Cookies: HTTP State Management Mechanism
draft-ietf-httpbis-rfc6265bis-07

HTTP M. West, Ed.
Internet-Draft Google, Inc
Obsoletes: 6265 (if approved) J. Wilander, Ed.
Intended status: Standards Track Apple, Inc
Expires: 10 June 2021 7 December 2020

 Cookies: HTTP State Management Mechanism
 draft-ietf-httpbis-rfc6265bis-07

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 infelicities that degrade their security
 and privacy, the Cookie and Set-Cookie header fields are widely used
 on the Internet. This document obsoletes RFC 6265.

Note to Readers

 Discussion of this draft takes place on the HTTP working group
 mailing list (ietf-http-wg@w3.org), which is archived at
 https://lists.w3.org/Archives/Public/ietf-http-wg/
 (https://lists.w3.org/Archives/Public/ietf-http-wg/).

 Working Group information can be found at http://httpwg.github.io/
 (http://httpwg.github.io/); source code and issues list for this
 draft can be found at https://github.com/httpwg/http-extensions/
 labels/6265bis (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/.

 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."

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 This Internet-Draft will expire on 10 June 2021.

Copyright Notice

 Copyright (c) 2020 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
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 as described in Section 4.e of the Trust Legal Provisions and are
 provided without warranty as described in the Simplified 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
 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 7
 3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 7
 4. Server Requirements . . . . . . . . . . . . . . . . . . . . . 9
 4.1. Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . 9
 4.1.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . 9
 4.1.2. Semantics (Non-Normative) . . . . . . . . . . . . . . 11
 4.1.3. Cookie Name Prefixes . . . . . . . . . . . . . . . . 14
 4.2. Cookie . . . . . . . . . . . . . . . . . . . . . . . . . 16
 4.2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . 16
 4.2.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 16
 5. User Agent Requirements . . . . . . . . . . . . . . . . . . . 16
 5.1. Subcomponent Algorithms . . . . . . . . . . . . . . . . . 17
 5.1.1. Dates . . . . . . . . . . . . . . . . . . . . . . . . 17

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 5.1.2. Canonicalized Host Names . . . . . . . . . . . . . . 18
 5.1.3. Domain Matching . . . . . . . . . . . . . . . . . . . 19
 5.1.4. Paths and Path-Match . . . . . . . . . . . . . . . . 19
 5.2. "Same-site" and "cross-site" Requests . . . . . . . . . . 20
 5.2.1. Document-based requests . . . . . . . . . . . . . . . 21
 5.2.2. Worker-based requests . . . . . . . . . . . . . . . . 22
 5.3. The Set-Cookie Header . . . . . . . . . . . . . . . . . . 23
 5.3.1. The Expires Attribute . . . . . . . . . . . . . . . . 25
 5.3.2. The Max-Age Attribute . . . . . . . . . . . . . . . . 25
 5.3.3. The Domain Attribute . . . . . . . . . . . . . . . . 26
 5.3.4. The Path Attribute . . . . . . . . . . . . . . . . . 26
 5.3.5. The Secure Attribute . . . . . . . . . . . . . . . . 27
 5.3.6. The HttpOnly Attribute . . . . . . . . . . . . . . . 27
 5.3.7. The SameSite Attribute . . . . . . . . . . . . . . . 27
 5.4. Storage Model . . . . . . . . . . . . . . . . . . . . . . 28
 5.5. The Cookie Header . . . . . . . . . . . . . . . . . . . . 33
 6. Implementation Considerations . . . . . . . . . . . . . . . . 36
 6.1. Limits . . . . . . . . . . . . . . . . . . . . . . . . . 36
 6.2. Application Programming Interfaces . . . . . . . . . . . 36
 6.3. IDNA Dependency and Migration . . . . . . . . . . . . . . 36
 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 37
 7.1. Third-Party Cookies . . . . . . . . . . . . . . . . . . . 37
 7.2. User Controls . . . . . . . . . . . . . . . . . . . . . . 38
 7.3. Expiration Dates . . . . . . . . . . . . . . . . . . . . 38
 8. Security Considerations . . . . . . . . . . . . . . . . . . . 38
 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 38
 8.2. Ambient Authority . . . . . . . . . . . . . . . . . . . . 39
 8.3. Clear Text . . . . . . . . . . . . . . . . . . . . . . . 39
 8.4. Session Identifiers . . . . . . . . . . . . . . . . . . . 40
 8.5. Weak Confidentiality . . . . . . . . . . . . . . . . . . 41
 8.6. Weak Integrity . . . . . . . . . . . . . . . . . . . . . 41
 8.7. Reliance on DNS . . . . . . . . . . . . . . . . . . . . . 42
 8.8. SameSite Cookies . . . . . . . . . . . . . . . . . . . . 42
 8.8.1. Defense in depth . . . . . . . . . . . . . . . . . . 43
 8.8.2. Top-level Navigations . . . . . . . . . . . . . . . . 43
 8.8.3. Mashups and Widgets . . . . . . . . . . . . . . . . . 44
 8.8.4. Server-controlled . . . . . . . . . . . . . . . . . . 44
 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
 9.1. Cookie . . . . . . . . . . . . . . . . . . . . . . . . . 44
 9.2. Set-Cookie . . . . . . . . . . . . . . . . . . . . . . . 44
 9.3. Cookie Attribute Registry . . . . . . . . . . . . . . . . 45
 9.3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 45
 9.3.2. Registration . . . . . . . . . . . . . . . . . . . . 45
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
 10.1. Normative References . . . . . . . . . . . . . . . . . . 46
 10.2. Informative References . . . . . . . . . . . . . . . . . 48
 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 49
 A.1. draft-ietf-httpbis-rfc6265bis-00 . . . . . . . . . . . . 49

