Cookies: HTTP State Management Mechanism
draft-ietf-httpbis-rfc6265bis-22

HTTP S. Bingler, Ed.
Internet-Draft 
Obsoletes: 6265 (if approved) M. West, Ed.
Intended status: Standards Track Google LLC
Expires: 4 June 2026 J. Wilander, Ed.
 Apple, Inc
 1 December 2025
 Cookies: HTTP State Management Mechanism
 draft-ietf-httpbis-rfc6265bis-22
Abstract
 This document defines the HTTP Cookie and Set-Cookie header fields.
 These header fields can be used by HTTP servers to store state
 (called cookies) at HTTP user agents, letting the servers maintain a
 stateful session over the mostly stateless HTTP protocol. Although
 cookies have many historical flaws that degrade their security and
 privacy, the Cookie and Set-Cookie header fields are widely used on
 the Internet. This document obsoletes RFC 6265.
About This Document
 This note is to be removed before publishing as an RFC.
 Status information for this document may be found at
 https://datatracker.ietf.org/doc/draft-ietf-httpbis-rfc6265bis/.
 Discussion of this document takes place on the HTTP Working Group
 mailing list (mailto:ietf-http-wg@w3.org), which is archived at
 https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group
 information can be found at https://httpwg.org/.
 Source for this draft and an issue tracker can be found at
 https://github.com/httpwg/http-extensions/labels/6265bis.
Status of This Memo
 This Internet-Draft is submitted in full conformance with the
 provisions of BCP 78 and BCP 79.
 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF). Note that other groups may also distribute
 working documents as Internet-Drafts. The list of current Internet-
 Drafts is at https://datatracker.ietf.org/drafts/current/.
Bingler, et al. Expires 4 June 2026 [Page 1]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time. It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as "work in progress."
 This Internet-Draft will expire on 4 June 2026.
Copyright Notice
 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents (https://trustee.ietf.org/
 license-info) in effect on the date of publication of this document.
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document. Code Components
 extracted from this document must include Revised BSD License text as
 described in Section 4.e of the Trust Legal Provisions and are
 provided without warranty as described in the Revised BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008. The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.
Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
 2.1. Conformance Criteria . . . . . . . . . . . . . . . . . . 5
 2.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
 2.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
 2.4. Name Resolution System . . . . . . . . . . . . . . . . . 7
 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 7
 3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
 3.2. Which Requirements to Implement . . . . . . . . . . . . . 10
 3.2.1. Cookie Producing Implementations . . . . . . . . . . 11
 3.2.2. Cookie Consuming Implementations . . . . . . . . . . 11
 4. Server Requirements . . . . . . . . . . . . . . . . . . . . . 12
 4.1. Set-Cookie Header . . . . . . . . . . . . . . . . . . . . 12
Bingler, et al. Expires 4 June 2026 [Page 2]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 4.1.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . 12
 4.1.2. Semantics (Non-Normative) . . . . . . . . . . . . . . 15
 4.1.3. Cookie Name Prefixes . . . . . . . . . . . . . . . . 18
 4.2. Cookie Header . . . . . . . . . . . . . . . . . . . . . . 19
 4.2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . 19
 4.2.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 20
 5. User Agent Requirements . . . . . . . . . . . . . . . . . . . 20
 5.1. Subcomponent Algorithms . . . . . . . . . . . . . . . . . 21
 5.1.1. Dates . . . . . . . . . . . . . . . . . . . . . . . . 21
 5.1.2. Canonicalized Host Names . . . . . . . . . . . . . . 23
 5.1.3. Domain Matching . . . . . . . . . . . . . . . . . . . 23
 5.1.4. Paths and Path-Match . . . . . . . . . . . . . . . . 23
 5.2. "Same-site" and "cross-site" Requests . . . . . . . . . . 24
 5.2.1. Document-based requests . . . . . . . . . . . . . . . 25
 5.2.2. Worker-based requests . . . . . . . . . . . . . . . . 26
 5.3. Ignoring Set-Cookie Header Fields . . . . . . . . . . . . 27
 5.4. Cookie Name Prefixes . . . . . . . . . . . . . . . . . . 27
 5.5. Cookie Lifetime Limits . . . . . . . . . . . . . . . . . 28
 5.6. The Set-Cookie Header Field . . . . . . . . . . . . . . . 29
 5.6.1. The Expires Attribute . . . . . . . . . . . . . . . . 31
 5.6.2. The Max-Age Attribute . . . . . . . . . . . . . . . . 32
 5.6.3. The Domain Attribute . . . . . . . . . . . . . . . . 32
 5.6.4. The Path Attribute . . . . . . . . . . . . . . . . . 33
 5.6.5. The Secure Attribute . . . . . . . . . . . . . . . . 33
 5.6.6. The HttpOnly Attribute . . . . . . . . . . . . . . . 33
 5.6.7. The SameSite Attribute . . . . . . . . . . . . . . . 33
 5.7. Storage Model . . . . . . . . . . . . . . . . . . . . . . 35
 5.8. Retrieval Model . . . . . . . . . . . . . . . . . . . . . 41
 5.8.1. The Cookie Header Field . . . . . . . . . . . . . . . 41
 5.8.2. Non-HTTP APIs . . . . . . . . . . . . . . . . . . . . 42
 5.8.3. Retrieval Algorithm . . . . . . . . . . . . . . . . . 42
 6. Implementation Considerations . . . . . . . . . . . . . . . . 44
 6.1. Limits . . . . . . . . . . . . . . . . . . . . . . . . . 44
 6.2. Application Programming Interfaces . . . . . . . . . . . 45
 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 45
 7.1. Third-Party Cookies . . . . . . . . . . . . . . . . . . . 46
 7.2. Cookie Policy . . . . . . . . . . . . . . . . . . . . . . 47
 7.3. User Controls . . . . . . . . . . . . . . . . . . . . . . 47
 7.4. Expiration Dates . . . . . . . . . . . . . . . . . . . . 48
 8. Security Considerations . . . . . . . . . . . . . . . . . . . 48
 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 48
 8.2. Ambient Authority . . . . . . . . . . . . . . . . . . . . 49
 8.3. Clear Text . . . . . . . . . . . . . . . . . . . . . . . 49
 8.4. Session Identifiers . . . . . . . . . . . . . . . . . . . 50
 8.5. Weak Confidentiality . . . . . . . . . . . . . . . . . . 51
 8.6. Weak Integrity . . . . . . . . . . . . . . . . . . . . . 51
 8.7. Reliance on DNS . . . . . . . . . . . . . . . . . . . . . 52
 8.8. SameSite Cookies . . . . . . . . . . . . . . . . . . . . 52
Bingler, et al. Expires 4 June 2026 [Page 3]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 8.8.1. Defense in depth . . . . . . . . . . . . . . . . . . 52
 8.8.2. Top-level Navigations . . . . . . . . . . . . . . . . 53
 8.8.3. Mashups and Widgets . . . . . . . . . . . . . . . . . 54
 8.8.4. Server-controlled . . . . . . . . . . . . . . . . . . 54
 8.8.5. Reload navigations . . . . . . . . . . . . . . . . . 54
 8.8.6. Top-level requests with "unsafe" methods . . . . . . 55
 8.9. Public Suffix List . . . . . . . . . . . . . . . . . . . 56
 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 56
 9.1. Cookie . . . . . . . . . . . . . . . . . . . . . . . . . 56
 9.2. Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . 56
 9.3. "Cookie Attributes" Registry . . . . . . . . . . . . . . 57
 9.3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 57
 9.3.2. Registration . . . . . . . . . . . . . . . . . . . . 57
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 57
 10.1. Normative References . . . . . . . . . . . . . . . . . . 58
 10.2. Informative References . . . . . . . . . . . . . . . . . 59
 Appendix A. Changes from RFC 6265 . . . . . . . . . . . . . . . 61
 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 62
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
1. Introduction
 This document defines the HTTP Cookie and Set-Cookie header fields.
 Using the Set-Cookie header field, an HTTP server can pass name/value
 pairs and associated metadata (called cookies) to a user agent. When
 the user agent makes subsequent requests to the server, the user
 agent uses the metadata and other information to determine whether to
 return the name/value pairs in the Cookie header field.
 Although simple on their surface, cookies have a number of
 complexities. For example, the server indicates a scope for each
 cookie when sending it to the user agent. The scope indicates the
 maximum amount of time in which the user agent should return the
 cookie, the servers to which the user agent should return the cookie,
 and the connection types for which the cookie is applicable.
 For historical reasons, cookies contain a number of security and
 privacy flaws. For example, a server can indicate that a given
 cookie is intended for "secure" connections, but the Secure attribute
 does not provide integrity in the presence of an active network
 attacker. Similarly, cookies for a given host are shared across all
 the ports on that host, even though the usual "same-origin policy"
 used by web browsers isolates content retrieved via different ports.
 This specification applies to developers of both cookie-producing
 servers and cookie-consuming user agents. Section 3.2 helps to
 clarify the intended target audience for each implementation type.
Bingler, et al. Expires 4 June 2026 [Page 4]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 To maximize interoperability with user agents, servers SHOULD limit
 themselves to the well-behaved profile defined in Section 4 when
 generating cookies.
 User agents MUST implement the more liberal processing rules defined
 in Section 5, in order to maximize interoperability with existing
 servers that do not conform to the well-behaved profile defined in
 Section 4.
 This document specifies the syntax and semantics of these header
 fields as they are actually used on the Internet. In particular,
 this document does not create new syntax or semantics beyond those in
 use today. The recommendations for cookie generation provided in
 Section 4 represent a preferred subset of current server behavior,
 and even the more liberal cookie processing algorithm provided in
 Section 5 does not recommend all of the syntactic and semantic
 variations in use today. Where some existing software differs from
 the recommended protocol in significant ways, the document contains a
 note explaining the difference.
 This document obsoletes [RFC6265].
2. Conventions
2.1. Conformance Criteria
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in BCP
 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
 Requirements phrased in the imperative as part of algorithms (such as
 "strip any leading space characters" or "return false and abort this
 algorithm") are to be interpreted with the meaning of the key word
 ("MUST", "SHOULD", "MAY", etc.) used in introducing the algorithm.
 Conformance requirements phrased as algorithms or specific steps can
 be implemented in any manner, so long as the end result is
 equivalent. In particular, the algorithms defined in this
 specification are intended to be easy to understand and are not
 intended to be performant.
2.2. Syntax Notation
 This specification uses the Augmented Backus-Naur Form (ABNF)
 notation of [RFC5234].
Bingler, et al. Expires 4 June 2026 [Page 5]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 The following core rules are included by reference, as defined in
 [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
 (CR LF), CTLs (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), NUL (null octet),
 OCTET (any 8-bit sequence of data except NUL), SP (space), HTAB
 (horizontal tab), CHAR (any [USASCII] character), VCHAR (any visible
 [USASCII] character), and WSP (whitespace).
 The OWS (optional whitespace) and BWS (bad whitespace) rules are
 defined in Section 5.6.3 of [HTTP].
2.3. Terminology
 The terms "user agent", "client", "server", "proxy", and "origin
 server" have the same meaning as in the HTTP/1.1 specification
 (Section 3 of [HTTP]).
 The request-host is the name of the host, as known by the user agent,
 to which the user agent is sending an HTTP request or from which it
 is receiving an HTTP response (i.e., the name of the host to which it
 sent the corresponding HTTP request).
 The term request-uri refers to "target URI" as defined in Section 7.1
 of [HTTP].
 Two sequences of octets are said to case-insensitively match each
 other if and only if they are equivalent under the i;ascii-casemap
 collation defined in [RFC4790].
 The term string means a sequence of non-NUL octets.
 The terms "active browsing context", "active document", "ancestor
 navigables", "container document", "content navigable", "dedicated
 worker", "Document", "inclusive ancestor navigables", "navigable",
 "navigate", "opaque origin", "sandboxed origin browsing context
 flag", "shared worker", "the worker's Documents", "top-level
 traversable", and "WorkerGlobalScope" are defined in [HTML].
 "Service Workers" are defined in the Service Workers specification
 [SERVICE-WORKERS].
 The term "origin", the mechanism of deriving an origin from a URI,
 and the "the same" matching algorithm for origins are defined in
 [RFC6454].
 "Safe" HTTP methods include GET, HEAD, OPTIONS, and TRACE, as defined
 in Section 9.2.1 of [HTTP].
Bingler, et al. Expires 4 June 2026 [Page 6]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 A domain's "public suffix" is the portion of a domain that is
 controlled by a public registry, such as "com", "co.uk", and
 "pvt.k12.wy.us". A domain's "registrable domain" is the domain's
 public suffix plus the label to its left. That is, for
 https://www.site.example, the public suffix is example, and the
 registrable domain is site.example. See Section 8.9 for security
 considerations.
 The term "request", as well as a request's "client", "current url",
 "method", "target browsing context", and "url list", are defined in
 [FETCH].
 The term "non-HTTP APIs" refers to non-HTTP mechanisms used to set
 and retrieve cookies, such as a web browser API that exposes cookies
 to scripts.
 The term "top-level navigation" refers to a navigation of a top-level
 traversable.
2.4. Name Resolution System
 While this document does not strictly prescribe any specific name
 resolution system for use with cookies it does require that the
 system uses only [USASCII] characters or uses an ASCII-compatible
 encoding (ACE). As the Domain Name System (DNS) is a typical and
 conventional example this document will reference name resolution in
 terms of DNS.
 Name resolution systems that directly expose non-ASCII characters,
 such as Unicode, are out of scope of this document.
3. Overview
 This section outlines a way for an origin server to send state
 information to a user agent and for the user agent to return the
 state information to the origin server.
 To store state, the origin server includes a Set-Cookie header field
 in an HTTP response. In subsequent requests, the user agent returns
 a Cookie request header field to the origin server. The Cookie
 header field contains cookies the user agent received in previous
 Set-Cookie header fields. The origin server is free to ignore the
 Cookie header field or use its contents for an application-defined
 purpose.
