→HTTPS), but isn't sent to less secure destinations (HTTPS→HTTP).

There is effort from browsers in moving to a stricter default value, namely strict-origin-when-cross-origin (see https://github.com/whatwg/fetch/pull/952), consider using this value (or a stricter one), if possible, when changing the Referrer-Policy.

origin

Only send the origin of the document as the referrer.
For example, a document at https://example.com/page.html will send the referrer https://example.com/.

origin-when-cross-origin

Send the origin, path, and query string when performing a same-origin request, but only send the origin of the document for other cases.

same-origin

A referrer will be sent for same-site origins, but cross-origin requests will send no referrer information.

strict-origin

Only send the origin of the document as the referrer when the protocol security level stays the same (HTTPS→HTTPS), but don't send it to a less secure destination (HTTPS→HTTP).

strict-origin-when-cross-origin

Send the origin, path, and querystring when performing a same-origin request, only send the origin when the protocol security level stays the same while performing a cross-origin request (HTTPS→HTTPS), and send no header to any less-secure destinations (HTTPS→HTTP).

unsafe-url

Send the origin, path, and query string when performing any request, regardless of security.

This policy will leak potentially-private information from HTTPS resource URLs to insecure origins. Carefully consider the impact of this setting.

You can also set referrer policies inside HTML. For example, you can set the referrer policy for the entire document with a <meta> element with a name of referrer:

<meta name="referrer" content="origin">

Or set it for individual requests with the referrerpolicy attribute on <a>, <area>, <img>, <iframe>, <script>, or <link> elements:

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

Alternatively, a noreferrer link relation on an a, area, or link element can be set:

<a href="http://example.com" rel="noreferrer">

CSS can fetch resources referenced from stylesheets. These resources follow a referrer policy as well:

• External CSS stylesheets use the default policy (no-referrer-when-downgrade), unless it's overwritten via a Referrer-Policy HTTP header on the CSS stylesheet’s response.
• For <style> elements or style attributes, the owner document's referrer policy is used.

If you want to specify a fallback policy in any case the desired policy hasn't got wide enough browser support, use a comma-separated list with the desired policy specified last:

Referrer-Policy: no-referrer, strict-origin-when-cross-origin

In the above scenario, no-referrer will only be used if strict-origin-when-cross-origin is not supported by the browser.

Specifying multiple values is only supported in the Referrer-Policy HTTP header, and not in the referrerpolicy attribute.

• HTTP referer on Wikipedia
• When using Fetch: Request.referrerPolicy
• The obsolete Content-Security-Policy referrer directive.
• Same-origin policy

• Tighter Control Over Your Referrers – Mozilla Security Blog