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 A.2. draft-ietf-httpbis-rfc6265bis-01 . . . . . . . . . . . . 50
 A.3. draft-ietf-httpbis-rfc6265bis-02 . . . . . . . . . . . . 50
 A.4. draft-ietf-httpbis-rfc6265bis-03 . . . . . . . . . . . . 51
 A.5. draft-ietf-httpbis-rfc6265bis-04 . . . . . . . . . . . . 51
 A.6. draft-ietf-httpbis-rfc6265bis-05 . . . . . . . . . . . . 52
 A.7. draft-ietf-httpbis-rfc6265bis-06 . . . . . . . . . . . . 52
 A.8. draft-ietf-httpbis-rfc6265bis-07 . . . . . . . . . . . . 52
 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 53
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 53

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.

 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 URI schemes for which the cookie is applicable.

 For historical reasons, cookies contain a number of security and
 privacy infelicities. 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.

 There are two audiences for this specification: developers of cookie-
 generating servers and developers of cookie-consuming user agents.

 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.

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 This document specifies the syntax and semantics of these headers 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", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

 Requirements phrased in the imperative as part of algorithms (such as
 "strip any leading space characters" or "return false and abort these
 steps") 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].

 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 3.2.3 of [RFC7230].

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2.3. Terminology

 The terms "user agent", "client", "server", "proxy", and "origin
 server" have the same meaning as in the HTTP/1.1 specification
 ([RFC7230], Section 2).

 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 "request-target" as defined in
 Section 5.3 of [RFC7230].

 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 document", "ancestor browsing context", "browsing
 context", "dedicated worker", "Document", "WorkerGlobalScope",
 "sandboxed origin browsing context flag", "parent browsing context",
 "shared worker", "the worker's Documents", "nested browsing context",
 and "top-level browsing context" 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 4.2.1 of [RFC7231].

 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". Whenever possible, user agents
 SHOULD use an up-to-date public suffix list, such as the one
 maintained by the Mozilla project at [PSL].

 The term "request", as well as a request's "client", "current url",
 "method", and "target browsing context", are defined in [FETCH].

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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 in an
 HTTP response. In subsequent requests, the user agent returns a
 Cookie request header to the origin server. The Cookie header
 contains cookies the user agent received in previous Set-Cookie
 headers. The origin server is free to ignore the Cookie header or
 use its contents for an application-defined purpose.

 Origin servers MAY send a Set-Cookie response header with any
 response. User agents MAY ignore Set-Cookie headers contained in
 responses with 100-level status codes but MUST process Set-Cookie
 headers contained in other responses (including responses with 400-
 and 500-level status codes). 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 SHOULD NOT fold multiple Set-Cookie header fields into
 a single header field. The usual mechanism for folding HTTP headers
 fields (i.e., as defined in Section 3.2.2 of [RFC7230]) 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
 folding.

3.1. Examples

 Using the Set-Cookie header, 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

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 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 above contains two cookies, one named
 SID and one named lang. 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=Wed, 09 Jun 2021 10:18:14 GMT

 == User Agent -> Server ==

 Cookie: SID=31d4d96e407aad42; lang=en-US

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 Finally, to remove a cookie, the server returns a Set-Cookie header
 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 match the values used when the cookie was
 created.

 == Server -> User Agent ==

 Set-Cookie: lang=; Expires=Sun, 06 Nov 1994 08:49:37 GMT

 == User Agent -> Server ==

 Cookie: SID=31d4d96e407aad42

4. Server Requirements

 This section describes the syntax and semantics of a well-behaved
 profile of the Cookie and Set-Cookie headers.

4.1. Set-Cookie

 The Set-Cookie HTTP response header is used to send cookies from the
 server to the user agent.

4.1.1. Syntax

 Informally, the Set-Cookie response header contains the header name
 "Set-Cookie" followed by a ":" and a cookie. Each cookie begins with
 a name-value-pair, followed by zero or more attribute-value pairs.
 Servers SHOULD NOT send Set-Cookie headers that fail to conform to
 the following grammar:

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 set-cookie-header = "Set-Cookie:" SP BWS set-cookie-string
 set-cookie-string = BWS cookie-pair *( BWS ";" OWS cookie-av )
 cookie-pair = cookie-name BWS "=" BWS cookie-value
 cookie-name = 1*cookie-octet
 cookie-value = *cookie-octet / ( DQUOTE *cookie-octet DQUOTE )
 cookie-octet = %x21 / %x23-2B / %x2D-3A / %x3C-5B / %x5D-7E
 / %x80-FF
 ; octets excluding CTLs,
 ; whitespace DQUOTE, comma, semicolon,
 ; and backslash

 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 [RFC7231], Section 7.1.1.1>
 max-age-av = "Max-Age" BWS "=" BWS non-zero-digit *DIGIT
 ; In practice, both expires-av and max-age-av
 ; are limited to dates representable by the
 ; user agent.
 non-zero-digit = %x31-39
 ; digits 1 through 9
 domain-av = "Domain" BWS "=" BWS domain-value
 domain-value = <subdomain>
 ; defined in [RFC1034], Section 3.5, as
 ; enhanced by [RFC1123], Section 2.1
 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 = *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]).