Bingler, et al. Expires 4 June 2026 [Page 7]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Origin servers MAY send a Set-Cookie response header field with any
 response. An origin server can include multiple Set-Cookie header
 fields in a single response. The presence of a Cookie or a Set-
 Cookie header field does not preclude HTTP caches from storing and
 reusing a response.
 Origin servers and intermediaries MUST NOT combine multiple Set-
 Cookie header fields into a single header field. The usual mechanism
 for combining HTTP headers fields (i.e., as defined in Section 5.3 of
 [HTTP]) might change the semantics of the Set-Cookie header field
 because the %x2C (",") character is used by Set-Cookie in a way that
 conflicts with such combining.
 For example,
 Set-Cookie: a=b;path=/c,d=e
 is ambiguous. It could be intended as two cookies, a=b and d=e, or a
 single cookie with a path of /c,d=e.
 User agents MAY ignore Set-Cookie header fields based on response
 status codes or the user agent's cookie policy (see Section 5.3).
 Note: A cookie's octets MUST be processed as [USASCII] characters.
 While it's possible a non-HTTP API could pass a set-cookie-string
 with one or more non-[USASCII] characters, no attempt should be made
 to interpret these octets as anything other than [USASCII]
 characters.
3.1. Examples
 Using the Set-Cookie header field, a server can send the user agent a
 short string in an HTTP response that the user agent will return in
 future HTTP requests that are within the scope of the cookie. For
 example, the server can send the user agent a "session identifier"
 named SID with the value 31d4d96e407aad42. The user agent then
 returns the session identifier in subsequent requests.
 == Server -> User Agent ==
 Set-Cookie: SID=31d4d96e407aad42
 == User Agent -> Server ==
 Cookie: SID=31d4d96e407aad42
Bingler, et al. Expires 4 June 2026 [Page 8]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 The server can alter the default scope of the cookie using the Path
 and Domain attributes. For example, the server can instruct the user
 agent to return the cookie to every path and every subdomain of
 site.example.
 == Server -> User Agent ==
 Set-Cookie: SID=31d4d96e407aad42; Path=/; Domain=site.example
 == User Agent -> Server ==
 Cookie: SID=31d4d96e407aad42
 As shown in the next example, the server can store multiple cookies
 at the user agent. For example, the server can store a session
 identifier as well as the user's preferred language by returning two
 Set-Cookie header fields. Notice that the server uses the Secure and
 HttpOnly attributes to provide additional security protections for
 the more sensitive session identifier (see Section 4.1.2).
 == Server -> User Agent ==
 Set-Cookie: SID=31d4d96e407aad42; Path=/; Secure; HttpOnly
 Set-Cookie: lang=en-US; Path=/; Domain=site.example
 == User Agent -> Server ==
 Cookie: SID=31d4d96e407aad42; lang=en-US
 Notice that the Cookie header field above contains two cookies, one
 named SID and one named lang.
 Cookie names are case-sensitive, meaning that if a server sends the
 user agent two Set-Cookie header fields that differ only in their
 name's case the user agent will store and return both of those
 cookies in subsequent requests.
 == Server -> User Agent ==
 Set-Cookie: SID=31d4d96e407aad42
 Set-Cookie: sid=31d4d96e407aad42
 == User Agent -> Server ==
 Cookie: SID=31d4d96e407aad42; sid=31d4d96e407aad42
Bingler, et al. Expires 4 June 2026 [Page 9]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 If the server wishes the user agent to persist the cookie over
 multiple "sessions" (e.g., user agent restarts), the server can
 specify an expiration date in the Expires attribute. Note that the
 user agent might delete the cookie before the expiration date if the
 user agent's cookie store exceeds its quota or if the user manually
 deletes the server's cookie.
 == Server -> User Agent ==
 Set-Cookie: lang=en-US; Expires=2026年6月09日 10:18:14 GMT
 == User Agent -> Server ==
 Cookie: SID=31d4d96e407aad42; lang=en-US
 Finally, to remove a cookie, the server returns a Set-Cookie header
 field with an expiration date in the past. The server will be
 successful in removing the cookie only if the Path and the Domain
 attribute in the Set-Cookie header field match the values used when
 the cookie was created.
 == Server -> User Agent ==
 Set-Cookie: lang=; Expires=1994年11月06日 08:49:37 GMT
 == User Agent -> Server ==
 Cookie: SID=31d4d96e407aad42
3.2. Which Requirements to Implement
 The upcoming two sections, Section 4 and Section 5, discuss the set
 of requirements for two distinct types of implementations. This
 section is meant to help guide implementers in determining which set
 of requirements best fits their goals. Choosing the wrong set of
 requirements could result in a lack of compatibility with other
 cookie implementations.
 It's important to note that being compatible means different things
 depending on the implementer's goals. These differences have built
 up over time due to both intentional and unintentional spec changes,
 spec interpretations, and historical implementation differences.
 This section roughly divides implementers of the cookie spec into two
 types, producers and consumers. These are not official terms and are
 only used here to help readers develop an intuitive understanding of
 the use cases.
Bingler, et al. Expires 4 June 2026 [Page 10]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
3.2.1. Cookie Producing Implementations
 An implementer should choose Section 4 whenever cookies are created
 and will be sent to a user agent, such as a web browser. These
 implementations are frequently referred to as servers by the spec but
 that term includes anything which primarily produces cookies. Some
 potential examples:
 * Server applications hosting a website or API
 * Programming languages or software frameworks that support cookies
 * Integrated third-party web applications, such as a business
 management suite
 All these benefit from not only supporting as many user agents as
 possible but also supporting other servers. This is useful if a
 cookie is produced by a software framework and is later sent back to
 a server application which needs to read it. Section 4 advises best
 practices that help maximize this sense of compatibility.
 See Section 3.2.2.1 for more details on programming languages and
 software frameworks.
3.2.2. Cookie Consuming Implementations
 An implementer should choose Section 5 whenever cookies are primarily
 received from another source. These implementations are referred to
 as user agents. Some examples:
 * Web browsers
 * Tools that support stateful HTTP
 * Programming languages or software frameworks that support cookies
 Because user agents don't know which servers a user will access, and
 whether or not that server is following best practices, users agents
 are advised to implement a more lenient set of requirements and to
 accept some things that servers are warned against producing.
 Section 5 advises best practices that help maximize this sense of
 compatibility.
 See Section 3.2.2.1 for more details on programming languages and
 software frameworks.
Bingler, et al. Expires 4 June 2026 [Page 11]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
3.2.2.1. Programming Languages & Software Frameworks
 A programming language or software framework with support for cookies
 could reasonably be used to create an application that acts as a
 cookie producer, cookie consumer, or both. Because a developer may
 want to maximize their compatibility as either a producer or
 consumer, these languages or frameworks should strongly consider
 supporting both sets of requirements, Section 4 and Section 5, behind
 a compatibility mode toggle. This toggle should default to
 Section 4's requirements.
 Doing so will reduce the chances that a developer's application can
 inadvertently create cookies that cannot be read by other servers.
4. Server Requirements
 This section describes the syntax and semantics of a well-behaved
 profile of the Cookie and Set-Cookie header fields.
4.1. Set-Cookie Header
 The Set-Cookie HTTP response header field is used to send cookies
 from the server to the user agent.
4.1.1. Syntax
 Informally, the Set-Cookie response header field contains a cookie,
 which begins with a name-value-pair, followed by zero or more
 attribute-value pairs. Servers conforming to this profile MUST NOT
 send Set-Cookie header fields that deviate from the following
 grammar:
Bingler, et al. Expires 4 June 2026 [Page 12]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 set-cookie = set-cookie-string
 set-cookie-string = BWS cookie-pair *( BWS ";" OWS cookie-av )
 cookie-pair = cookie-name BWS "=" BWS cookie-value
 cookie-name = token
 cookie-value = *cookie-octet / ( DQUOTE *cookie-octet DQUOTE )
 cookie-octet = %x21 / %x23-2B / %x2D-3A / %x3C-5B / %x5D-7E
 ; US-ASCII characters excluding CTLs,
 ; whitespace, DQUOTE, comma, semicolon,
 ; and backslash
 token = <token, defined in [HTTP], Section 5.6.2>
 cookie-av = expires-av / max-age-av / domain-av /
 path-av / secure-av / httponly-av /
 samesite-av / extension-av
 expires-av = "Expires" BWS "=" BWS sane-cookie-date
 sane-cookie-date =
 <IMF-fixdate, defined in [HTTP], Section 5.6.7>
 max-age-av = "Max-Age" BWS "=" BWS non-zero-digit *DIGIT
 non-zero-digit = %x31-39
 ; digits 1 through 9
 domain-av = "Domain" BWS "=" BWS domain-value
 domain-value = <subdomain>
 ; see details below
 path-av = "Path" BWS "=" BWS path-value
 path-value = *av-octet
 secure-av = "Secure"
 httponly-av = "HttpOnly"
 samesite-av = "SameSite" BWS "=" BWS samesite-value
 samesite-value = "Strict" / "Lax" / "None"
 extension-av = 1*av-octet
 av-octet = %x20-3A / %x3C-7E
 ; any CHAR except CTLs or ";"
 Note that some of the grammatical terms above reference documents
 that use different grammatical notations than this document (which
 uses ABNF from [RFC5234]).
 Per the grammar above, servers MUST NOT produce nameless cookies
 (i.e.: an empty cookie-name) as such cookies may be unpredictably
 serialized by UAs when sent back to the server.
 The semantics of the cookie-value are not defined by this document.
 To maximize compatibility with user agents, servers that wish to
 store arbitrary data in a cookie-value SHOULD encode that data, for
 example, using Base64 [RFC4648].
Bingler, et al. Expires 4 June 2026 [Page 13]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Per the grammar above, the cookie-value MAY be wrapped in DQUOTE
 characters. Note that in this case, the initial and trailing DQUOTE
 characters are not stripped. They are part of the cookie-value, and
 will be included in Cookie header fields sent to the server.
 Per the grammar above, extension-av MUST NOT contain leading or
 trailing WSP characters as they will be interpreted as BWS and
 removed.
 The domain-value is a subdomain as defined by Section 3.5 of
 [RFC1034], and as enhanced by Section 2.1 of [RFC1123]. Thus,
 domain-value is a string of [USASCII] characters, such as an
 "A-label" as defined in Section 2.3.2.1 of [RFC5890].
 The portions of the set-cookie-string produced by the cookie-av term
 are known as attributes. To maximize compatibility with user agents,
 servers MUST NOT produce two attributes with the same name in the
 same set-cookie-string. (See Section 5.7 for how user agents handle
 this case.)
 Note: The name of an attribute-value pair is not case-sensitive. So
 while they are presented here in CamelCase, such as "HttpOnly" or
 "SameSite", any case is accepted. E.x.: "httponly", "Httponly",
 "hTTPoNLY", etc.
 Servers MUST NOT include more than one Set-Cookie header field in the
 same response with the same cookie-name. (See Section 5.6 for how
 user agents handle this case.)
 If a server sends multiple responses containing Set-Cookie header
 fields concurrently to the user agent (e.g., when communicating with
 the user agent over multiple sockets), these responses create a "race
 condition" that can lead to unpredictable behavior.
 Note: Some existing user agents differ in their interpretation of
 two-digit years. To avoid compatibility issues, servers SHOULD use
 the rfc1123-date format, which requires a four-digit year.
 Note: Some user agents store and process dates in cookies as 32-bit
 UNIX time_t values. Implementation bugs in the libraries supporting
 time_t processing on some systems might cause such user agents to
 process dates after the year 2038 incorrectly.
Bingler, et al. Expires 4 June 2026 [Page 14]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
4.1.2. Semantics (Non-Normative)
 This section describes simplified semantics of the Set-Cookie header
 field. These semantics are detailed enough to be useful for
 understanding the most common uses of cookies by servers. The full
 semantics are described in Section 5.
 When the user agent receives a Set-Cookie header field, the user
 agent stores the cookie together with its attributes. Subsequently,
 when the user agent makes an HTTP request, the user agent includes
 the applicable, non-expired cookies in the Cookie header field.
 If the user agent receives a new cookie with the same cookie-name,
 domain-value, and path-value as a cookie that it has already stored,
 the existing cookie is evicted and replaced with the new cookie.
 Notice that servers can delete cookies by sending the user agent a
 new cookie with an Expires attribute with a value in the past.
 Unless the cookie's attributes indicate otherwise, the cookie is
 returned only to the origin server (and not, for example, to any
 subdomains), and it expires at the end of the current session (as
 defined by the user agent). User agents ignore unrecognized cookie
 attributes (but not the entire cookie).
4.1.2.1. The Expires Attribute
 The Expires attribute indicates the maximum lifetime of the cookie,
 represented as the date and time at which the cookie expires. The
 user agent may adjust the specified date and is not required to
 retain the cookie until that date has passed. In fact, user agents
 often evict cookies due to memory pressure or privacy concerns.
 The cookie's lifetime is based on the user agent's clock which may
 differ from the server's clock. Servers MUST NOT depend on cookies
 being evicted exactly at the specified date and time of the server's
 clock.
4.1.2.2. The Max-Age Attribute
 The Max-Age attribute indicates the maximum lifetime of the cookie,
 represented as the number of seconds until the cookie expires. The
 user agent may adjust the specified duration and is not required to
 retain the cookie for that duration. In fact, user agents often
 evict cookies due to memory pressure or privacy concerns.
 Note: Some existing user agents do not support the Max-Age attribute.
 User agents that do not support the Max-Age attribute ignore the
 attribute.
