Set-Cookie

<cookie-name>=<cookie-value>

Defines the cookie name and its value. A cookie definition begins with a name-value pair.

A <cookie-name> can contain any US-ASCII characters except for: control characters (ASCII characters 0 up to 31 and ASCII character 127) or separator characters (space, tab and the characters: ( ) < > @ , ; : \ " / [ ] ? = { })

A <cookie-value> can optionally be wrapped in double quotes and include any US-ASCII character excluding control characters (ASCII characters 0 up to 31 and ASCII character 127), Whitespace, double quotes, commas, semicolons, and backslashes.

Encoding: Many implementations perform percent-encoding on cookie values. However, this is not required by the RFC specification. The percent-encoding does help to satisfy the requirements of the characters allowed for <cookie-value>.

Note: Some <cookie-name> have a specific semantic:

__Secure- prefix: Cookies with names starting with __Secure- (dash is part of the prefix) must be set with the secure flag from a secure page (HTTPS).

__Host- prefix: Cookies with names starting with __Host- are sent only to the host subdomain or domain that set them, and not to any other host. They must be set with the secure flag, must be from a secure page (HTTPS), must not have a domain specified, and the path must be /.

Domain=<domain-value> Optional

Defines the host to which the cookie will be sent.

Only the current domain can be set as the value, or a domain of a higher order, unless it is a public suffix. Setting the domain will make the cookie available to it, as well as to all its subdomains.

If omitted, this attribute defaults to the host of the current document URL, not including subdomains.

Contrary to earlier specifications, leading dots in domain names (.example.com) are ignored.

Multiple host/domain values are not allowed, but if a domain is specified, then subdomains are always included.

Expires=<date> Optional

Indicates the maximum lifetime of the cookie as an HTTP-date timestamp. See Date for the required formatting.

If unspecified, the cookie becomes a session cookie. A session finishes when the client shuts down, after which the session cookie is removed.

Warning: Many web browsers have a session restore feature that will save all tabs and restore them the next time the browser is used. Session cookies will also be restored, as if the browser was never closed.

When an Expires date is set, the deadline is relative to the client the cookie is being set on, not the server.

HttpOnly Optional

Forbids JavaScript from accessing the cookie, for example, through the Document.cookie property. Note that a cookie that has been created with HttpOnly will still be sent with JavaScript-initiated requests, for example, when calling XMLHttpRequest.send() or fetch(). This mitigates attacks against cross-site scripting (XSS).

Max-Age=<number> Optional

Indicates the number of seconds until the cookie expires. A zero or negative number will expire the cookie immediately. If both Expires and Max-Age are set, Max-Age has precedence.

Partitioned Optional

Indicates that the cookie should be stored using partitioned storage. Note that if this is set, the Secure directive must also be set. See Cookies Having Independent Partitioned State (CHIPS) for more details.

Path=<path-value> Optional

Indicates the path that must exist in the requested URL for the browser to send the Cookie header.

The forward slash (/) character is interpreted as a directory separator, and subdirectories are matched as well. For example, for Path=/docs,

• the request paths /docs, /docs/, /docs/Web/, and /docs/Web/HTTP will all match.
• the request paths /, /docsets, /fr/docs will not match.

SameSite=<samesite-value> Optional

Controls whether or not a cookie is sent with cross-site requests, providing some protection against cross-site request forgery attacks (CSRF).

The possible attribute values are:

Strict

Means that the browser sends the cookie only for same-site requests, that is, requests originating from the same site that set the cookie. If a request originates from a different domain or scheme (even with the same domain), no cookies with the SameSite=Strict attribute are sent.

Lax

Means that the cookie is not sent on cross-site requests, such as on requests to load images or frames, but is sent when a user is navigating to the origin site from an external site (for example, when following a link). This is the default behavior if the SameSite attribute is not specified.

None

Means that the browser sends the cookie with both cross-site and same-site requests. The Secure attribute must also be set when setting this value, like so SameSite=None; Secure. If Secure is missing an error will be logged:

Cookie "myCookie" rejected because it has the "SameSite=None" attribute but is missing the "secure" attribute.

This Set-Cookie was blocked because it had the "SameSite=None" attribute but did not have the "Secure" attribute, which is required in order to use "SameSite=None".

Note: A Secure cookie is only sent to the server with an encrypted request over the HTTPS protocol. Note that insecure sites (http:) can't set cookies with the Secure directive, and therefore can't use SameSite=None.

Warning: Cookies with the SameSite=None; Secure that do not also have the Partitioned attribute may be blocked in cross-site contexts on future browser versions. This behavior protects user data from cross-site tracking. See Cookies Having Independent Partitioned State (CHIPS) and Third-party cookies.

Secure Optional

Indicates that the cookie is sent to the server only when a request is made with the https: scheme (except on localhost), and therefore, is more resistant to man-in-the-middle attacks.

Note: Do not assume that Secure prevents all access to sensitive information in cookies (session keys, login details, etc.). Cookies with this attribute can still be read/modified either with access to the client's hard disk or from JavaScript if the HttpOnly cookie attribute is not set.

Insecure sites (http:) cannot set cookies with the Secure attribute (since Chrome 52 and Firefox 52). The https: requirements are ignored when the Secure attribute is set by localhost (since Chrome 89 and Firefox 75).

Session cookies are removed when the client shuts down. Cookies are session cookies if they do not specify the Expires or Max-Age attribute.

Set-Cookie: sessionId=38afes7a8

Permanent cookies are removed at a specific date (Expires) or after a specific length of time (Max-Age) and not when the client is closed.

Set-Cookie: id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT

Set-Cookie: id=a3fWa; Max-Age=2592000

A cookie for a domain that does not include the server that set it should be rejected by the user agent.

The following cookie will be rejected if set by a server hosted on original-company.com:

Set-Cookie: qwerty=219ffwef9w0f; Domain=some-company.co.uk

A cookie for a subdomain of the serving domain will be rejected.

The following cookie will be rejected if set by a server hosted on example.com:

Set-Cookie: sessionId=e8bb43229de9; Domain=foo.example.com

Cookie names prefixed with __Secure- or __Host- can be used only if they are set with the secure attribute from a secure (HTTPS) origin.

In addition, cookies with the __Host- prefix must have a path of / (meaning any path at the host) and must not have a Domain attribute.

// Both accepted when from a secure origin (HTTPS)
Set-Cookie: __Secure-ID=123; Secure; Domain=example.com
Set-Cookie: __Host-ID=123; Secure; Path=/

// Rejected due to missing Secure attribute
Set-Cookie: __Secure-id=1

// Rejected due to the missing Path=/ attribute
Set-Cookie: __Host-id=1; Secure

// Rejected due to setting a Domain
Set-Cookie: __Host-id=1; Secure; Path=/; Domain=example.com

• Starting with Chrome 52 and Firefox 52, insecure sites (http:) can't set cookies with the Secure attribute anymore.

• HTTP cookies
• Cookie
• Document.cookie
• Samesite cookies explained (web.dev blog)