 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].

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 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 headers sent to the server.

 The portions of the set-cookie-string produced by the cookie-av term
 are known as attributes. To maximize compatibility with user agents,
 servers SHOULD NOT produce two attributes with the same name in the
 same set-cookie-string. (See Section 5.4 for how user agents handle
 this case.)

 Servers SHOULD NOT include more than one Set-Cookie header field in
 the same response with the same cookie-name. (See Section 5.3 for
 how user agents handle this case.)

 If a server sends multiple responses containing Set-Cookie headers
 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.

4.1.2. Semantics (Non-Normative)

 This section describes simplified semantics of the Set-Cookie header.
 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, 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.

 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.

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 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 is not required to retain the cookie until the specified
 date has passed. In fact, user agents often evict cookies due to
 memory pressure or privacy concerns.

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 is not required to retain the cookie for the specified
 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.

 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 when making HTTP requests to site.example, www.site.example,
 and www.corp.site.example. (Note that a leading %x2E ("."), if
 present, is ignored even though that character is not permitted, but
 a trailing %x2E ("."), if present, will cause the user agent to
 ignore the attribute.) If the server omits the Domain attribute, the
 user agent will return the cookie only to the origin server.

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 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
 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 5.4 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.

 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)
 [RFC2818]).

 Although seemingly useful for protecting cookies from active network
 attackers, the Secure attribute protects only the cookie's
 confidentiality. An active network attacker can overwrite Secure
 cookies from an insecure channel, disrupting their integrity (see
 Section 8.6 for more details).

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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
 (such as a web browser API that exposes cookies to scripts).

 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 5.3.7.1. 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".

 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 normative requirements for the prefixes described below are
 detailed in the storage model algorithm defined in Section 5.4.

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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 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 would be accepted:

 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 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=/

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4.2. Cookie

4.2.1. Syntax

 The user agent sends stored cookies to the origin server in the
 Cookie header. 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 that conforms to
 the following grammar:

 cookie-header = "Cookie:" SP cookie-string
 cookie-string = cookie-pair *( ";" SP cookie-pair )

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.

 Notice that the cookie attributes are not returned. In particular,
 the server cannot determine from the Cookie header 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 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,
 servers SHOULD NOT rely upon the serialization order. In particular,
 if the Cookie header 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.

5. User Agent Requirements

 This section specifies the Cookie and Set-Cookie headers 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).

 A user agent could enforce more restrictions than those specified
 herein (e.g., for the sake of improved security); however,
 experiments have shown that such strictness reduces the likelihood
 that a user agent will be able to interoperate with existing servers.

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5.1. Subcomponent Algorithms

 This section defines some algorithms used by user agents to process
 specific subcomponents of the Cookie and Set-Cookie headers.

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.

 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.

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 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 these steps 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 these steps and fail to parse the cookie-date.

 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:

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 1. Convert the host name to a sequence of individual domain name
 labels.

 2. Convert each label that is not a Non-Reserved LDH (NR-LDH) label,
 to an A-label (see Section 2.3.2.1 of [RFC5890] for the former
 and latter), or to a "punycode label" (a label resulting from the
 "ToASCII" conversion in Section 4 of [RFC3490]), as appropriate
 (see Section 6.3 of this specification).

 3. Concatenate the resulting labels, separated by a %x2E (".")
 character.

5.1.3. Domain Matching

 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:

 1. Let uri-path be the path portion of the request-uri if such a
 portion exists (and empty otherwise). For example, if the
 request-uri contains just a path (and optional query string),
 then the uri-path is that path (without the %x3F ("?") character
 or query string), and if the request-uri contains a full
 absoluteURI, the uri-path is the path component of that URI.

 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.

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 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, A and B, are considered same-site if the following
 algorithm returns true:

 1. If A and B are both the same globally unique identifier, return
 true.

 2. If A and B are both scheme/host/port triples:

 1. If A's scheme does not equal B's scheme, return false.

 2. Let hostA be A's host, and hostB be B's host.

 3. If hostA equals hostB and hostA's registrable domain is null,
 return true.

 4. If hostA's registrable domain equals hostB's registrable
 domain and is non-null, return true.

 3. Return false.

 Note: The port component of the origins is not considered.

 A request is "same-site" if its target's URI'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. The request is otherwise "cross-
 site".

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 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.
 We'll define this origin, the top-level browsing context's active
 document's origin, as the "top-level origin".

 For a document displayed in a top-level browsing context, we can stop
 here: the document's "site for cookies" is the top-level origin.

 For documents which are displayed in nested browsing contexts, we
 need to audit the origins of each of a document's ancestor browsing
 contexts' active documents 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 a globally unique identifier.

 Given a Document ("document"), the following algorithm returns its
 "site for cookies":

 1. Let "top-document" be the active document in "document"'s
 browsing context's top-level browsing context.

 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 containing "document" and each of
 "document"'s ancestor browsing contexts' active documents.