Bingler, et al. Expires 4 June 2026 [Page 15]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 If a cookie has both the Max-Age and the Expires attribute, the Max-
 Age attribute has precedence and controls the expiration date of the
 cookie. If a cookie has neither the Max-Age nor the Expires
 attribute, the user agent will retain the cookie until "the current
 session is over" (as defined by the user agent).
4.1.2.3. The Domain Attribute
 The Domain attribute specifies those hosts to which the cookie will
 be sent. For example, if the value of the Domain attribute is
 "site.example", the user agent will include the cookie in the Cookie
 header field when making HTTP requests to site.example,
 www.site.example, and www.corp.site.example. If the server omits the
 Domain attribute, the user agent will return the cookie only to the
 origin server.
 WARNING: Some existing user agents treat an absent Domain attribute
 as if the Domain attribute were present and contained the current
 host name. For example, if site.example returns a Set-Cookie header
 field without a Domain attribute, these user agents will erroneously
 send the cookie to www.site.example as well.
 The user agent will reject cookies unless the Domain attribute
 specifies a scope for the cookie that would include the origin
 server. For example, the user agent will accept a cookie with a
 Domain attribute of "site.example" or of "foo.site.example" from
 foo.site.example, but the user agent will not accept a cookie with a
 Domain attribute of "bar.site.example" or of "baz.foo.site.example".
 Note: For security reasons, many user agents are configured to reject
 Domain attributes that correspond to "public suffixes". For example,
 some user agents will reject Domain attributes of "com" or "co.uk".
 (See Section 5.7 for more information.)
4.1.2.4. The Path Attribute
 The scope of each cookie is limited to a set of paths, controlled by
 the Path attribute. If the server omits the Path attribute, the user
 agent will use the "directory" of the request-uri's path component as
 the default value. (See Section 5.1.4 for more details.)
 The user agent will include the cookie in an HTTP request only if the
 path portion of the request-uri matches (or is a subdirectory of) the
 cookie's Path attribute, where the %x2F ("/") character is
 interpreted as a directory separator.
Bingler, et al. Expires 4 June 2026 [Page 16]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Although seemingly useful for isolating cookies between different
 paths within a given host, the Path attribute cannot be relied upon
 for security (see Section 8).
4.1.2.5. The Secure Attribute
 The Secure attribute limits the scope of the cookie to "secure"
 channels (where "secure" is defined by the user agent). When a
 cookie has the Secure attribute, the user agent will include the
 cookie in an HTTP request only if the request is transmitted over a
 secure channel (typically HTTP over Transport Layer Security (TLS
 [TLS13]) [HTTP]).
4.1.2.6. The HttpOnly Attribute
 The HttpOnly attribute limits the scope of the cookie to HTTP
 requests. In particular, the attribute instructs the user agent to
 omit the cookie when providing access to cookies via non-HTTP APIs.
 Note that the HttpOnly attribute is independent of the Secure
 attribute: a cookie can have both the HttpOnly and the Secure
 attribute.
4.1.2.7. The SameSite Attribute
 The "SameSite" attribute limits the scope of the cookie such that it
 will only be attached to requests if those requests are same-site, as
 defined by the algorithm in Section 5.2. For example, requests for
 https://site.example/sekrit-image will attach same-site cookies if
 and only if initiated from a context whose "site for cookies" is an
 origin with a scheme and registered domain of "https" and
 "site.example" respectively.
 If the "SameSite" attribute's value is "Strict", the cookie will only
 be sent along with "same-site" requests. If the value is "Lax", the
 cookie will be sent with same-site requests, and with "cross-site"
 top-level navigations, as described in Section 5.6.7.1. If the value
 is "None", the cookie will be sent with same-site and cross-site
 requests. If the "SameSite" attribute's value is something other
 than these three known keywords, the attribute's value will be
 subject to a default enforcement mode that is equivalent to "Lax".
 If a user agent uses "Lax-allowing-unsafe" enforcement (See
 Section 5.6.7.2) then this default enforcement mode will instead be
 equivalent to "Lax-allowing-unsafe".
Bingler, et al. Expires 4 June 2026 [Page 17]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 The "SameSite" attribute affects cookie creation as well as delivery.
 Cookies which assert "SameSite=Lax" or "SameSite=Strict" cannot be
 set in responses to cross-site subresource requests, or cross-site
 nested navigations. They can be set along with any top-level
 navigation, cross-site or otherwise.
4.1.3. Cookie Name Prefixes
 Section 8.5 and Section 8.6 of this document spell out some of the
 drawbacks of cookies' historical implementation. In particular, it
 is impossible for a server to have confidence that a given cookie was
 set with a particular set of attributes. In order to provide such
 confidence in a backwards-compatible way, two common sets of
 requirements can be inferred from the first few characters of the
 cookie's name.
 The user agent requirements for the prefixes described below are
 detailed in Section 5.4.
 To maximize compatibility with user agents servers SHOULD use
 prefixes as described below.
4.1.3.1. The "__Secure-" Prefix
 If a cookie's name begins with a case-sensitive match for the string
 __Secure-, then the cookie will have been set with a Secure
 attribute.
 For example, the following Set-Cookie header field would be rejected
 by a conformant user agent, as it does not have a Secure attribute.
 Set-Cookie: __Secure-SID=12345; Domain=site.example
 Whereas the following Set-Cookie header field would be accepted if
 set from a secure origin (e.g. "https://site.example/"), and rejected
 otherwise:
 Set-Cookie: __Secure-SID=12345; Domain=site.example; Secure
4.1.3.2. The "__Host-" Prefix
 If a cookie's name begins with a case-sensitive match for the string
 __Host-, then the cookie will have been set with a Secure attribute,
 a Path attribute with a value of /, and no Domain attribute.
 This combination yields a cookie that hews as closely as a cookie can
 to treating the origin as a security boundary. The lack of a Domain
 attribute ensures that the cookie's host-only-flag is true, locking
Bingler, et al. Expires 4 June 2026 [Page 18]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 the cookie to a particular host, rather than allowing it to span
 subdomains. Setting the Path to / means that the cookie is effective
 for the entire host, and won't be overridden for specific paths. The
 Secure attribute ensures that the cookie is unaltered by non-secure
 origins, and won't span protocols.
 Ports are the only piece of the origin model that __Host- cookies
 continue to ignore.
 For example, the following cookies would always be rejected:
 Set-Cookie: __Host-SID=12345
 Set-Cookie: __Host-SID=12345; Secure
 Set-Cookie: __Host-SID=12345; Domain=site.example
 Set-Cookie: __Host-SID=12345; Domain=site.example; Path=/
 Set-Cookie: __Host-SID=12345; Secure; Domain=site.example; Path=/
 While the following would be accepted if set from a secure origin
 (e.g. "https://site.example/"), and rejected otherwise:
 Set-Cookie: __Host-SID=12345; Secure; Path=/
4.2. Cookie Header
4.2.1. Syntax
 The user agent sends stored cookies to the origin server in the
 Cookie header field. If the server conforms to the requirements in
 Section 4.1 (and the user agent conforms to the requirements in
 Section 5), the user agent will send a Cookie header field that
 conforms to the following grammar:
 cookie = cookie-string
 cookie-string = cookie-pair *( ";" SP cookie-pair )
 While Section 5.4 of [HTTP] does not define a length limit for header
 fields it is likely that the web server's implementation does impose
 a limit; many popular implementations have default limits of 8192
 octets. Servers SHOULD avoid setting a large number of large cookies
 such that the final cookie-string would exceed their header field
 limit. Not doing so could result in requests to the server failing.
Bingler, et al. Expires 4 June 2026 [Page 19]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Servers MUST be tolerant of multiple cookie headers. For example, an
 HTTP/2 [RFC9113] or HTTP/3 [RFC9114] client or intermediary might
 split a cookie header to improve compression. Servers are free to
 determine what form this tolerance takes. For example, the server
 could process each cookie header individually or the server could
 concatenate all the cookie headers into one and then process that
 final, single, header. The server should be mindful of any header
 field limits when deciding which approach to take.
 Note: Since intermediaries can modify cookie headers they should also
 be mindful of common server header field limits in order to avoid
 sending servers headers that they cannot process. For example,
 concatenating multiple cookie headers into a single header might
 exceed a server's size limit.
4.2.2. Semantics
 Each cookie-pair represents a cookie stored by the user agent. The
 cookie-pair contains the cookie-name and cookie-value the user agent
 received in the Set-Cookie header field.
 Notice that the cookie attributes are not returned. In particular,
 the server cannot determine from the Cookie field alone when a cookie
 will expire, for which hosts the cookie is valid, for which paths the
 cookie is valid, or whether the cookie was set with the Secure or
 HttpOnly attributes.
 The semantics of individual cookies in the Cookie header field are
 not defined by this document. Servers are expected to imbue these
 cookies with application-specific semantics.
 Although cookies are serialized linearly in the Cookie header field,
 servers SHOULD NOT rely upon the serialization order. In particular,
 if the Cookie header field contains two cookies with the same name
 (e.g., that were set with different Path or Domain attributes),
 servers SHOULD NOT rely upon the order in which these cookies appear
 in the header field.
5. User Agent Requirements
 This section specifies the Cookie and Set-Cookie header fields in
 sufficient detail that a user agent implementing these requirements
 precisely can interoperate with existing servers (even those that do
 not conform to the well-behaved profile described in Section 4).
Bingler, et al. Expires 4 June 2026 [Page 20]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 A user agent could enforce more restrictions than those specified
 herein (e.g., restrictions specified by its cookie policy, described
 in Section 7.2). However, such additional restrictions may reduce
 the likelihood that a user agent will be able to interoperate with
 existing servers.
5.1. Subcomponent Algorithms
 This section defines some algorithms used by user agents to process
 specific subcomponents of the Cookie and Set-Cookie header fields.
5.1.1. Dates
 The user agent MUST use an algorithm equivalent to the following
 algorithm to parse a cookie-date. Note that the various boolean
 flags defined as a part of the algorithm (i.e., found-time, found-
 day-of-month, found-month, found-year) are initially "not set".
 1. Using the grammar below, divide the cookie-date into date-tokens.
 cookie-date = *delimiter date-token-list *delimiter
 date-token-list = date-token *( 1*delimiter date-token )
 date-token = 1*non-delimiter
 delimiter = %x09 / %x20-2F / %x3B-40 / %x5B-60 / %x7B-7E
 non-delimiter = %x00-08 / %x0A-1F / DIGIT / ":" / ALPHA
 / %x7F-FF
 non-digit = %x00-2F / %x3A-FF
 day-of-month = 1*2DIGIT [ non-digit *OCTET ]
 month = ( "jan" / "feb" / "mar" / "apr" /
 "may" / "jun" / "jul" / "aug" /
 "sep" / "oct" / "nov" / "dec" ) *OCTET
 year = 2*4DIGIT [ non-digit *OCTET ]
 time = hms-time [ non-digit *OCTET ]
 hms-time = time-field ":" time-field ":" time-field
 time-field = 1*2DIGIT
 2. Process each date-token sequentially in the order the date-tokens
 appear in the cookie-date:
 1. If the found-time flag is not set and the token matches the
 time production, set the found-time flag and set the hour-
 value, minute-value, and second-value to the numbers denoted
 by the digits in the date-token, respectively. Skip the
 remaining sub-steps and continue to the next date-token.
Bingler, et al. Expires 4 June 2026 [Page 21]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 2. If the found-day-of-month flag is not set and the date-token
 matches the day-of-month production, set the found-day-of-
 month flag and set the day-of-month-value to the number
 denoted by the date-token. Skip the remaining sub-steps and
 continue to the next date-token.
 3. If the found-month flag is not set and the date-token matches
 the month production, set the found-month flag and set the
 month-value to the month denoted by the date-token. Skip the
 remaining sub-steps and continue to the next date-token.
 4. If the found-year flag is not set and the date-token matches
 the year production, set the found-year flag and set the
 year-value to the number denoted by the date-token. Skip the
 remaining sub-steps and continue to the next date-token.
 3. If the year-value is greater than or equal to 70 and less than or
 equal to 99, increment the year-value by 1900.
 4. If the year-value is greater than or equal to 0 and less than or
 equal to 69, increment the year-value by 2000.
 1. Note: Some existing user agents interpret two-digit years
 differently.
 5. Abort this algorithm and fail to parse the cookie-date if:
 * at least one of the found-day-of-month, found-month, found-
 year, or found-time flags is not set,
 * the day-of-month-value is less than 1 or greater than 31,
 * the year-value is less than 1601,
 * the hour-value is greater than 23,
 * the minute-value is greater than 59, or
 * the second-value is greater than 59.
 (Note that leap seconds cannot be represented in this syntax.)
 6. Let the parsed-cookie-date be the date whose day-of-month, month,
 year, hour, minute, and second (in UTC) are the day-of-month-
 value, the month-value, the year-value, the hour-value, the
 minute-value, and the second-value, respectively. If no such
 date exists, abort this algorithm and fail to parse the cookie-
 date.
Bingler, et al. Expires 4 June 2026 [Page 22]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 7. Return the parsed-cookie-date as the result of this algorithm.
5.1.2. Canonicalized Host Names
 A canonicalized host name is the string generated by the following
 algorithm:
 1. Convert the host name to a sequence of individual domain name
 labels.
 2. All labels must be one of U-label, A-label, or Non-Reserved LDH
 (NR-LDH) label (see Section 2.3.1 of [RFC5890]). If any label is
 not one of these then abort this algorithm and fail to
 canonicalize the host name.
 3. Convert each U-label to an A-label (see Section 2.3.2.1 of
 [RFC5890]).
 4. If any label is a Fake A-label then abort this algorithm and fail
 to canonicalize the host name.
 5. Concatenate the resulting labels, separated by a %x2E (".")
 character.
5.1.3. Domain Matching
 Note: This algorithm expects that both inputs are canonicalized.