 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 a globally unique identifier.

 5. Return "top-origin".

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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 browsing
 context 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.

 Note: The descriptions below assume that workers must be same-origin
 with the documents that instantiate them. If this invariant changes,
 we'll need to take the worker's script's URI into account when
 determining their status.

5.2.2.1. Dedicated and Shared Workers

 Dedicated workers are simple, as each dedicated worker is bound to
 one and only one document. Requests generated from a dedicated
 worker (via "importScripts", "XMLHttpRequest", "fetch()", etc) define
 their "site for cookies" as that document's "site for cookies".

 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 a globally unique identifier 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 a globally unique identifier.

 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.

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 Requests which simply pass through a Service Worker will be handled
 as described above: the request's client will be the Document or
 Worker which initiated the request, and its "site for cookies" will
 be those defined in Section 5.2.1 and Section 5.2.2.1

 Requests which are initiated by the Service Worker itself (via a
 direct call to "fetch()", for instance), on the other hand, will have
 a client which is a ServiceWorkerGlobalScope. Its "site for cookies"
 will be the Service Worker's URI's origin.

 Given a ServiceWorkerGlobalScope ("worker"), the following algorithm
 returns its "site for cookies":

 1. Return "worker"'s origin.

5.3. The Set-Cookie Header

 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. For example, the user agent might wish to block
 responses to "third-party" requests from setting cookies (see
 Section 7.1).

 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 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.

 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 %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:

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 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.

 2. 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 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.

 3. Remove any leading or trailing WSP characters from the name
 string and the value string.

 4. 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.

 4. If the cookie-av string contains a %x3D ("=") character:

 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.

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 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. Process the attribute-name and attribute-value according to the
 requirements in the following subsections. (Notice that
 attributes with unrecognized attribute-names are ignored.)

 7. 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 5.4 for additional requirements triggered by
 receiving a cookie.)

5.3.1. 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. If the expiry-time is later than the last date the user agent can
 represent, the user agent MAY replace the expiry-time with the
 last representable date.

 4. 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.

 5. Append an attribute to the cookie-attribute-list with an
 attribute-name of Expires and an attribute-value of expiry-time.

5.3.2. 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.

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 1. If the first character of the attribute-value is not a DIGIT or a
 "-" character, ignore the cookie-av.

 2. If the remainder of attribute-value contains a non-DIGIT
 character, ignore the cookie-av.

 3. Let delta-seconds be the attribute-value converted to an integer.

 4. 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.

 5. Append an attribute to the cookie-attribute-list with an
 attribute-name of Max-Age and an attribute-value of expiry-time.

5.3.3. The Domain Attribute

 If the attribute-name case-insensitively matches the string "Domain",
 the user agent MUST process the cookie-av as follows.

 1. If the attribute-value is empty, the behavior is undefined.
 However, the user agent SHOULD ignore the cookie-av entirely.

 2. If the first character of the attribute-value string is %x2E
 ("."):

 1. Let cookie-domain be the attribute-value without the leading
 %x2E (".") character.

 Otherwise:

 1. Let cookie-domain be the entire attribute-value.

 3. Convert the cookie-domain to lower case.

 4. Append an attribute to the cookie-attribute-list with an
 attribute-name of Domain and an attribute-value of cookie-domain.

5.3.4. 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.

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 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.

5.3.5. 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.

5.3.6. 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.

5.3.7. 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".

 5. Append an attribute to the cookie-attribute-list with an
 attribute-name of "SameSite" and an attribute-value of
 "enforcement".

 Note: This algorithm maps the "None" value, as well as any unknown
 value, to the "None" behavior, which is helpful for backwards
 compatibility when introducing new variants.

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5.3.7.1. "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 [RFC7231] sense) HTTP
 method. (Note that a request's method may be changed from POST to
 GET for some redirects (see sections 6.4.2 and 6.4.3 of [RFC7231]);
 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.

 When possible, developers should use a session management mechanism
 such as that described in Section 8.8.2 to mitigate the risk of CSRF
 more completely.

5.4. 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. For
 example, the user agent might wish to block receiving cookies
 from "third-party" responses or the user agent might not wish to
 store cookies that exceed some size.

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 2. If cookie-name is empty and cookie-value is empty, abort these
 steps and ignore the cookie entirely.

 3. 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.

 4. 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.

 5. 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 an attribute-
 name of "Domain".

 Otherwise:

 1. Let the domain-attribute be the empty string.

 6. If the user agent is configured to reject "public suffixes" and
 the domain-attribute is a public suffix:

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 1. If the domain-attribute is identical to the canonicalized
 request-host:

 1. Let the domain-attribute be the empty string.

 Otherwise:

 1. Ignore the cookie entirely and abort these steps.

 NOTE: This step prevents "attacker.example" from disrupting the
 integrity of "site.example" by setting a cookie with a Domain
 attribute of "example".

 7. If the domain-attribute is non-empty:

 1. If the canonicalized request-host does not domain-match the
 domain-attribute:

 1. Ignore the cookie entirely and abort these steps.

 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 the canonicalized request-host.

 8. 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 an
 attribute-name of "Path". Otherwise, set the cookie's path to
 the default-path of the request-uri.

 9. 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.