 A string domain-matches a given domain string if at least one of the
 following conditions hold:
 * The domain string and the string are identical. (Note that both
 the domain string and the string will have been canonicalized to
 lower case at this point.)
 * All of the following conditions hold:
 - The domain string is a suffix of the string.
 - The last character of the string that is not included in the
 domain string is a %x2E (".") character.
 - The string is a host name (i.e., not an IP address).
5.1.4. Paths and Path-Match
 The user agent MUST use an algorithm equivalent to the following
 algorithm to compute the default-path of a cookie:
Bingler, et al. Expires 4 June 2026 [Page 23]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 1. Let uri-path be the path portion of the request-uri if such a
 portion exists (and empty otherwise).
 2. If the uri-path is empty or if the first character of the uri-
 path is not a %x2F ("/") character, output %x2F ("/") and skip
 the remaining steps.
 3. If the uri-path contains no more than one %x2F ("/") character,
 output %x2F ("/") and skip the remaining step.
 4. Output the characters of the uri-path from the first character up
 to, but not including, the right-most %x2F ("/").
 A request-path path-matches a given cookie-path if at least one of
 the following conditions holds:
 * The cookie-path and the request-path are identical.
 Note that this differs from the rules in [RFC3986] for equivalence
 of the path component, and hence two equivalent paths can have
 different cookies.
 * The cookie-path is a prefix of the request-path, and the last
 character of the cookie-path is %x2F ("/").
 * The cookie-path is a prefix of the request-path, and the first
 character of the request-path that is not included in the cookie-
 path is a %x2F ("/") character.
5.2. "Same-site" and "cross-site" Requests
 Two origins are same-site if they satisfy the "same site" criteria
 defined in [SAMESITE]. A request is "same-site" if the following
 criteria are true:
 1. The request is not the result of a reload navigation triggered
 through a user interface element (as defined by the user agent;
 e.g., a request triggered by the user clicking a refresh button
 on a toolbar).
 2. The request's current url's origin is same-site with the
 request's client's "site for cookies" (which is an origin), or if
 the request has no client or the request's client is null.
 Requests which are the result of a reload navigation triggered
 through a user interface element are same-site if the reloaded
 document was originally navigated to via a same-site request. A
 request that is not "same-site" is instead "cross-site".
Bingler, et al. Expires 4 June 2026 [Page 24]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 The request's client's "site for cookies" is calculated depending
 upon its client's type, as described in the following subsections:
5.2.1. Document-based requests
 The URI displayed in a user agent's address bar is the only security
 context directly exposed to users, and therefore the only signal
 users can reasonably rely upon to determine whether or not they trust
 a particular website. The origin of that URI represents the context
 in which a user most likely believes themselves to be interacting.
 This origin, the top-level traversable's active document's origin, is
 defined as the "top-level origin".
 For a document displayed in a top-level traversable, the document's
 "site for cookies" is the top-level origin.
 For container documents, the origins of each of a document's ancestor
 navigables' active documents must be audited in order to account for
 the "multiple-nested scenarios" described in Section 4 of [RFC7034].
 A document's "site for cookies" is the top-level origin if and only
 if the top-level origin is same-site with the document's origin, and
 with each of the document's ancestor documents' origins. Otherwise
 its "site for cookies" is an origin set to an opaque origin.
 Given a Document (document), the following algorithm returns its
 "site for cookies":
 1. Let top-document be the active document in document's navigable's
 top-level traversable.
 2. Let top-origin be the origin of top-document's URI if top-
 document's sandboxed origin browsing context flag is set, and
 top-document's origin otherwise.
 3. Let documents be a list consisting of the active documents of
 document's inclusive ancestor navigables.
 4. For each item in documents:
 1. Let origin be the origin of item's URI if item's sandboxed
 origin browsing context flag is set, and item's origin
 otherwise.
 2. If origin is not same-site with top-origin, return an origin
 set to an opaque origin.
 5. Return top-origin.
Bingler, et al. Expires 4 June 2026 [Page 25]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Note: This algorithm only applies when the entire chain of documents
 from top-document to document are all active.
5.2.2. Worker-based requests
 Worker-driven requests aren't as clear-cut as document-driven
 requests, as there isn't a clear link between a top-level traversable
 and a worker. This is especially true for Service Workers
 [SERVICE-WORKERS], which may execute code in the background, without
 any document visible at all.
5.2.2.1. Dedicated and Shared Workers
 Dedicated workers are simple, as each dedicated worker is bound to
 one and only one document. The worker's "site for cookies" is the
 document's "site for cookies" if the worker's origin is same-site
 with the document's "site for cookies", otherwise its "site for
 cookies" is an origin set to an opaque origin.
 Shared workers may be bound to multiple documents at once. As it is
 quite possible for those documents to have distinct "site for
 cookies" values, the worker's "site for cookies" will be an origin
 set to an opaque origin in cases where the values are not all same-
 site with the worker's origin, and the worker's origin in cases where
 the values agree.
 Given a WorkerGlobalScope (worker), the following algorithm returns
 its "site for cookies":
 1. Let site be worker's origin.
 2. For each document in worker's Documents:
 1. Let document-site be document's "site for cookies" (as
 defined in Section 5.2.1).
 2. If document-site is not same-site with site, return an origin
 set to an opaque origin.
 3. Return site.
5.2.2.2. Service Workers
 Service Workers are more complicated, as they act as a completely
 separate execution context with only tangential relationship to the
 Document which registered them.
Bingler, et al. Expires 4 June 2026 [Page 26]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 How user agents handle Service Workers may differ, but user agents
 SHOULD match the [SERVICE-WORKERS] specification.
5.3. Ignoring Set-Cookie Header Fields
 User agents MAY ignore Set-Cookie header fields contained in
 responses with 100-level status codes or based on its cookie policy
 (see Section 7.2).
 All other Set-Cookie header fields SHOULD be processed according to
 Section 5.6. That is, Set-Cookie header fields contained in
 responses with non-100-level status codes (including those in
 responses with 400- and 500-level status codes) SHOULD be processed
 unless ignored according to the user agent's cookie policy.
5.4. Cookie Name Prefixes
 User agents' requirements for cookie name prefixes differ slightly
 from servers' (Section 4.1.3) in that UAs MUST match the prefix
 string case-insensitively.
 The normative requirements for the prefixes are detailed in the
 storage model algorithm defined in Section 5.7.
 This is because some servers will process cookies case-insensitively,
 resulting in them unintentionally miscapitalizing and accepting
 miscapitalized prefixes.
 For example, if a server sends the following Set-Cookie header field
 Set-Cookie: __SECURE-SID=12345
 to a UA which checks prefixes case-sensitively it will accept this
 cookie and the server would incorrectly believe the cookie is subject
 the same guarantees as one spelled __Secure-.
 Additionally the server is vulnerable to an attacker that
 purposefully miscapitalizes a cookie in order to impersonate a
 prefixed cookie. For example, a site already has a cookie __Secure-
 SID=12345 and by some means an attacker sends the following Set-
 Cookie header field for the site to a UA which checks prefixes case-
 sensitively.
 Set-Cookie: __SeCuRe-SID=evil
 The next time a user visits the site the UA will send both cookies:
 Cookie: __Secure-SID=12345; __SeCuRe-SID=evil
Bingler, et al. Expires 4 June 2026 [Page 27]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 The server, being case-insensitive, won't be able to tell the
 difference between the two cookies allowing the attacker to
 compromise the site.
 To prevent these issues, UAs MUST match cookie name prefixes case-
 insensitively.
 Note: Cookies with different names are still considered separate by
 UAs. So both __Secure-foo=bar and __secure-foo=baz can exist as
 distinct cookies simultaneously and both would have the requirements
 of the __Secure- prefix applied.
 The following are examples of Set-Cookie header fields that would be
 rejected by a conformant user agent.
 Set-Cookie: __Secure-SID=12345; Domain=site.example
 Set-Cookie: __secure-SID=12345; Domain=site.example
 Set-Cookie: __SECURE-SID=12345; Domain=site.example
 Set-Cookie: __Host-SID=12345
 Set-Cookie: __host-SID=12345; Secure
 Set-Cookie: __host-SID=12345; Domain=site.example
 Set-Cookie: __HOST-SID=12345; Domain=site.example; Path=/
 Set-Cookie: __Host-SID=12345; Secure; Domain=site.example; Path=/
 Set-Cookie: __host-SID=12345; Secure; Domain=site.example; Path=/
 Set-Cookie: __HOST-SID=12345; Secure; Domain=site.example; Path=/
 Whereas the following Set-Cookie header fields would be accepted if
 set from a secure origin.
 Set-Cookie: __Secure-SID=12345; Domain=site.example; Secure
 Set-Cookie: __secure-SID=12345; Domain=site.example; Secure
 Set-Cookie: __SECURE-SID=12345; Domain=site.example; Secure
 Set-Cookie: __Host-SID=12345; Secure; Path=/
 Set-Cookie: __host-SID=12345; Secure; Path=/
 Set-Cookie: __HOST-SID=12345; Secure; Path=/
5.5. Cookie Lifetime Limits
 When processing cookies with a specified lifetime, either with the
 Expires or with the Max-Age attribute, the user agent MUST limit the
 maximum age of the cookie. The limit SHOULD NOT be greater than 400
 days (34560000 seconds) in the future. The RECOMMENDED limit is 400
 days in the future, but the user agent MAY adjust the limit (see
 Section 7.2). Expires or Max-Age attributes that specify a lifetime
 longer than the limit MUST be reduced to the limit.
Bingler, et al. Expires 4 June 2026 [Page 28]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
5.6. The Set-Cookie Header Field
 When a user agent receives a Set-Cookie header field in an HTTP
 response, the user agent MAY ignore the Set-Cookie header field in
 its entirety (see Section 5.3).
 If the user agent does not ignore the Set-Cookie header field in its
 entirety, the user agent MUST parse the field-value of the Set-Cookie
 header field as a set-cookie-string (defined below).
 Note: The algorithm below is more permissive than the grammar in
 Section 4.1. For example, the algorithm allows cookie-name to be
 comprised of cookie-octets instead of being a token as specified in
 Section 4.1 and the algorithm accommodates some characters that are
 not cookie-octets according to the grammar in Section 4.1. In
 addition, the algorithm below also strips leading and trailing
 whitespace from the cookie name and value (but maintains internal
 whitespace), whereas the grammar in Section 4.1 forbids whitespace in
 these positions. User agents use this algorithm so as to
 interoperate with servers that do not follow the recommendations in
 Section 4.
 Note: As set-cookie-string may originate from a non-HTTP API, it is
 not guaranteed to be free of CTL characters, so this algorithm
 handles them explicitly. Horizontal tab (%x09) is excluded from the
 CTL characters that lead to set-cookie-string rejection, as it is
 considered whitespace, which is handled separately.
 Note: The set-cookie-string may contain octet sequences that appear
 percent-encoded as per Section 2.1 of [RFC3986]. However, a user
 agent MUST NOT decode these sequences and instead parse the
 individual octets as specified in this algorithm.
 A user agent MUST use an algorithm equivalent to the following
 algorithm to parse a set-cookie-string:
 1. If the set-cookie-string contains a %x00-08 / %x0A-1F / %x7F
 character (CTL characters excluding HTAB): Abort this algorithm
 and ignore the set-cookie-string entirely.
 2. If the set-cookie-string contains a %x3B (";") character:
 1. The name-value-pair string consists of the characters up to,
 but not including, the first %x3B (";"), and the unparsed-
 attributes consist of the remainder of the set-cookie-string
 (including the %x3B (";") in question).
 Otherwise:
Bingler, et al. Expires 4 June 2026 [Page 29]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 1. The name-value-pair string consists of all the characters
 contained in the set-cookie-string, and the unparsed-
 attributes is the empty string.
 3. If the name-value-pair string lacks a %x3D ("=") character, then
 the name string is empty, and the value string is the value of
 name-value-pair.
 Otherwise, the (possibly empty) name string consists of the
 characters up to, but not including, the first %x3D ("=")
 character, and the (possibly empty) value string consists of the
 characters after the first %x3D ("=") character.
 4. Remove any leading or trailing WSP characters from the name
 string and the value string.
 5. If the sum of the lengths of the name string and the value string
 is more than 4096 octets, abort this algorithm and ignore the
 set-cookie-string entirely.
 6. The cookie-name is the name string, and the cookie-value is the
 value string.
 The user agent MUST use an algorithm equivalent to the following
 algorithm to parse the unparsed-attributes:
 1. If the unparsed-attributes string is empty, skip the rest of
 these steps.
 2. Discard the first character of the unparsed-attributes (which
 will be a %x3B (";") character).
 3. If the remaining unparsed-attributes contains a %x3B (";")
 character:
 1. Consume the characters of the unparsed-attributes up to, but
 not including, the first %x3B (";") character.
 Otherwise:
 1. Consume the remainder of the unparsed-attributes.
 Let the cookie-av string be the characters consumed in this step;
 unparsed-attributes now contains any remaining characters.
 4. If the cookie-av string contains a %x3D ("=") character:
Bingler, et al. Expires 4 June 2026 [Page 30]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 1. The (possibly empty) attribute-name string consists of the
 characters up to, but not including, the first %x3D ("=")
 character, and the (possibly empty) attribute-value string
 consists of the characters after the first %x3D ("=")
 character.
 Otherwise:
 1. The attribute-name string consists of the entire cookie-av
 string, and the attribute-value string is empty.
 5. Remove any leading or trailing WSP characters from the attribute-
 name string and the attribute-value string.
 6. If the attribute-value is longer than 1024 octets, ignore the
 cookie-av string and return to Step 1 of this algorithm.
 7. Process the attribute-name and attribute-value according to the
 requirements in the following subsections. (Notice that
 attributes with unrecognized attribute-names are ignored.)
 8. Return to Step 1 of this algorithm.
 When the user agent finishes parsing the set-cookie-string, the user
 agent is said to "receive a cookie" from the request-uri with name
 cookie-name, value cookie-value, and attributes cookie-attribute-
 list. (See Section 5.7 for additional requirements triggered by
 receiving a cookie.)
5.6.1. The Expires Attribute
 If the attribute-name case-insensitively matches the string
 "Expires", the user agent MUST process the cookie-av as follows.
 1. Let the expiry-time be the result of parsing the attribute-value
 as cookie-date (see Section 5.1.1).
 2. If the attribute-value failed to parse as a cookie date, ignore
 the cookie-av.
 3. Let cookie-age-limit be the maximum age of the cookie (which
 SHOULD be 400 days in the future or sooner, see Section 5.5).
 4. If the expiry-time is more than cookie-age-limit, the user agent
 MUST set the expiry time to cookie-age-limit in seconds.
Bingler, et al. Expires 4 June 2026 [Page 31]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 5. If the expiry-time is earlier than the earliest date the user
 agent can represent, the user agent MAY replace the expiry-time
 with the earliest representable date.
 6. Append an attribute to the cookie-attribute-list with an
 attribute-name of Expires and an attribute-value of expiry-time.
5.6.2. The Max-Age Attribute
 If the attribute-name case-insensitively matches the string "Max-
 Age", the user agent MUST process the cookie-av as follows.
 1. If the attribute-value is empty, ignore the cookie-av.
 2. If the first character of the attribute-value is neither a DIGIT,
 nor a "-" character followed by a DIGIT, ignore the cookie-av.
 3. If the remainder of attribute-value contains a non-DIGIT
 character, ignore the cookie-av.
 4. Let delta-seconds be the attribute-value converted to a base 10
 integer.
 5. Let cookie-age-limit be the maximum age of the cookie (which
 SHOULD be 400 days or less, see Section 5.5).
 6. Set delta-seconds to the smaller of its present value and cookie-
 age-limit.
 7. If delta-seconds is less than or equal to zero (0), let expiry-
 time be the earliest representable date and time. Otherwise, let
 the expiry-time be the current date and time plus delta-seconds
 seconds.
 8. Append an attribute to the cookie-attribute-list with an
 attribute-name of Max-Age and an attribute-value of expiry-time.
5.6.3. The Domain Attribute
 If the attribute-name case-insensitively matches the string "Domain",
 the user agent MUST process the cookie-av as follows.
 1. Let cookie-domain be the attribute-value.
 2. If cookie-domain starts with %x2E ("."), let cookie-domain be
 cookie-domain without its leading %x2E (".").
 3. Convert the cookie-domain to lower case.
Bingler, et al. Expires 4 June 2026 [Page 32]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 4. Append an attribute to the cookie-attribute-list with an
 attribute-name of Domain and an attribute-value of cookie-domain.
5.6.4. The Path Attribute
 If the attribute-name case-insensitively matches the string "Path",
 the user agent MUST process the cookie-av as follows.
 1. If the attribute-value is empty or if the first character of the
 attribute-value is not %x2F ("/"):
 1. Let cookie-path be the default-path.
 Otherwise:
 1. Let cookie-path be the attribute-value.
 2. Append an attribute to the cookie-attribute-list with an
 attribute-name of Path and an attribute-value of cookie-path.
5.6.5. The Secure Attribute
 If the attribute-name case-insensitively matches the string "Secure",
 the user agent MUST append an attribute to the cookie-attribute-list
 with an attribute-name of Secure and an empty attribute-value.
5.6.6. The HttpOnly Attribute
 If the attribute-name case-insensitively matches the string
 "HttpOnly", the user agent MUST append an attribute to the cookie-
 attribute-list with an attribute-name of HttpOnly and an empty
 attribute-value.
5.6.7. The SameSite Attribute
 If the attribute-name case-insensitively matches the string
 "SameSite", the user agent MUST process the cookie-av as follows:
 1. Let enforcement be "Default".
 2. If cookie-av's attribute-value is a case-insensitive match for
 "None", set enforcement to "None".
 3. If cookie-av's attribute-value is a case-insensitive match for
 "Strict", set enforcement to "Strict".
 4. If cookie-av's attribute-value is a case-insensitive match for
 "Lax", set enforcement to "Lax".
Bingler, et al. Expires 4 June 2026 [Page 33]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 5. Append an attribute to the cookie-attribute-list with an
 attribute-name of "SameSite" and an attribute-value of
 enforcement.
5.6.7.1. "Strict" and "Lax" enforcement
 Same-site cookies in "Strict" enforcement mode will not be sent along
 with top-level navigations which are triggered from a cross-site
 document context. As discussed in Section 8.8.2, this might or might
 not be compatible with existing session management systems. In the
 interests of providing a drop-in mechanism that mitigates the risk of
 CSRF attacks, developers may set the SameSite attribute in a "Lax"
 enforcement mode that carves out an exception which sends same-site
 cookies along with cross-site requests if and only if they are top-
 level navigations which use a "safe" (in the [HTTP] sense) HTTP
 method. (Note that a request's method may be changed from POST to
 GET for some redirects (see Sections 15.4.2 and 15.4.3 of [HTTP]); in
 these cases, a request's "safe"ness is determined based on the method
 of the current redirect hop.)
 Lax enforcement provides reasonable defense in depth against CSRF
 attacks that rely on unsafe HTTP methods (like POST), but does not
 offer a robust defense against CSRF as a general category of attack:
 1. Attackers can still pop up new windows or trigger top-level
 navigations in order to create a "same-site" request (as
 described in Section 5.2.1), which is only a speedbump along the
 road to exploitation.
 2. Features like <link rel='prerender'> [prerendering] can be
 exploited to create "same-site" requests without the risk of user
 detection.
 Developers can more completely mitigate CSRF through a session
 management mechanism such as that described in Section 8.8.2.
5.6.7.2. "Lax-Allowing-Unsafe" enforcement
 As discussed in Section 8.8.6, compatibility concerns may necessitate
 the use of a "Lax-allowing-unsafe" enforcement mode that allows
 cookies to be sent with a cross-site HTTP request if and only if it
 is a top-level request, regardless of request method. That is, the
 "Lax-allowing-unsafe" enforcement mode waives the requirement for the
 HTTP request's method to be "safe" in the SameSite enforcement step
 of the retrieval algorithm in Section 5.8.3. (All cookies,
 regardless of SameSite enforcement mode, may be set for top-level
 navigations, regardless of HTTP request method, as specified in
 Section 5.7.)
Bingler, et al. Expires 4 June 2026 [Page 34]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 "Lax-allowing-unsafe" is not a distinct value of the SameSite
 attribute. Rather, user agents MAY apply "Lax-allowing-unsafe"
 enforcement only to cookies that did not explicitly specify a
 SameSite attribute (i.e., those whose same-site-flag was set to
 "Default" by default). To limit the scope of this compatibility
 mode, user agents which apply "Lax-allowing-unsafe" enforcement
 SHOULD restrict the enforcement to cookies which were created
 recently. Deployment experience has shown a cookie age of 2 minutes
 or less to be a reasonable limit.
 If the user agent uses "Lax-allowing-unsafe" enforcement, it MUST
 apply the following modification to the retrieval algorithm defined
 in Section 5.8.3:
 Replace the condition in the penultimate bullet point of step 1 of
 the retrieval algorithm reading
 * The HTTP request associated with the retrieval uses a "safe"
 method.
 with
 * At least one of the following is true:
 1. The HTTP request associated with the retrieval uses a "safe"
 method.
 2. The cookie's same-site-flag is "Default" and the amount of
 time elapsed since the cookie's creation-time is at most a
 duration of the user agent's choosing.
5.7. Storage Model
 The user agent stores the following fields about each cookie: name,
 value, expiry-time, domain, path, creation-time, last-access-time,
 persistent-flag, host-only-flag, secure-only-flag, http-only-flag,
 and same-site-flag.
 When the user agent "receives a cookie" from a request-uri with name
 cookie-name, value cookie-value, and attributes cookie-attribute-
 list, the user agent MUST process the cookie as follows:
 1. A user agent MAY ignore a received cookie in its entirety. See
 Section 5.3.
 2. If cookie-name is empty and cookie-value is empty, abort this
 algorithm and ignore the cookie entirely.
Bingler, et al. Expires 4 June 2026 [Page 35]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 3. If the cookie-name or the cookie-value contains a %x00-08 /
 %x0A-1F / %x7F character (CTL characters excluding HTAB), abort
 this algorithm and ignore the cookie entirely.
 4. If the sum of the lengths of cookie-name and cookie-value is
 more than 4096 octets, abort this algorithm and ignore the
 cookie entirely.
 5. Create a new cookie with name cookie-name, value cookie-value.
 Set the creation-time and the last-access-time to the current
 date and time.
 6. If the cookie-attribute-list contains an attribute with an
 attribute-name of "Max-Age":
 1. Set the cookie's persistent-flag to true.
 2. Set the cookie's expiry-time to attribute-value of the last
 attribute in the cookie-attribute-list with an attribute-
 name of "Max-Age".
 Otherwise, if the cookie-attribute-list contains an attribute
 with an attribute-name of "Expires" (and does not contain an
 attribute with an attribute-name of "Max-Age"):
 1. Set the cookie's persistent-flag to true.
 2. Set the cookie's expiry-time to attribute-value of the last
 attribute in the cookie-attribute-list with an attribute-
 name of "Expires".
 Otherwise:
 1. Set the cookie's persistent-flag to false.
 2. Set the cookie's expiry-time to the latest representable
 date.
 7. If the cookie-attribute-list contains an attribute with an
 attribute-name of "Domain":
 1. Let the domain-attribute be the attribute-value of the last
 attribute in the cookie-attribute-list with both an
 attribute-name of "Domain" and an attribute-value whose
 length is no more than 1024 octets. (Note that a leading
 %x2E ("."), if present, is ignored even though that
 character is not permitted.)
Bingler, et al. Expires 4 June 2026 [Page 36]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Otherwise:
 1. Let the domain-attribute be the empty string.
 8. If the domain-attribute contains a character that is not in
 CHAR, abort this algorithm and ignore the cookie entirely.
 9. If the user agent is configured to reject "public suffixes" and
 the domain-attribute is a public suffix:
 1. Let request-host-canonical be the canonicalized request-
 host.
 2. If request-host fails to be canonicalized then abort this
 algorithm and ignore the cookie entirely.
 3. If the domain-attribute is identical to the request-host-
 canonical:
 1. Let the domain-attribute be the empty string.
 Otherwise:
 1. Abort this algorithm and ignore the cookie entirely.
 Note: This step prevents attacker.example from disrupting the
 integrity of site.example by setting a cookie with a Domain
 attribute of "example".
 10. If the domain-attribute is non-empty:
 1. If request-host-canonical does not domain-match (see
 Section 5.1.3) the domain-attribute:
 1. Abort this algorithm and ignore the cookie entirely.
 Otherwise:
 1. Set the cookie's host-only-flag to false.
 2. Set the cookie's domain to the domain-attribute.
 Otherwise:
 1. Set the cookie's host-only-flag to true.
 2. Set the cookie's domain to request-host-canonical.
Bingler, et al. Expires 4 June 2026 [Page 37]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 11. If the cookie-attribute-list contains an attribute with an
 attribute-name of "Path", set the cookie's path to attribute-
 value of the last attribute in the cookie-attribute-list with
 both an attribute-name of "Path" and an attribute-value whose
 length is no more than 1024 octets. Otherwise, set the cookie's
 path to the default-path of the request-uri.
 12. If the cookie-attribute-list contains an attribute with an
 attribute-name of "Secure", set the cookie's secure-only-flag to
 true. Otherwise, set the cookie's secure-only-flag to false.
 13. If the request-uri does not denote a "secure" connection (as
 defined by the user agent), and the cookie's secure-only-flag is
 true, then abort these steps and ignore the cookie entirely.
 14. If the cookie-attribute-list contains an attribute with an
 attribute-name of "HttpOnly", set the cookie's http-only-flag to
 true. Otherwise, set the cookie's http-only-flag to false.
 15. If the cookie was received from a "non-HTTP" API and the
 cookie's http-only-flag is true, abort this algorithm and ignore
 the cookie entirely.
 16. If the cookie's secure-only-flag is false, and the request-uri
 does not denote a "secure" connection, then abort this algorithm
 and ignore the cookie entirely if the cookie store contains one
 or more cookies that meet all of the following criteria:
 1. Their name matches the name of the newly-created cookie.
 2. Their secure-only-flag is true.
 3. Their domain domain-matches (see Section 5.1.3) the domain
 of the newly-created cookie, or vice-versa.
 4. The path of the newly-created cookie path-matches the path
 of the existing cookie.
 Note: The path comparison is not symmetric, ensuring only that a
 newly-created, non-secure cookie does not overlay an existing
 secure cookie, providing some mitigation against cookie-fixing
 attacks. That is, given an existing secure cookie named 'a'
 with a path of '/login', a non-secure cookie named 'a' could be
 set for a path of '/' or '/foo', but not for a path of '/login'
 or '/login/en'.
Bingler, et al. Expires 4 June 2026 [Page 38]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 17. If the cookie-attribute-list contains an attribute with an
 attribute-name of "SameSite", and an attribute-value of
 "Strict", "Lax", or "None", set the cookie's same-site-flag to
 the attribute-value of the last attribute in the cookie-
 attribute-list with an attribute-name of "SameSite". Otherwise,
 set the cookie's same-site-flag to "Default".
 18. If the cookie's same-site-flag is not "None":
 1. If the cookie was received from a "non-HTTP" API, and the
 API was called from a navigable's active document whose
 "site for cookies" is not same-site with the top-level
 origin, then abort this algorithm and ignore the newly
 created cookie entirely.
 2. If the cookie was received from a "same-site" request (as
 defined in Section 5.2), skip the remaining substeps and
 continue processing the cookie.