 10. If the scheme component of the request-uri does not denote a
 "secure" protocol (as defined by the user agent), and the
 cookie's secure-only-flag is true, then abort these steps and
 ignore the cookie entirely.

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 11. 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.

 12. If the cookie was received from a "non-HTTP" API and the
 cookie's http-only-flag is true, abort these steps and ignore
 the cookie entirely.

 13. If the cookie's secure-only-flag is false, and the scheme
 component of request-uri does not denote a "secure" protocol,
 then abort these steps 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 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'.

 14. 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".

 15. 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 browsing context's active document
 whose "site for cookies" is not same-site with the top-level
 origin, then abort these steps and ignore the newly created
 cookie entirely.

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 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 browsing context [HTML] (e.g. if the
 request's "reserved client" is either "null" or an
 environment whose "target browsing context" is a top-level
 browing context), 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 these steps and ignore the newly created cookie
 entirely.

 16. If the cookie's "same-site-flag" is "None", abort these steps
 and ignore the cookie entirely unless the cookie's secure-only-
 flag is true.

 17. If the cookie-name begins with a case-sensitive match for the
 string "__Secure-", abort these steps and ignore the cookie
 entirely unless the cookie's secure-only-flag is true.

 18. If the cookie-name begins with a case-sensitive match for the
 string "__Host-", abort these steps and ignore the cookie
 entirely unless the cookie meets all the following criteria:

 1. The cookie's secure-only-flag is true.

 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 "/".

 19. 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.)

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 2. If the newly-created cookie was received from a "non-HTTP"
 API and the old-cookie's http-only-flag is true, abort these
 steps 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.

 20. 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).

 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.

5.5. The Cookie Header

 The user agent includes stored cookies in the Cookie HTTP request
 header.

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 When the user agent generates an HTTP request, the user agent MUST
 NOT attach more than one Cookie header field.

 A user agent MAY omit the Cookie header in its entirety. For
 example, the user agent might wish to block sending cookies during
 "third-party" requests from setting cookies (see Section 7.1).

 If the user agent does attach a Cookie header field to an HTTP
 request, the user agent MUST send the cookie-string (defined below)
 as the value of the header field.

 The user agent MUST use an algorithm equivalent to the following
 algorithm to compute the cookie-string from a cookie store and a
 request-uri:

 1. 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 the canonicalized
 request-host is identical to the cookie's domain.

 Or:

 - The cookie's host-only-flag is false and the canonicalized
 request-host domain-matches the cookie's domain.

 * The request-uri's path path-matches the cookie's path.

 * If the cookie's secure-only-flag is true, then the request-
 uri's scheme must denote a "secure" protocol (as defined by
 the user agent).

 NOTE: The notion of a "secure" protocol is not defined by this
 document. Typically, user agents consider a protocol secure
 if the protocol makes use of transport-layer security, such as
 SSL or TLS. For example, most user agents consider "https" to
 be a scheme that denotes a secure protocol.

 * If the cookie's http-only-flag is true, then exclude the
 cookie if the cookie-string is being generated for a "non-
 HTTP" API (as defined by the user agent).

 * If the cookie's same-site-flag is not "None", and the HTTP
 request is cross-site (as defined in Section 5.2) then exclude
 the cookie unless all of the following statements hold:

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 1. The same-site-flag is "Lax" or "Default".

 2. The HTTP request's method is "safe".

 3. The HTTP request's target browsing context is a top-level
 browsing context.

 2. The user agent SHOULD sort the cookie-list in the following
 order:

 * 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.

 3. Update the last-access-time of each cookie in the cookie-list to
 the current date and time.

 4. 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 there is an unprocessed cookie in the cookie-list, output
 the characters %x3B and %x20 ("; ").

 NOTE: Despite its name, the cookie-string is actually a sequence of
 octets, not a sequence of characters. To convert the cookie-string
 (or components thereof) into a sequence of characters (e.g., for
 presentation to the user), the user agent might wish to try using the
 UTF-8 character encoding [RFC3629] to decode the octet sequence.
 This decoding might fail, however, because not every sequence of
 octets is valid UTF-8.

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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 4096 bytes per cookie (as measured by the sum of the
 length of the cookie's name, value, and attributes).

 * At least 50 cookies per domain.

 * At least 3000 cookies total.

 Servers SHOULD use as few and as small cookies as possible to avoid
 reaching these implementation limits and to minimize network
 bandwidth due to the Cookie header being included in every request.

 Servers SHOULD gracefully degrade if the user agent fails to return
 one or more cookies in the Cookie header because the user agent might
 evict any cookie at any time on orders from the user.

6.2. Application Programming Interfaces

 One reason the Cookie and Set-Cookie headers use such esoteric syntax
 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 headers, 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.

6.3. IDNA Dependency and Migration

 IDNA2008 [RFC5890] supersedes IDNA2003 [RFC3490]. However, there are
 differences between the two specifications, and thus there can be
 differences in processing (e.g., converting) domain name labels that
 have been registered under one from those registered under the other.
 There will be a transition period of some time during which
 IDNA2003-based domain name labels will exist in the wild. User
 agents SHOULD implement IDNA2008 [RFC5890] and MAY implement [UTS46]

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 or [RFC5895] in order to facilitate their IDNA transition. If a user
 agent does not implement IDNA2008, the user agent MUST implement
 IDNA2003 [RFC3490].