 3. If the cookie was received from a request which is
 navigating a top-level traversable [HTML] (e.g. if the
 request's "reserved client" is either null or an environment
 whose "target browsing context"'s navigable is a top-level
 traversable), skip the remaining substeps and continue
 processing the cookie.
 Note: Top-level navigations can create a cookie with any
 SameSite value, even if the new cookie wouldn't have been
 sent along with the request had it already existed prior to
 the navigation.
 4. Abort this algorithm and ignore the newly created cookie
 entirely.
 19. If the cookie's "same-site-flag" is "None", abort this algorithm
 and ignore the cookie entirely unless the cookie's secure-only-
 flag is true.
 20. If the cookie-name begins with a case-insensitive match for the
 string "__Secure-", abort this algorithm and ignore the cookie
 entirely unless the cookie's secure-only-flag is true.
 21. If the cookie-name begins with a case-insensitive match for the
 string "__Host-", abort this algorithm and ignore the cookie
 entirely unless the cookie meets all the following criteria:
 1. The cookie's secure-only-flag is true.
Bingler, et al. Expires 4 June 2026 [Page 39]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 2. The cookie's host-only-flag is true.
 3. The cookie-attribute-list contains an attribute with an
 attribute-name of "Path", and the cookie's path is /.
 22. If the cookie-name is empty and either of the following
 conditions are true, abort this algorithm and ignore the cookie
 entirely:
 * the cookie-value begins with a case-insensitive match for the
 string "__Secure-"
 * the cookie-value begins with a case-insensitive match for the
 string "__Host-"
 23. If the cookie store contains a cookie with the same name,
 domain, host-only-flag, and path as the newly-created cookie:
 1. Let old-cookie be the existing cookie with the same name,
 domain, host-only-flag, and path as the newly-created
 cookie. (Notice that this algorithm maintains the invariant
 that there is at most one such cookie.)
 2. If the newly-created cookie was received from a "non-HTTP"
 API and the old-cookie's http-only-flag is true, abort this
 algorithm and ignore the newly created cookie entirely.
 3. Update the creation-time of the newly-created cookie to
 match the creation-time of the old-cookie.
 4. Remove the old-cookie from the cookie store.
 24. Insert the newly-created cookie into the cookie store.
 A cookie is "expired" if the cookie has an expiry date in the past.
 The user agent MUST evict all expired cookies from the cookie store
 if, at any time, an expired cookie exists in the cookie store.
 At any time, the user agent MAY "remove excess cookies" from the
 cookie store if the number of cookies sharing a domain field exceeds
 some implementation-defined upper bound (such as 50 cookies).
 At any time, the user agent MAY "remove excess cookies" from the
 cookie store if the cookie store exceeds some predetermined upper
 bound (such as 3000 cookies).
Bingler, et al. Expires 4 June 2026 [Page 40]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 When the user agent removes excess cookies from the cookie store, the
 user agent MUST evict cookies in the following priority order:
 1. Expired cookies.
 2. Cookies whose secure-only-flag is false, and which share a domain
 field with more than a predetermined number of other cookies.
 3. Cookies that share a domain field with more than a predetermined
 number of other cookies.
 4. All cookies.
 If two cookies have the same removal priority, the user agent MUST
 evict the cookie with the earliest last-access-time first.
 When "the current session is over" (as defined by the user agent),
 the user agent MUST remove from the cookie store all cookies with the
 persistent-flag set to false.
5.8. Retrieval Model
 This section defines how cookies are retrieved from a cookie store in
 the form of a cookie-string. A "retrieval" is any event which
 requires generating a cookie-string. For example, a retrieval may
 occur in order to build a Cookie header field for an HTTP request, or
 may be required in order to return a cookie-string from a call to a
 "non-HTTP" API that provides access to cookies. A retrieval has an
 associated URI, same-site status, and type, which are defined below
 depending on the type of retrieval.
5.8.1. The Cookie Header Field
 The user agent includes stored cookies in the Cookie HTTP request
 header field.
 A user agent MAY omit the Cookie header field in its entirety. For
 example, the user agent might wish to block sending cookies during
 "third-party" requests (see Section 7.1).
 If the user agent does attach a Cookie header field to an HTTP
 request, the user agent MUST generate a single cookie-string and the
 user agent MUST compute the cookie-string following the algorithm
 defined in Section 5.8.3, where the retrieval's URI is the request-
 uri, the retrieval's same-site status is computed for the HTTP
 request as defined in Section 5.2, and the retrieval's type is
 "HTTP".
Bingler, et al. Expires 4 June 2026 [Page 41]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Note: Previous versions of this specification required that only one
 Cookie header field be sent in requests. This is no longer a
 requirement. While this specification requires that a single cookie-
 string be produced, some user agents may split that string across
 multiple cookie header fields. For examples, see Section 8.2.3 of
 [RFC9113] and Section 4.2.1 of [RFC9114].
5.8.2. Non-HTTP APIs
 The user agent MAY implement "non-HTTP" APIs that can be used to
 access stored cookies.
 A user agent MAY return an empty cookie-string in certain contexts,
 such as when a retrieval occurs within a third-party context (see
 Section 7.1).
 If a user agent does return cookies for a given call to a "non-HTTP"
 API with an associated Document, then the user agent MUST compute the
 cookie-string following the algorithm defined in Section 5.8.3, where
 the retrieval's URI is defined by the caller (see
 [DOM-DOCUMENT-COOKIE]), the retrieval's same-site status is "same-
 site" if the Document's "site for cookies" is same-site with the top-
 level origin as defined in Section 5.2.1 (otherwise it is "cross-
 site"), and the retrieval's type is "non-HTTP".
5.8.3. Retrieval Algorithm
 Given a cookie store and a retrieval, the following algorithm returns
 a cookie-string from a given cookie store.
 1. Let retrieval-host-canonical be the canonicalized host of the
 retrieval's URI.
 2. If the host of the retrieval's URI fails to be canonicalized then
 abort this algorithm.
 3. Let cookie-list be the set of cookies from the cookie store that
 meets all of the following requirements:
 * Either:
 - The cookie's host-only-flag is true and retrieval-host-
 canonical is identical to the cookie's domain.
 Or:
Bingler, et al. Expires 4 June 2026 [Page 42]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 - The cookie's host-only-flag is false and retrieval-host-
 canonical domain-matches (see Section 5.1.3) the cookie's
 domain.
 - The cookie's domain is not a public suffix, for user agents
 configured to reject "public suffixes".
 Note: It's possible that the public suffix list was changed
 since the cookie was created. If this change resulted in the
 cookie's domain becoming a public suffix then that cookie
 would have been rejected during creation if it had been
 created now. (See Section 5.7 step 9).
 * The retrieval's URI's path path-matches the cookie's path.
 * If the cookie's secure-only-flag is true, then the retrieval's
 URI must denote a "secure" connection (as defined by the user
 agent).
 Note: The notion of a "secure" connection is not defined by
 this document. Typically, user agents consider a connection
 secure if the connection makes use of transport-layer
 security, such as SSL or TLS [TLS13], or if the host is
 trusted. For example, most user agents consider "https" to be
 a scheme that denotes a secure protocol and "localhost" to be
 trusted host.
 * If the cookie's http-only-flag is true, then exclude the
 cookie if the retrieval's type is "non-HTTP".
 * If the cookie's same-site-flag is not "None" and the
 retrieval's same-site status is "cross-site", then exclude the
 cookie unless all of the following conditions are met:
 - The retrieval's type is "HTTP".
 - The same-site-flag is "Lax" or "Default".
 - The HTTP request associated with the retrieval uses a
 "safe" method.
 - The target browsing context of the HTTP request associated
 with the retrieval is the active browsing context or a top-
 level traversable.
 4. The user agent SHOULD sort the cookie-list in the following
 order:
Bingler, et al. Expires 4 June 2026 [Page 43]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 * Cookies with longer paths are listed before cookies with
 shorter paths.
 * Among cookies that have equal-length path fields, cookies with
 earlier creation-times are listed before cookies with later
 creation-times.
 Note: Not all user agents sort the cookie-list in this order, but
 this order reflects common practice when this document was
 written, and, historically, there have been servers that
 (erroneously) depended on this order.
 5. Update the last-access-time of each cookie in the cookie-list to
 the current date and time.
 6. Serialize the cookie-list into a cookie-string by processing each
 cookie in the cookie-list in order:
 1. If the cookies' name is not empty, output the cookie's name
 followed by the %x3D ("=") character.
 2. If the cookies' value is not empty, output the cookie's
 value.
 3. If the cookie was not the last cookie in the cookie-list,
 output the characters %x3B and %x20 ("; ").
6. Implementation Considerations
6.1. Limits
 Practical user agent implementations have limits on the number and
 size of cookies that they can store. General-use user agents SHOULD
 provide each of the following minimum capabilities:
 * At least 50 cookies per domain.
 * At least 3000 cookies total.
 User agents MAY limit the maximum number of cookies they store, and
 may evict any cookie at any time (whether at the request of the user
 or due to implementation limitations).
 Note that a limit on the maximum number of cookies also limits the
 total size of the stored cookies, due to the length limits which MUST
 be enforced in Section 5.6.
Bingler, et al. Expires 4 June 2026 [Page 44]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Servers SHOULD use as few and as small cookies as possible to avoid
 reaching these implementation limits, minimize network bandwidth due
 to the Cookie header field being included in every request, and to
 avoid reaching server header field limits (See Section 4.2.1).
 Servers SHOULD gracefully degrade if the user agent fails to return
 one or more cookies in the Cookie header field because the user agent
 might evict any cookie at any time.
6.2. Application Programming Interfaces
 One reason the Cookie and Set-Cookie header fields use such esoteric
 handling is that many platforms (both in servers and user agents)
 provide a string-based application programming interface (API) to
 cookies, requiring application-layer programmers to generate and
 parse the syntax used by the Cookie and Set-Cookie header fields,
 which many programmers have done incorrectly, resulting in
 interoperability problems.
 Instead of providing string-based APIs to cookies, platforms would be
 well-served by providing more semantic APIs. It is beyond the scope
 of this document to recommend specific API designs, but there are
 clear benefits to accepting an abstract "Date" object instead of a
 serialized date string.
7. Privacy Considerations
 Cookies' primary privacy risk is their ability to correlate user
 activity. This can happen on a single site, but is most problematic
 when activity is tracked across different, seemingly unconnected Web
 sites to build a user profile.
 Over time, this capability (warned against explicitly in [RFC2109]
 and all of its successors) has become widely used for varied reasons
 including:
 * authenticating users across sites,
 * assembling information on users,
 * protecting against fraud and other forms of undesirable traffic,
 * targeting advertisements at specific users or at users with
 specified attributes,
 * measuring how often ads are shown to users, and
 * recognizing when an ad resulted in a change in user behavior.
Bingler, et al. Expires 4 June 2026 [Page 45]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 While not every use of cookies is necessarily problematic for
 privacy, their potential for abuse has become a widespread concern in
 the Internet community and broader society. In response to these
 concerns, user agents have actively constrained cookie functionality
 in various ways (as allowed and encouraged by previous
 specifications), while avoiding disruption to features they judge
 desirable for the health of the Web.
 It is too early to declare consensus on which specific mechanism(s)
 should be used to mitigate cookies' privacy impact; user agents'
 ongoing changes to how they are handled are best characterised as
 experiments that can provide input into that eventual consensus.
 Instead, this document describes limited, general mitigations against
 the privacy risks associated with cookies that enjoy wide deployment
 at the time of writing. It is expected that implementations will
 continue to experiment and impose stricter, more well-defined
 limitations on cookies over time. Future versions of this document
 might codify those mechanisms based upon deployment experience. If
 functions that currently rely on cookies can be supported by
 separate, targeted mechanisms, they might be documented in separate
 specifications and stricter limitations on cookies might become
 feasible.
 Note that cookies are not the only mechanism that can be used to
 track users across sites, so while these mitigations are necessary to
 improve Web privacy, they are not sufficient on their own.
7.1. Third-Party Cookies
 A "third-party" or cross-site cookie is one that is associated with
 embedded content (such as scripts, images, stylesheets, frames) that
 is obtained from a different server than the one that hosts the
 primary resource (usually, the Web page that the user is viewing).
 Third-party cookies are often used to correlate users' activity on
 different sites.
 Because of their inherent privacy issues, most user agents now limit
 third-party cookies in a variety of ways. Some completely block
 third-party cookies by refusing to process third-party Set-Cookie
 header fields and refusing to send third-party Cookie header fields.
 Some partition cookies based upon the first-party context, so that
 different cookies are sent depending on the site being browsed. Some
 block cookies based upon user agent cookie policy and/or user
 controls.
Bingler, et al. Expires 4 June 2026 [Page 46]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 While this document does not endorse or require a specific approach,
 it is RECOMMENDED that user agents adopt a policy for third-party
 cookies that is as restrictive as compatibility constraints permit.
 Consequently, resources cannot rely upon third-party cookies being
 treated consistently by user agents for the foreseeable future.
7.2. Cookie Policy
 User agents MAY enforce a cookie policy consisting of restrictions on
 how cookies may be used or ignored (see Section 5.3).
 A cookie policy may govern which domains or parties, as in first and
 third parties (see Section 7.1), for which the user agent will allow
 cookie access. The policy can also define limits on cookie size,
 cookie expiry (see Section 5.5), and the number of cookies per domain
 or in total.
 The recommended cookie expiry upper limit is 400 days. User agents
 may set a lower limit to enforce shorter data retention timelines, or
 set the limit higher to support longer retention when appropriate
 (e.g., server-to-server communication over HTTPS).
 The goal of a restrictive cookie policy is often to improve security
 or privacy. User agents often allow users to change the cookie
 policy (see Section 7.3).
7.3. User Controls
 User agents SHOULD provide users with a mechanism for managing the
 cookies stored in the cookie store. For example, a user agent might
 let users delete all cookies received during a specified time period
 or all the cookies related to a particular domain. In addition, many
 user agents include a user interface element that lets users examine
 the cookies stored in their cookie store.
 User agents SHOULD provide users with a mechanism for disabling
 cookies. When cookies are disabled, the user agent MUST NOT include
 a Cookie header field in outbound HTTP requests and the user agent
 MUST NOT process Set-Cookie header fields in inbound HTTP responses.
 User agents MAY offer a way to change the cookie policy (see
 Section 7.2).
 User agents MAY provide users the option of preventing persistent
 storage of cookies across sessions. When configured thusly, user
 agents MUST treat all received cookies as if the persistent-flag were
 set to false. Some popular user agents expose this functionality via
 "private browsing" mode [Aggarwal2010].
Bingler, et al. Expires 4 June 2026 [Page 47]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
7.4. Expiration Dates
 Although servers can set the expiration date for cookies to the
 distant future, most user agents do not actually retain cookies for
 multiple decades. Rather than choosing gratuitously long expiration
 periods, servers SHOULD promote user privacy by selecting reasonable
 cookie expiration periods based on the purpose of the cookie. For
 example, a typical session identifier might reasonably be set to
 expire in two weeks.
8. Security Considerations
8.1. Overview
 Cookies have a number of security pitfalls. This section overviews a
 few of the more salient issues.
 In particular, cookies encourage developers to rely on ambient
 authority for authentication, often becoming vulnerable to attacks
 such as cross-site request forgery [CSRF]. Also, when storing
 session identifiers in cookies, developers often create session
 fixation vulnerabilities.
 Transport-layer encryption, such as that employed in HTTPS, offers a
 significant layer of defense against network attacks on cookies.
 However, it is insufficient in fully preventing a networking attacker
 from obtaining or altering a victim's cookies because of inherent
 vulnerabilities in the cookie protocol itself (see "Weak
 Confidentiality" and "Weak Integrity", below). In addition, by
 default, cookies do not provide confidentiality or integrity from
 network attackers, even when used in conjunction with HTTPS. This
 means that a cookie needs to explicitly specify any protective
 attributes. For example, the cookie:
 Set-Cookie: a=b
 doesn't specify the Secure attribute and will therefore be accessible
 on both secure and insecure connections, regardless of the original
 connection type it was created on. This behavior could allow an
 attacker to read or modify the cookie.
Bingler, et al. Expires 4 June 2026 [Page 48]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
8.2. Ambient Authority
 A server that uses cookies to authenticate users can suffer security
 vulnerabilities because some user agents let remote parties issue
 HTTP requests from the user agent (e.g., via HTTP redirects or HTML
 forms). When issuing those requests, user agents attach cookies even
 if the remote party does not know the contents of the cookies,
 potentially letting the remote party exercise authority at an unwary
 server.
 Although this security concern goes by a number of names (e.g.,
 cross-site request forgery, confused deputy), the issue stems from
 cookies being a form of ambient authority. Cookies encourage server
 operators to separate designation (in the form of URLs) from
 authorization (in the form of cookies). Consequently, the user agent
 might supply the authorization for a resource designated by the
 attacker, possibly causing the server or its clients to undertake
 actions designated by the attacker as though they were authorized by
 the user.
 Instead of using cookies for authorization, server operators might
 wish to consider entangling designation and authorization by treating
 URLs as capabilities. Instead of storing secrets in cookies, this
 approach stores secrets in URLs, requiring the remote entity to
 supply the secret itself. Although this approach is not a panacea,
 judicious application of these principles can lead to more robust
 security.
8.3. Clear Text
 Unless sent over a secure channel (such as TLS [TLS13]), the
 information in the Cookie and Set-Cookie header fields is transmitted
 in the clear.
 1. All sensitive information conveyed in these header fields is
 exposed to an eavesdropper.
 2. A malicious intermediary could alter the header fields as they
 travel in either direction, with unpredictable results.
 Servers SHOULD encrypt and sign the contents of cookies (using
 whatever format the server desires) when transmitting them to the
 user agent (even when sending the cookies over a secure channel).
 However, encrypting and signing cookie contents does not prevent an
 attacker from transplanting a cookie from one user agent to another
 or from replaying the cookie at a later time.
Bingler, et al. Expires 4 June 2026 [Page 49]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 In addition to encrypting and signing the contents of every cookie,
 servers that require a higher level of security SHOULD use the Cookie
 and Set-Cookie header fields only over a secure channel. When using
 cookies over a secure channel, servers SHOULD set the Secure
 attribute (see Section 4.1.2.5) for every cookie. If a server does
 not set the Secure attribute, the protection provided by the secure
 channel will be largely moot.
 For example, consider a webmail server that stores a session
 identifier in a cookie and is typically accessed over HTTPS. If the
 server does not set the Secure attribute on its cookies, an active
 network attacker can intercept any outbound HTTP request from the
 user agent and redirect that request to the webmail server over HTTP.
 Even if the webmail server is not listening for HTTP connections, the
 user agent will still include cookies in the request. The active
 network attacker can intercept these cookies, replay them against the
 server, and learn the contents of the user's email. If, instead, the
 server had set the Secure attribute on its cookies, the user agent
 would not have included the cookies in the clear-text request.
8.4. Session Identifiers
 Instead of storing session information directly in a cookie (where it
 might be exposed to or replayed by an attacker), servers commonly
 store a nonce (or "session identifier") in a cookie. When the server
 receives an HTTP request with a nonce, the server can look up state
 information associated with the cookie using the nonce as a key.
 Using session identifier cookies limits the damage an attacker can
 cause if the attacker learns the contents of a cookie because the
 nonce is useful only for interacting with the server (unlike non-
 nonce cookie content, which might itself be sensitive). Furthermore,
 using a single nonce prevents an attacker from "splicing" together
 cookie content from two interactions with the server, which could
 cause the server to behave unexpectedly.
 Using session identifiers is not without risk. For example, the
 server SHOULD take care to avoid "session fixation" vulnerabilities.
 A session fixation attack proceeds in three steps. First, the
 attacker transplants a session identifier from his or her user agent
 to the victim's user agent. Second, the victim uses that session
 identifier to interact with the server, possibly imbuing the session
 identifier with the user's credentials or confidential information.
 Third, the attacker uses the session identifier to interact with
 server directly, possibly obtaining the user's authority or
 confidential information.
Bingler, et al. Expires 4 June 2026 [Page 50]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
8.5. Weak Confidentiality
 Cookies do not provide isolation by port. If a cookie is readable by
 a service running on one port, the cookie is also readable by a
 service running on another port of the same server. If a cookie is
 writable by a service on one port, the cookie is also writable by a
 service running on another port of the same server. For this reason,
 servers SHOULD NOT both run mutually distrusting services on
 different ports of the same host and use cookies to store security-
 sensitive information.
 Cookies do not provide isolation by scheme. Although most commonly
 used with the http and https schemes, the cookies for a given host
 might also be available to other schemes, such as ftp and gopher.
 Although this lack of isolation by scheme is most apparent in non-
 HTTP APIs that permit access to cookies (e.g., HTML's document.cookie
 API), the lack of isolation by scheme is actually present in
 requirements for processing cookies themselves (e.g., consider
 retrieving a URI with the gopher scheme via HTTP).
 Cookies do not always provide isolation by path. Although the
 network-level protocol does not send cookies stored for one path to
 another, some user agents expose cookies via non-HTTP APIs, such as
 HTML's document.cookie API. Because some of these user agents (e.g.,
 web browsers) do not isolate resources received from different paths,
 a resource retrieved from one path might be able to access cookies
 stored for another path.
8.6. Weak Integrity
 Cookies do not provide integrity guarantees for sibling domains (and
 their subdomains). For example, consider foo.site.example and
 bar.site.example. The foo.site.example server can set a cookie with
 a Domain attribute of "site.example" (possibly overwriting an
 existing "site.example" cookie set by bar.site.example), and the user
 agent will include that cookie in HTTP requests to bar.site.example.
 In the worst case, bar.site.example will be unable to distinguish
 this cookie from a cookie it set itself. The foo.site.example server
 might be able to leverage this ability to mount an attack against
 bar.site.example.
Bingler, et al. Expires 4 June 2026 [Page 51]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Even though the Set-Cookie header field supports the Path attribute,
 the Path attribute does not provide any integrity protection because
 the user agent will accept an arbitrary Path attribute in a Set-
 Cookie header field. For example, an HTTP response to a request for
 http://site.example/foo/bar can set a cookie with a Path attribute of
 "/qux". Consequently, servers SHOULD NOT both run mutually
 distrusting services on different paths of the same host and use
 cookies to store security-sensitive information.
 An active network attacker can also inject cookies into the Cookie
 header field sent to https://site.example/ by impersonating a
 response from http://site.example/ and injecting a Set-Cookie header
 field. The HTTPS server at site.example will be unable to
 distinguish these cookies from cookies that it set itself in an HTTPS
 response. An active network attacker might be able to leverage this
 ability to mount an attack against site.example even if site.example
 uses HTTPS exclusively.
 Servers can partially mitigate these attacks by encrypting and
 signing the contents of their cookies, or by naming the cookie with
 the __Secure- prefix. However, using cryptography does not mitigate
 the issue completely because an attacker can replay a cookie he or
 she received from the authentic site.example server in the user's
 session, with unpredictable results.
 Finally, an attacker might be able to force the user agent to delete
 cookies by storing a large number of cookies. Once the user agent
 reaches its storage limit, the user agent will be forced to evict
 some cookies. Servers SHOULD NOT rely upon user agents retaining
 cookies.
8.7. Reliance on DNS
 Cookies rely upon the Domain Name System (DNS) for security. If the
 DNS is partially or fully compromised, the cookie protocol might fail
 to provide the security properties required by applications.
8.8. SameSite Cookies
8.8.1. Defense in depth
 "SameSite" cookies offer a robust defense against CSRF attack when
 deployed in strict mode, and when supported by the client. It is,
 however, prudent to ensure that this designation is not the extent of
 a site's defense against CSRF, as same-site navigations and
 submissions can certainly be executed in conjunction with other
 attack vectors such as cross-site scripting or abuse of page
 redirections.
Bingler, et al. Expires 4 June 2026 [Page 52]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 Understanding how and when a request is considered same-site is also
 important in order to properly design a site for SameSite cookies.
 For example, if a cross-site top-level request is made to a sensitive
 page that request will be considered cross-site and SameSite=Strict
 cookies won't be sent; that page's sub-resources requests, however,
 are same-site and would receive SameSite=Strict cookies. Sites can
 avoid inadvertently allowing access to these sub-resources by
 returning an error for the initial page request if it doesn't include
 the appropriate cookies.
 Developers are strongly encouraged to deploy the usual server-side
 defenses (CSRF tokens, ensuring that "safe" HTTP methods are
 idempotent, etc) to mitigate the risk more fully.
 Additionally, client-side techniques such as those described in
 [app-isolation] may also prove effective against CSRF, and are
 certainly worth exploring in combination with "SameSite" cookies.
8.8.2. Top-level Navigations
 Setting the SameSite attribute in "strict" mode provides robust
 defense in depth against CSRF attacks, but has the potential to
 confuse users unless sites' developers carefully ensure that their
 cookie-based session management systems deal reasonably well with
 top-level navigations.
 Consider the scenario in which a user reads their email at MegaCorp
 Inc's webmail provider https://site.example/. They might expect that
 clicking on an emailed link to https://projects.example/secret/
 project would show them the secret project that they're authorized to
 see, but if https://projects.example has marked their session cookies
 as SameSite=Strict, then this cross-site navigation won't send them
 along with the request. https://projects.example will render a 404
 error to avoid leaking secret information, and the user will be quite
 confused.
 Developers can avoid this confusion by adopting a session management
 system that relies on not one, but two cookies: one conceptually
 granting "read" access, another granting "write" access. The latter
 could be marked as SameSite=Strict, and its absence would prompt a
 reauthentication step before executing any non-idempotent action.
 The former could be marked as SameSite=Lax, in order to allow users
 access to data via top-level navigation, or SameSite=None, to permit
 access in all contexts (including cross-site embedded contexts).
Bingler, et al. Expires 4 June 2026 [Page 53]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
8.8.3. Mashups and Widgets
 The Lax and Strict values for the SameSite attribute are
 inappropriate for some important use-cases. In particular, note that
 content intended for embedding in cross-site contexts (social
 networking widgets or commenting services, for instance) will not
 have access to same-site cookies. Cookies which are required in
 these situations should be marked with SameSite=None to allow access
 in cross-site contexts.
 Likewise, some forms of Single-Sign-On might require cookie-based
 authentication in a cross-site context; these mechanisms will not
 function as intended with same-site cookies and will also require
 SameSite=None.
8.8.4. Server-controlled
 SameSite cookies in and of themselves don't do anything to address
 the general privacy concerns outlined in Section 7.1 of [RFC6265].
 The "SameSite" attribute is set by the server, and serves to mitigate
 the risk of certain kinds of attacks that the server is worried
 about. The user is not involved in this decision. Moreover, a
 number of side-channels exist which could allow a server to link
 distinct requests even in the absence of cookies (for example,
 connection and/or socket pooling between same-site and cross-site
 requests).
8.8.5. Reload navigations
 Requests issued for reloads triggered through user interface elements
 (such as a refresh button on a toolbar) are same-site only if the
 reloaded document was originally navigated to via a same-site
 request. This differs from the handling of other reload navigations,
 which are always same-site if top-level, since the source navigable's
 active document is precisely the document being reloaded.