7. Privacy Considerations

 Cookies are often criticized for letting servers track users. For
 example, a number of "web analytics" companies use cookies to
 recognize when a user returns to a web site or visits another web
 site. Although cookies are not the only mechanism servers can use to
 track users across HTTP requests, cookies facilitate tracking because
 they are persistent across user agent sessions and can be shared
 between hosts.

7.1. Third-Party Cookies

 Particularly worrisome are so-called "third-party" cookies. In
 rendering an HTML document, a user agent often requests resources
 from other servers (such as advertising networks). These third-party
 servers can use cookies to track the user even if the user never
 visits the server directly. For example, if a user visits a site
 that contains content from a third party and then later visits
 another site that contains content from the same third party, the
 third party can track the user between the two sites.

 Given this risk to user privacy, some user agents restrict how third-
 party cookies behave, and those restrictions vary widly. For
 instance, user agents might block third-party cookies entirely by
 refusing to send Cookie headers or process Set-Cookie headers during
 third-party requests. They might take a less draconian approach by
 partitioning cookies based on the first-party context, sending one
 set of cookies to a given third party in one first-party context, and
 another to the same third party in another.

 This document grants user agents wide latitude to experiment with
 third-party cookie policies that balance the privacy and
 compatibility needs of their users. However, this document does not
 endorse any particular third-party cookie policy.

 Third-party cookie blocking policies are often ineffective at
 achieving their privacy goals if servers attempt to work around their
 restrictions to track users. In particular, two collaborating
 servers can often track users without using cookies at all by
 injecting identifying information into dynamic URLs.

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7.2. 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 in outbound HTTP requests and the user agent MUST NOT
 process Set-Cookie headers in inbound HTTP responses.

 Some user agents 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].

 Some user agents provide users with the ability to approve individual
 writes to the cookie store. In many common usage scenarios, these
 controls generate a large number of prompts. However, some privacy-
 conscious users find these controls useful nonetheless.

7.3. 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.

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 Transport-layer encryption, such as that employed in HTTPS, is
 insufficient to prevent a network attacker from obtaining or altering
 a victim's cookies because the cookie protocol itself has various
 vulnerabilities (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.

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), the information in
 the Cookie and Set-Cookie headers is transmitted in the clear.

 1. All sensitive information conveyed in these headers is exposed to
 an eavesdropper.

 2. A malicious intermediary could alter the headers as they travel
 in either direction, with unpredictable results.

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 3. A malicious client could alter the Cookie header before
 transmission, 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.

 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 headers 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.

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 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.

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

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 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.

 Even though the Set-Cookie header 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. 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 sent to https://site.example/ by impersonating a response from
 http://site.example/ and injecting a Set-Cookie header. 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. 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

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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.

 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 "projects.example" has marked their session cookies as
 "SameSite", then this cross-site navigation won't send them along
 with the request. "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", and its absence would prompt a
 reauthentication step before executing any non-idempotent action.
 The former could drop the "SameSite" attribute entirely, or choose
 the "Lax" version of enforcement, in order to allow users access to
 data via top-level navigation.

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8.8.3. Mashups and Widgets

 The "SameSite" attribute is inappropriate for some important use-
 cases. In particular, note that content intended for embedding in a
 cross-site contexts (social networking widgets or commenting
 services, for instance) will not have access to same-site cookies.
 Cookies may be required for requests triggered in these cross-site
 contexts in order to provide seamless functionality that relies on a
 user's state.

 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.

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).

9. IANA Considerations

9.1. Cookie

 The permanent message header field registry (see [RFC3864]) 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 5.5)

9.2. Set-Cookie

 The permanent message header field registry (see [RFC3864]) needs to
 be updated with the following registration:

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 Header field name: Set-Cookie

 Applicable protocol: http

 Status: standard

 Author/Change controller: IETF

 Specification document: this specification (Section 5.3)

9.3. Cookie Attribute Registry

 The "Cookie Attribute Registry" defines the name space of attribute
 used to control cookies' behavior. The registry is maintained 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 technically accepted
 case-insensitively.

9.3.2. Registration

 The "Cookie Attribute Registry" will be updated with the
 registrations below:

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 +==========+==================================+
 | 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

10.1. Normative References

 [FETCH] van Kesteren, A., "Fetch", n.d.,
 <https://fetch.spec.whatwg.org/>.

 [HTML] Hickson, I., Pieters, S., van Kesteren, A., Jägenstedt,
 P., and D. Denicola, "HTML", n.d.,
 <https://html.spec.whatwg.org/>.

 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
 <https://www.rfc-editor.org/info/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/info/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/info/rfc2119>.

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 [RFC3490] Costello, A., "Internationalizing Domain Names in
 Applications (IDNA)", RFC 3490, March 2003,
 <https://www.rfc-editor.org/rfc/rfc3490>. See Section 6.3
 for an explanation why the normative reference to an
 obsoleted specification is needed.

 [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/info/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/info/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/info/rfc5890>.

 [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
 DOI 10.17487/RFC6454, December 2011,
 <https://www.rfc-editor.org/info/rfc6454>.

 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
 Protocol (HTTP/1.1): Message Syntax and Routing",
 RFC 7230, DOI 10.17487/RFC7230, June 2014,
 <https://www.rfc-editor.org/info/rfc7230>.