 This special handling of reloads triggered through a user interface
 element avoids sending SameSite cookies on user-initiated reloads if
 they were withheld on the original navigation (i.e., if the initial
 navigation were cross-site). If the reload navigation were instead
 considered same-site, and sent all the initially withheld SameSite
 cookies, the security benefits of withholding the cookies in the
 first place would be nullified. This is especially important given
 that the absence of SameSite cookies withheld on a cross-site
 navigation request may lead to visible site breakage, prompting the
 user to trigger a reload.
Bingler, et al. Expires 4 June 2026 [Page 54]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 For example, suppose the user clicks on a link from
 https://attacker.example/ to https://victim.example/. This is a
 cross-site request, so SameSite=Strict cookies are withheld. Suppose
 this causes https://victim.example/ to appear broken, because the
 site only displays its sensitive content if a particular SameSite
 cookie is present in the request. The user, frustrated by the
 unexpectedly broken site, presses refresh on their browser's toolbar.
 To now consider the reload request same-site and send the initially
 withheld SameSite cookie would defeat the purpose of withholding it
 in the first place, as the reload navigation triggered through the
 user interface may replay the original (potentially malicious)
 request. Thus, the reload request should be considered cross-site,
 like the request that initially navigated to the page.
 Because requests issued for, non-user initiated, reloads attach all
 SameSite cookies, developers should be careful and thoughtful about
 when to initiate a reload in order to avoid a CSRF attack. For
 example, the page could only initiate a reload if a CSRF token is
 present on the initial request.
8.8.6. Top-level requests with "unsafe" methods
 The "Lax" enforcement mode described in Section 5.6.7.1 allows a
 cookie to be sent with a cross-site HTTP request if and only if it is
 a top-level navigation with a "safe" HTTP method. Implementation
 experience shows that this is difficult to apply as the default
 behavior, as some sites may rely on cookies not explicitly specifying
 a SameSite attribute being included on top-level cross-site requests
 with "unsafe" HTTP methods (as was the case prior to the introduction
 of the SameSite attribute).
 For example, the concluding step of a login flow may involve a cross-
 site top-level POST request to an endpoint; this endpoint expects a
 recently created cookie containing transactional state information,
 necessary to securely complete the login. For such a cookie, "Lax"
 enforcement is not appropriate, as it would cause the cookie to be
 excluded due to the unsafe HTTP request method, resulting in an
 unrecoverable failure of the whole login flow.
 The "Lax-allowing-unsafe" enforcement mode described in
 Section 5.6.7.2 retains some of the protections of "Lax" enforcement
 (as compared to "None") while still allowing recently created cookies
 to be sent cross-site with unsafe top-level requests.
Bingler, et al. Expires 4 June 2026 [Page 55]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 As a more permissive variant of "Lax" mode, "Lax-allowing-unsafe"
 mode necessarily provides fewer protections against CSRF.
 Ultimately, the provision of such an enforcement mode should be seen
 as a temporary, transitional measure to ease adoption of "Lax"
 enforcement by default.
8.9. Public Suffix List
 The boundaries of cookies depend on a site's "registrable domain"
 which in turn depends on the public suffix of the domain.
 Whenever possible, user agents SHOULD use an up-to-date public suffix
 list, such as the one maintained by the Mozilla project at [PSL].
 Failure to do so could allow malicious or sensitive cookies to leak
 between registrable domains.
9. IANA Considerations
9.1. Cookie
 The HTTP Field Name Registry (see [HttpFieldNameRegistry]) needs to
 be updated with the following registration:
 Header field name: Cookie
 Applicable protocol: http
 Status: standard
 Author/Change controller: IETF
 Specification document: this specification (Section 5.8.1)
9.2. Set-Cookie
 The HTTP Field Name Registry (see [HttpFieldNameRegistry]) needs to
 be updated with the following registration:
 Header field name: Set-Cookie
 Applicable protocol: http
 Status: standard
 Author/Change controller: IETF
 Specification document: this specification (Section 5.6)
Bingler, et al. Expires 4 June 2026 [Page 56]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
9.3. "Cookie Attributes" Registry
 IANA is requested to create the "Cookie Attributes" registry,
 defining the name space of attributes used to control cookies'
 behavior. The registry should be maintained in a new registry group
 called "Hypertext Transfer Protocol (HTTP) Cookie Attributes" at
 https://www.iana.org/assignments/cookie-attribute-names
 (https://www.iana.org/assignments/cookie-attribute-names).
9.3.1. Procedure
 Each registered attribute name is associated with a description, and
 a reference detailing how the attribute is to be processed and
 stored.
 New registrations happen on a "RFC Required" basis (see Section 4.7
 of [RFC8126]). The attribute to be registered MUST match the
 extension-av syntax defined in Section 4.1.1. Note that attribute
 names are generally defined in CamelCase but MUST be recognized case-
 insensitively. Two attribute names that case-insensitively match
 MUST NOT be registered.
9.3.2. Registration
 The "Cookie Attributes" registry should be created with the
 registrations below:
 +==========+==================================+
 | Name | Reference |
 +==========+==================================+
 | Domain | Section 4.1.2.3 of this document |
 +----------+----------------------------------+
 | Expires | Section 4.1.2.1 of this document |
 +----------+----------------------------------+
 | HttpOnly | Section 4.1.2.6 of this document |
 +----------+----------------------------------+
 | Max-Age | Section 4.1.2.2 of this document |
 +----------+----------------------------------+
 | Path | Section 4.1.2.4 of this document |
 +----------+----------------------------------+
 | SameSite | Section 4.1.2.7 of this document |
 +----------+----------------------------------+
 | Secure | Section 4.1.2.5 of this document |
 +----------+----------------------------------+
 Table 1
10. References
Bingler, et al. Expires 4 June 2026 [Page 57]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
10.1. Normative References
 [DOM-DOCUMENT-COOKIE]
 van Kesteren, A., Denicola, D., Farolino, D., Hickson, I.,
 Jägenstedt, P., and S. Pieters, "HTML - Living Standard",
 n.d., <https://html.spec.whatwg.org/#dom-document-cookie>.
 WHATWG
 [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
 Ed., "HTTP Semantics", STD 97, RFC 9110,
 DOI 10.17487/RFC9110, June 2022,
 <https://www.rfc-editor.org/rfc/rfc9110>.
 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
 <https://www.rfc-editor.org/rfc/rfc1034>.
 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
 Application and Support", STD 3, RFC 1123,
 DOI 10.17487/RFC1123, October 1989,
 <https://www.rfc-editor.org/rfc/rfc1123>.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/rfc/rfc2119>.
 [RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
 Application Protocol Collation Registry", RFC 4790,
 DOI 10.17487/RFC4790, March 2007,
 <https://www.rfc-editor.org/rfc/rfc4790>.
 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
 Specifications: ABNF", STD 68, RFC 5234,
 DOI 10.17487/RFC5234, January 2008,
 <https://www.rfc-editor.org/rfc/rfc5234>.
 [RFC5890] Klensin, J., "Internationalized Domain Names for
 Applications (IDNA): Definitions and Document Framework",
 RFC 5890, DOI 10.17487/RFC5890, August 2010,
 <https://www.rfc-editor.org/rfc/rfc5890>.
 [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
 DOI 10.17487/RFC6454, December 2011,
 <https://www.rfc-editor.org/rfc/rfc6454>.
Bingler, et al. Expires 4 June 2026 [Page 58]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
 Writing an IANA Considerations Section in RFCs", BCP 26,
 RFC 8126, DOI 10.17487/RFC8126, June 2017,
 <https://www.rfc-editor.org/rfc/rfc8126>.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
 [SAMESITE] van Kesteren, A., Denicola, D., Farolino, D., Hickson, I.,
 Jägenstedt, P., and S. Pieters, "HTML Living Standard",
 n.d., <https://html.spec.whatwg.org/#same-site>. WHATWG
 [USASCII] American National Standards Institute, "Coded Character
 Set -- 7-bit American Standard Code for Information
 Interchange", ANSI X3.4, 1986.
10.2. Informative References
 [Aggarwal2010]
 Aggarwal, G., Burzstein, E., Jackson, C., and D. Boneh,
 "An Analysis of Private Browsing Modes in Modern
 Browsers", 2010,
 <http://www.usenix.org/events/sec10/tech/full_papers/
 Aggarwal.pdf>.
 [app-isolation]
 Chen, E., Bau, J., Reis, C., Barth, A., and C. Jackson,
 "App Isolation - Get the Security of Multiple Browsers
 with Just One", 2011,
 <http://www.collinjackson.com/research/papers/
 appisolation.pdf>.
 [CSRF] Barth, A., Jackson, C., and J. Mitchell, "Robust Defenses
 for Cross-Site Request Forgery",
 DOI 10.1145/1455770.1455782, ISBN 978-1-59593-810-7,
 ACM CCS '08: Proceedings of the 15th ACM conference on
 Computer and communications security (pages 75-88),
 October 2008,
 <http://portal.acm.org/citation.cfm?id=1455770.1455782>.
 [FETCH] van Kesteren, A., "Fetch Living Standard", n.d.,
 <https://fetch.spec.whatwg.org/>. WHATWG
 [HTML] van Kesteren, A., Denicola, D., Farolino, D., Hickson, I.,
 Jägenstedt, P., and S. Pieters, "HTML Living Standard",
 n.d., <https://html.spec.whatwg.org/>. WHATWG
Bingler, et al. Expires 4 June 2026 [Page 59]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 [HttpFieldNameRegistry]
 "Hypertext Transfer Protocol (HTTP) Field Name Registry",
 n.d., <https://www.iana.org/assignments/http-fields/>.
 [prerendering]
 Bentzel, C., "Chrome Prerendering", n.d.,
 <https://www.chromium.org/developers/design-documents/
 prerender>.
 [PSL] "Public Suffix List", n.d.,
 <https://publicsuffix.org/list/>.
 [RFC2109] Kristol, D. and L. Montulli, "HTTP State Management
 Mechanism", RFC 2109, DOI 10.17487/RFC2109, February 1997,
 <https://www.rfc-editor.org/rfc/rfc2109>.
 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
 Resource Identifier (URI): Generic Syntax", STD 66,
 RFC 3986, DOI 10.17487/RFC3986, January 2005,
 <https://www.rfc-editor.org/rfc/rfc3986>.
 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
 <https://www.rfc-editor.org/rfc/rfc4648>.
 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
 DOI 10.17487/RFC6265, April 2011,
 <https://www.rfc-editor.org/rfc/rfc6265>.
 [RFC7034] Ross, D. and T. Gondrom, "HTTP Header Field X-Frame-
 Options", RFC 7034, DOI 10.17487/RFC7034, October 2013,
 <https://www.rfc-editor.org/rfc/rfc7034>.
 [RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
 DOI 10.17487/RFC9113, June 2022,
 <https://www.rfc-editor.org/rfc/rfc9113>.
 [RFC9114] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
 June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.
 [SERVICE-WORKERS]
 Archibald, J. and M. Kruisselbrink, "Service Workers",
 n.d., <https://www.w3.org/TR/service-workers/>.
 [TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
 <https://www.rfc-editor.org/rfc/rfc8446>.
Bingler, et al. Expires 4 June 2026 [Page 60]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
Appendix A. Changes from RFC 6265
 * Adds the same-site concept and the SameSite attribute.
 (Section 5.2 and Section 4.1.2.7)
 * Introduces cookie prefixes and prohibits nameless cookies from
 setting a value that would mimic a cookie prefix. (Section 4.1.3
 and Section 5.7)
 * Prohibits non-secure origins from setting cookies with a Secure
 flag or overwriting cookies with this flag. (Section 5.7)
 * Limits maximum cookie size. (Section 5.7)
 * Limits maximum values for max-age and expire. (Section 5.6.1 and
 Section 5.6.2)
 * Includes the host-only-flag as part of a cookie's uniqueness
 computation. (Section 5.7)
 * Considers potentially trustworthy origins as "secure".
 (Section 5.7)
 * Improves cookie syntax
 - Treats Set-Cookie: token as creating the cookie ("", "token").
 (Section 5.6)
 - Rejects cookies without a name nor value. (Section 5.7)
 - Specifies how to serialize a nameless/valueless cookie.
 (Section 5.8.3)
 - Adjusts ABNF for cookie-pair and the Cookie header production
 to allow for spaces. (Section 4.1.1)
 - Explicitly handle control characters. (Section 5.6 and
 Section 5.7)
 - Specifies how to handle empty domain attributes. (Section 5.7)
 - Requires ASCII characters for the domain attribute.
 (Section 5.7)
 * Refactors cookie retrieval algorithm to support non-HTTP APIs.
 (Section 5.8.2)
Bingler, et al. Expires 4 June 2026 [Page 61]
Internet-Draft Cookies: HTTP State Management Mechanism December 2025
 * Specifies that the Set-Cookie line should not be decoded.
 (Section 5.6)
 * Adds an advisory section to assist implementers in deciding which
 requirements to implement. (Section 3.2)
 * Advises against sending invalid cookies due to public suffix list
 changes. (Section 5.8.3)
 * Removes the single cookie header requirement. (Section 5.8.1)
 * Address errata 3444 by updating the path-value andextension-av
 grammar, errata 4148 by updating the day-of-month, year, and time
 grammar, and errata 3663 by adding the requested note.
 (Section 4.1 and Section 5.1.4)
Acknowledgements
 RFC 6265 was written by Adam Barth. This document is an update of
 RFC 6265, adding features and aligning the specification with the
 reality of today's deployments. Here, we're standing upon the
 shoulders of a giant since the majority of the text is still Adam's.
 Thank you to both Lily Chen and Steven Englehardt, editors emeritus,
 for their significant contributions improving this draft.
Authors' Addresses
 Steven Bingler (editor)
 Email: bingler@chromium.org
 Mike West (editor)
 Google LLC
 Email: mkwst@google.com
 URI: https://mikewest.org/
 John Wilander (editor)
 Apple, Inc
 Email: wilander@apple.com
Bingler, et al. Expires 4 June 2026 [Page 62]