 [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
 Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
 DOI 10.17487/RFC7231, June 2014,
 <https://www.rfc-editor.org/info/rfc7231>.

 [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/info/rfc8126>.

 [SERVICE-WORKERS]
 Russell, A., Song, J., and J. Archibald, "Service
 Workers", n.d., <http://www.w3.org/TR/service-workers/>.

 [USASCII] American National Standards Institute, "Coded Character
 Set -- 7-bit American Standard Code for Information
 Interchange", ANSI X3.4, 1986.

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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>.

 [I-D.ietf-httpbis-cookie-alone]
 West, M., "Deprecate modification of 'secure' cookies from
 non-secure origins", Work in Progress, Internet-Draft,
 draft-ietf-httpbis-cookie-alone-01, 5 September 2016,
 <http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
 cookie-alone-01.txt>.

 [I-D.ietf-httpbis-cookie-prefixes]
 West, M., "Cookie Prefixes", Work in Progress, Internet-
 Draft, draft-ietf-httpbis-cookie-prefixes-00, 23 February
 2016, <http://www.ietf.org/internet-drafts/draft-ietf-
 httpbis-cookie-prefixes-00.txt>.

 [I-D.ietf-httpbis-cookie-same-site]
 West, M. and M. Goodwin, "Same-Site Cookies", Work in
 Progress, Internet-Draft, draft-ietf-httpbis-cookie-same-
 site-00, 20 June 2016, <http://www.ietf.org/internet-
 drafts/draft-ietf-httpbis-cookie-same-site-00.txt>.

 [prerendering]
 Bentzel, C., "Chrome Prerendering", n.d.,
 <https://www.chromium.org/developers/design-documents/
 prerender>.

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 [PSL] "Public Suffix List", n.d.,
 <https://publicsuffix.org/list/>.

 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
 DOI 10.17487/RFC2818, May 2000,
 <https://www.rfc-editor.org/info/rfc2818>.

 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
 2003, <https://www.rfc-editor.org/info/rfc3629>.

 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
 Procedures for Message Header Fields", BCP 90, RFC 3864,
 DOI 10.17487/RFC3864, September 2004,
 <https://www.rfc-editor.org/info/rfc3864>.

 [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/info/rfc3986>.

 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
 Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
 <https://www.rfc-editor.org/info/rfc4648>.

 [RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
 Internationalized Domain Names in Applications (IDNA)
 2008", RFC 5895, DOI 10.17487/RFC5895, September 2010,
 <https://www.rfc-editor.org/info/rfc5895>.

 [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
 DOI 10.17487/RFC6265, April 2011,
 <https://www.rfc-editor.org/info/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/info/rfc7034>.

 [UTS46] Davis, M. and M. Suignard, "Unicode IDNA Compatibility
 Processing", UNICODE Unicode Technical Standards # 46,
 June 2016, <http://unicode.org/reports/tr46/>.

Appendix A. Changes

A.1. draft-ietf-httpbis-rfc6265bis-00

 * Port [RFC6265] to Markdown. No (intentional) normative changes.

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A.2. draft-ietf-httpbis-rfc6265bis-01

 * Fixes to formatting caused by mistakes in the initial port to
 Markdown:

 - https://github.com/httpwg/http-extensions/issues/243
 (https://github.com/httpwg/http-extensions/issues/243)

 - https://github.com/httpwg/http-extensions/issues/246
 (https://github.com/httpwg/http-extensions/issues/246)

 * Addresses errata 3444 by updating the "path-value" and "extension-
 av" grammar, errata 4148 by updating the "day-of-month", "year",
 and "time" grammar, and errata 3663 by adding the requested note.
 https://www.rfc-editor.org/errata_search.php?rfc=6265
 (https://www.rfc-editor.org/errata_search.php?rfc=6265)

 * Dropped "Cookie2" and "Set-Cookie2" from the IANA Considerations
 section: https://github.com/httpwg/http-extensions/issues/247
 (https://github.com/httpwg/http-extensions/issues/247)

 * Merged the recommendations from [I-D.ietf-httpbis-cookie-alone],
 removing the ability for a non-secure origin to set cookies with a
 'secure' flag, and to overwrite cookies whose 'secure' flag is
 true.

 * Merged the recommendations from
 [I-D.ietf-httpbis-cookie-prefixes], adding "__Secure-" and
 "__Host-" cookie name prefix processing instructions.

A.3. draft-ietf-httpbis-rfc6265bis-02

 * Merged the recommendations from
 [I-D.ietf-httpbis-cookie-same-site], adding support for the
 "SameSite" attribute.

 * Closed a number of editorial bugs:

 - Clarified address bar behavior for SameSite cookies:
 https://github.com/httpwg/http-extensions/issues/201
 (https://github.com/httpwg/http-extensions/issues/201)

 - Added the word "Cookies" to the document's name:
 https://github.com/httpwg/http-extensions/issues/204
 (https://github.com/httpwg/http-extensions/issues/204)

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 - Clarified that the "__Host-" prefix requires an explicit "Path"
 attribute: https://github.com/httpwg/http-extensions/issues/222
 (https://github.com/httpwg/http-extensions/issues/222)

 - Expanded the options for dealing with third-party cookies to
 include a brief mention of partitioning based on first-party:
 https://github.com/httpwg/http-extensions/issues/248
 (https://github.com/httpwg/http-extensions/issues/248)

 - Noted that double-quotes in cookie values are part of the
 value, and are not stripped: https://github.com/httpwg/http-
 extensions/issues/295 (https://github.com/httpwg/http-
 extensions/issues/295)

 - Fixed the "site for cookies" algorithm to return something that
 makes sense: https://github.com/httpwg/http-extensions/
 issues/302 (https://github.com/httpwg/http-extensions/
 issues/302)

A.4. draft-ietf-httpbis-rfc6265bis-03

 * Clarified handling of invalid SameSite values:
 https://github.com/httpwg/http-extensions/issues/389
 (https://github.com/httpwg/http-extensions/issues/389)

 * Reflect widespread implementation practice of including a cookie's
 "host-only-flag" when calculating its uniqueness:
 https://github.com/httpwg/http-extensions/issues/199
 (https://github.com/httpwg/http-extensions/issues/199)

 * Introduced an explicit "None" value for the SameSite attribute:
 https://github.com/httpwg/http-extensions/issues/788
 (https://github.com/httpwg/http-extensions/issues/788)

A.5. draft-ietf-httpbis-rfc6265bis-04

 * Allow "SameSite" cookies to be set for all top-level navigations.
 https://github.com/httpwg/http-extensions/issues/594
 (https://github.com/httpwg/http-extensions/issues/594)

 * Treat "Set-Cookie: token" as creating the cookie "("", "token")":
 https://github.com/httpwg/http-extensions/issues/159
 (https://github.com/httpwg/http-extensions/issues/159)

 * Reject cookies with neither name nor value (e.g. "Set-Cookie: ="
 and "Set-Cookie:": https://github.com/httpwg/http-extensions/
 issues/159 (https://github.com/httpwg/http-extensions/issues/159)

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 * Clarified behavior of multiple "SameSite" attributes in a cookie
 string: https://github.com/httpwg/http-extensions/issues/901
 (https://github.com/httpwg/http-extensions/issues/901)

A.6. draft-ietf-httpbis-rfc6265bis-05

 * Typos and editorial fixes: https://github.com/httpwg/http-
 extensions/pull/1035 (https://github.com/httpwg/http-extensions/
 pull/1035), https://github.com/httpwg/http-extensions/pull/1038
 (https://github.com/httpwg/http-extensions/pull/1038),
 https://github.com/httpwg/http-extensions/pull/1040
 (https://github.com/httpwg/http-extensions/pull/1040),
 https://github.com/httpwg/http-extensions/pull/1047
 (https://github.com/httpwg/http-extensions/pull/1047).

A.7. draft-ietf-httpbis-rfc6265bis-06

 * Editorial fixes: https://github.com/httpwg/http-extensions/
 issues/1059 (https://github.com/httpwg/http-extensions/
 issues/1059), https://github.com/httpwg/http-extensions/
 issues/1158 (https://github.com/httpwg/http-extensions/
 issues/1158).

 * Created a registry for cookie attribute names:
 https://github.com/httpwg/http-extensions/pull/1060
 (https://github.com/httpwg/http-extensions/pull/1060).

 * Tweaks to ABNF for "cookie-pair" and the "Cookie" header
 production: https://github.com/httpwg/http-extensions/issues/1074
 (https://github.com/httpwg/http-extensions/issues/1074),
 https://github.com/httpwg/http-extensions/issues/1119
 (https://github.com/httpwg/http-extensions/issues/1119).

 * Fixed serialization for nameless/valueless cookies:
 https://github.com/httpwg/http-extensions/pull/1143
 (https://github.com/httpwg/http-extensions/pull/1143).

 * Converted a normative reference to Mozilla's Public Suffix List
 [PSL] into an informative reference: https://github.com/httpwg/
 http-extensions/issues/1159 (https://github.com/httpwg/http-
 extensions/issues/1159).

A.8. draft-ietf-httpbis-rfc6265bis-07

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 * Moved instruction to ignore cookies with empty cookie-name and
 cookie-value from Section 5.3 to Section 5.4 to ensure that they
 apply to cookies created without parsing a cookie string:
 https://github.com/httpwg/http-extensions/issues/1234
 (https://github.com/httpwg/http-extensions/issues/1234).

 * Add a default enforcement value to the "same-site-flag",
 equivalent to "SameSite=Lax": https://github.com/httpwg/http-
 extensions/pull/1325 (https://github.com/httpwg/http-extensions/
 pull/1325).

 * Require a Secure attribute for "SameSite=None":
 https://github.com/httpwg/http-extensions/pull/1323
 (https://github.com/httpwg/http-extensions/pull/1323).

 * Consider scheme when running the same-site algorithm:
 https://github.com/httpwg/http-extensions/pull/1324
 (https://github.com/httpwg/http-extensions/pull/1324).

Acknowledgements

 RFC 6265 was written by Adam Barth. This document is a minor update
 of RFC 6265, adding small 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.

Authors' Addresses

 Mike West (editor)
 Google, Inc

 Email: mkwst@google.com
 URI: https://mikewest.org/

 John Wilander (editor)
 Apple, Inc

 Email: wilander@apple.com

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